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Canada-United States Air Quality Agreement Progress Report 2012

Section 2: Scientific and Technical Cooperation and Research

Joint Efforts

Emission Inventories and Trends

The United States and Canada have updated and improved their emission inventories and projections on PM10, PM2.5, VOCs, NOX, and SO2 to reflect the latest information available. These inventories are also being used in U.S. and Canadian air quality models to support the technical assessment of air quality problems and for the development of air quality management strategies. In the United States, the most recent complete emission inventory data are for the year 2008. The 2010 emission data in this section of the 2012 Progress Report are estimated based on 2008 and projected 2012 inventory data for the mobile source sectors, reported 2010 data for EGUs, and holding 2008 emissions constant for other sectors. The 2008 emission inventory and 2010 emission trends data are available at The 2012 projected inventory was used for U.S. EPA rulemaking and is a product of the 2005 based modeling platform . For Canada, the 2010 emission inventory was developed using the latest emission estimation methods and statistics, and includes the pollutant emission information reported by more than 8,700 facilities to the NPRI for 2010. The Canadian inventories are available at

Figure 25 shows the distribution of emissions by source category grouping for SO2, NOX, and VOCs. The following observations can be made from this figure:

  • SO2 emissions in the United States stem primarily from coal-fired combustion in the electric power sector and industrial boilers.
  • Canadian SO2 emissions come mostly from the nonferrous smelting and refining industry, upstream petroleum industry, and electric power generation utilities. The relative contribution from electric power generation utilities is lower in Canada due to the large hydroelectric and nuclear capacity in place, and differences in population and demand.
  • The distribution of NOX emissions in the two countries is similar, with nonroad and on-road vehicles accounting for the greatest portion of NOX emissions.

VOC emissions are the most diverse of the emission profiles in each country. The most significant difference is that most VOCs (36 percent) in Canada come from the industrial sector. This is the result of the proportionately higher contribution of oil and gas production in Canada. In the United States, solvent utilization (23 percent) and other anthropogenic sources (23 percent) -- e.g., agricultural livestock waste and field burning, prescribed burns, and petroleum storage and transport -- contribute the highest percentage of VOCs.

Figure 25. U.S. and Canadian National Emissions by Sector for Selected Pollutants, 2010

U.S. and Canadian National Emissions by Sector for Selected Pollutants, 2010


  • Emissions exclude natural sources (biogenics and forest fires).
  • Percentages may not add up to 100 due to rounding

Source: U.S. EPA and Environment Canada, 2012

Figure 26, Figure 27, and Figure 28 for SO2, NOX, and VOCs, respectively, show emissions from 1990 through 2010. Both countries have seen major reductions in SO2 emissions. In the United States, there is an overall trend of emission reduction for both SO2 and NOX. The major reductions in SO2 emissions came from electric power generation sources as well as industrial and commercial fuel combustion sources. For NOX, the reductions came from on-road and nonroad mobile sources, electric power generation sources, and other industrial fuel combustion sources. For VOCs, the largest reductions were mainly from on-road and nonroad mobile sources, solvent utilization, and petroleum storage and transport. As noted earlier, the increase in VOC emissions around 2002 was due to improved characterization methods for nonroad mobile sources and residential fuel combustion, as well as more complete characterization and exclusion of wildfires to account for anthropogenic sources only.

In Canada, the reductions in SO2 emissions came from the non-ferrous smelting and refining industry and the electric power generation utilities. For NOX, the reductions were from on-road mobile sources, electric power generation utilities, and the mining and rock quarrying industry. The VOC reductions came from on-road mobile sources and the downstream petroleum industry, with additional reductions from various industrial sectors such as chemical, pulp and paper, wood products, and iron and steel industries.

Figure 26. National SO2 Emissions in the United States and Canada from All Sources, 1990–2010

National SO2 Emissions in the United States and Canada from All Sources, 1990–2010

Source: U.S. EPA and Environment Canada, 2012

Figure 27. National NOX Emissions in the United States and Canada from All Sources, 1990–2010

National NOX Emissions in the United States and Canada from All Sources, 1990–2010

Source: U.S. EPA and Environment Canada, 2012

Figure 28. National VOC Emissions in the United States and Canada from All Sources, 1990–2010

National VOC Emissions in the United States and Canada from All Sources, 1990–2010

Source: U.S. EPA and Environment Canada, 2012

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Air Quality Reporting and Mapping

Canada and the U.S. collaborate closely on real-time air quality reporting and mapping through the AIRNow program (, which was initiated by the U.S. EPA more than a decade ago. The AIRNow program provides current and forecasted air quality information for monitoring sites throughout the U.S. and Canada. Each country is responsible for ensuring instrument calibration and comparability of ambient measurements of ozone and PM2.5. In 2004, the AIRNow program was expanded to provide information on PM2.5 and ozone measurements on a continental scale year-round. Figure 29 is an example of the kind of maps available on the AIRNow website which display pollutant concentration data expressed in terms of the color-coded Air Quality Index (AQI).

AIRNow also distributes air quality data via web services and text files through AIRNow Gateway

Note:  The AQI for ozone reflects 8-hour average ozone concentrations. Areas shaded orange indicate values that are “unhealthy for sensitive groups.” More information on the AQI is available at

Figure 29. AIRNow Map Illustrating the AQI for 8-hour Ozone

AIRNow Map Illustrating the <abbr>AQI</abbr> for 8-hour Ozone

Note: This map is an illustration of the highest ozone concentrations reached throughout the region on a given day. It does not represent a snapshot at a particular time of the day, but is more like the daily high temperature portion of a weather forecast. The AQI shown in the legend is based on 8-hour average ozone. More information on the AQI is available at

Source: U.S. EPA, 2012


Air quality monitoring measures the level of pollutants present in the air. This information is then used for a variety of purposes, including evaluation of the effectiveness of emission reduction measures, trends, notification of smog advisories, health studies, and comparison with standards.

The National Air Pollution Surveillance (NAPS) network and the CAPMoN are the two major ambient air monitoring networks in Canada. The NAPSProgram is a joint federal, provincial, territorial, and municipal initiative. The purpose of this Program is to coordinate the collection of air quality data from existing provincial, territorial, and municipal air quality monitoring networks and provide accurate and long-term air quality data of a uniform standard in a unified Canada-wide air quality database. Information about these networks can be found at

The associated federal and provincial/territorial/regional monitoring networks reporting data to the Canada-wide database comprise 318 air monitoring stations located in 217 communities. In total, over 800 instruments, including continuous analyzers for SO2, CO, NO2, ozone, and fine particulate matter are used to provide continuous air quality measurements. Toxic substances such as polycyclic aromatic hydrocarbons, dioxins and furans, and heavy metals such as arsenic, lead, and mercury are also analyzed for 24 hour events at scheduled 1 in 3 or 1 in 6 day intervals.

CAPMoN consists of 30 stations located in rural or remote areas, including one station in the United States. The objectives of CAPMoN differ from those of NAPS in that CAPMoN measurements provide data for research into: (1) regional-scale spatial and temporal variations of air pollutants and deposition, (2) long range transport of air pollutants (including transboundary transport), and (3) atmospheric processes, and chemical transport model evaluation. To meet these objectives, the CAPMoN sites are located in rural and remote areas.

Figure 30 shows the number of PM2.5 and ozone sites reporting to the Canada-wide air quality database in 2010. These sites are located in over 100 communities including all communities with a population greater than 100,000. In total, these communities account for about 75 percent of the Canadian population.

Figure 30. Ozone and Continuous PM2.5 Monitors Reporting to the NAPS Canada-wide Air Quality Database, 2010

Ozone and Continuous PM2.5 Monitors Reporting to the NAPS Canada-wide Air Quality Database, 2010

Source: Environment Canada, 2010

In addition to the continuous PM2.5 monitors, there were 41 filter-based samplers in operation, which meet the NAPSPM2.5 Reference Method criteria. The mass concentrations from these samplers are used for comparison with the continuous PM2.5 instruments and the filter media also undergo chemical analysis. A subset of these sites (13) make up the PM2.5 speciation network which measure major ions, organic and elemental carbon, metals and gas phase species including ammonia (NH3) and nitric acid. The principle gaseous precursors to secondary PM2.5 and ozone formation, SO2, NOx, and VOC are monitored at 152, 176, and 53 sites, respectively, reporting to the unified database. Measurements from these instruments are used to analyze source attribution and for the development of effective management strategies.

Recent investments to the air monitoring networks include:

  • A multi-million dollar investment to ensure that monitoring instruments are properly maintained and replaced when an instrument has reached its end of life. This has reduced the average age of NAPSinstruments from over 15 years to 5 years over the past decade.
  • Conversion of the existing continuous PM2.5instruments to U.S. Class III Federal Equivalent Method (FEM) instruments.
  • Modernization of federal, provincial, and municipal data logging and data reporting systems to allow more timely reporting of data and improve quality of real time data, used to report air quality indices (e.g., the Canadian Air Quality Health Index [AQHI]) and for real-time mapping websites (e.g., AIRNOW).
  • New samplers have been purchased to update the existing PM2.5 chemical speciation network and existing measurement programs for VOCs have also been enhanced.
  • Expansion of the laboratories and analytical equipment used to carry out detailed chemical analysis such as VOC and PM2.5 speciation.

United States

The majority of air quality monitoring performed in the United States is carried out by state, local, and tribal agencies in four major networks of monitoring stations: State and Local Air Monitoring Stations (SLAMS), Photochemical Assessment Monitoring Stations (PAMS), PM2.5 Chemical Speciation Network (CSN), and air toxics monitoring stations. In addition, ambient air monitoring is performed by the federal government (U.S. EPA, NPS, NOAA, the U.S. Geological Survey, and the U. S. Department of Agriculture), tribes, and industry. Air quality monitoring in the United States supports several air quality management objectives:

  • NAAQS attainment/nonattainment determination
  • Human exposure assessment for health research studies
  • Public air quality reporting and forecasting (AQI/AIRNow)
  • Accountability of control programs (ARP, NOX SIP Call, NBP, and CAIR)
  • Model evaluation
  • Determination of source receptor relationships
  • Characterization of regional air masses and transport
  • Ecological exposure assessments (acidity; nutrients; ozone; mercury and other persistent, bioaccumulative, and toxic chemicals)
  • Assessments for toxic air pollutants, trends, hotspots, human health exposure, and research.

Table 4 provides a summary list of major routine operating air monitoring networks in the U.S. The majority of air quality monitoring performed in the U.S. is carried out by state, local and tribal agencies in four major networks of monitoring stations: State and Local Air Monitoring Stations (SLAMS). Photochemical Assessment and Monitoring Stations (PAMS), PM2.5 Chemical Speciation Network (CSN) and air toxics monitoring stations.

Table 4. U.S. Air Quality Monitoring Networks

Table 4.1 Urban/Human-Health Monitoring
Major Routine Operating Air Monitoring Networks:
State / Local / Tribal / Federal Networks
Network*SitesInitiatedMeasurement ParametersSource of Information and/or Data
National Core Monitoring Network (NCore)~802011Ground level ozone (O3), reactive oxidized nitrogen,  (NO)/NOy, SO2, CO, PM2.5/PM10-2.5, PM2.5speciation, Surface
SLAMS~30001978O3, NOx/NO2, SO2, PM2.5/PM10, CO, lead (Pb)
CSN~200 currently active1999PM2.5 mass, PM2.5 speciation, major ions,
PAMS751994O3, NOx/NOy, CO, speciated VOCs, carbonyls, surface meteorology, upper
Table 4.2 Rural/Regional Monitoring
Major Routine Operating Air Monitoring Networks:
State / Local / Tribal / Federal Networks
Network*SitesInitiatedMeasurement ParametersSource of Information and/or Data
IMPROVE110 plus 67 protocol sites1988PM2.5/PM10, major ions, metals, light extinction, scattering
CASTNET80+1987O3, weekly concentrations of SO2, nitric acid (HNO3), sulfate (SO42-), nitrate (NO3-), chlorine (Cl-), ammonium (NH4+), calcium ions (Ca2+), magnesium ion (Mg2+), sodium ion (Na+), potassium ion (K+) for dry and total deposition, surface
Gaseous Pollutant Monitoring Program (GPMP)331987O3, NOx/NO/NO2, SO2, CO, surface meteorology, enhanced monitoring of CO, NO, NOx, NOy and SO2, canister samples for VOC at three
NADP/NTN250+1978Precipitation chemistry and wet deposition for major Ions (SO42,NO3-, NH4+, Ca2+, 3 Mg2+, Na+, K+, hydrogen ion [H+] as the measure of the activity of the solvated hydrogen ion [pH])
NADP/Ammonia Monitoring Network (AMoN)572010Bi-weekly
Table 4.3 Air Toxics Monitoring
Major Routine Operating Air Monitoring Networks:
State / Local / Tribal / Federal Networks
Network*SitesInitiatedMeasurement ParametersSource of Information and/or Data
National Air Toxics Trends Stations (NATTS)272005VOCs, carbonyls, PM10metals**, mercury (Hg)
State/Local Air Toxics Monitoring250+1987VOCs, carbonyls, PM10metals**, Hg
National Dioxin Air Monitoring Network341998–2005Chlorinated dibenzo-p-dioxins (CDDs), furans (CDFs), dioxin-like
NADP/Mercury Deposition Network100+1996Mercury measured in precipitation and wet
NADP/AMNet212009Speciated ambient mercury concentrations, gaseous oxidized mercury (GOM), particulate bound mercury (PBM), gaseous elemental mercury (GEM)
Integrated Atmospheric Deposition Network (IADN)201990Polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyl (PCBs), and organochlorine compounds are measured in air and


* Some networks listed separately may also serve as subcomponents of other larger listed networks; as a result, some double counting of the number of individual monitors is likely. This list of networks is not meant to be totally inclusive of all routine monitoring in the United States.

**PM10 metals may include arsenic, beryllium, cadmium, chromium, lead, manganese, nickel, and others.

Source:  U.S. EPA, 2012

The U.S. EPA introduced a new multi-pollutant monitoring network referred to as NCore that became operational in 2011. Monitors at NCore sites measure particles (e.g., PM2.5, speciated PM2.5, PM10-2.5), ozone, SO2, CO, NO, NOy, Pb, and basic meteorological parameters. Sites are broadly representative of urban (about 60 sites) and rural (about 20 sites) locations across the U.S. Where possible, states locate urban NCore sites next to existing monitoring operations to leverage existing resources. The objective of the NCore network is to gather additional information needed to support emissions and air quality model development, air quality program accountability, and future health studies. General information on the NCore network is available at More specific information on each NCore site can be viewed or downloaded from

The U.S. EPA has completed transitioning of the carbon measurement at CSN-speciated PM2.5 stations to the IMPROVE protocol to support better comparability between CSN and IMPROVE networks. This effort was initiated in 2007.

The U.S. EPA finalized revisions to monitoring requirements for lead in 2008 to support the tightening of the lead NAAQS from 1.5 µg/m3 (quarterly average) to 0.15 µg/m3(rolling three-month average). New monitoring requirements included the establishment of source-oriented lead monitoring sites around lead sources emitting 1.0 or greater short tons (0.9 metric tons) of lead per year by January 1, 2010. Additional lead monitoring requirements were finalized at the end of 2010 including the addition of lead trends monitoring at urban NCore sites, and the establishment of a short-term monitoring study at 15 general aviation airports across the U.S. Information on changes to the lead NAAQS and associated monitoring requirements is available at:

New ambient monitoring requirements have been established for the recently revised NO2 and SO2, and CO NAAQS, including a near-roadway requirement for NO2 and CO monitoring. All new NO2 and SO2 monitors must begin operating no later than January 1, 2013, while new CO near-road monitors will be phased in to the NO2near-road sites between January 1, 2015 and January 1, 2017. Information on the near-roadway effort is available at: Additional details on the NO2, SO2, and CO monitoring requirements and the proposed changes to ozone monitoring requirements are available at

The NADP is operating the Ambient Mercury Network (AMNet), which measures ambient concentrations of speciated mercury at 21 sites throughout the U.S. and Canada. The data from this network will provide status and trends of ambient mercury concentrations, as well as information for model development including validation and source apportionment.

EPA collaborated with NADP to establish the AMoN as a NADP sub-network in 2010. The NADP operates AMoN, which uses passive devices to measure gaseous NH3 concentrations. Currently there are 57 sites collecting two-week samples of ambient NH3 concentrations. These measurements are needed to enhance atmospheric and deposition models, validate emission inventories, and understand the chemistry driving PM2.5formation. Both efforts aim to utilize the NADP committee structure as a platform for initiation and continued growth. The NADP website contains data, maps, and other program information ( In the past two years, EPA has collocated AMoN with more than twenty-five CASTNET sites, and the NPS has collocated AMoN with nine CASTNET sites. Other recent activities related to CASTNET include transitioning its ozone monitoring operations to fully meet the regulatory quality requirements applicable to SLAMS air monitoring data and real-time reporting of hourly ozone and meteorological data to the AIRNow system for use in forecasting and mapping current air quality conditions. In addition, CASTNET is evaluating monitoring methods that provide highly time-resolved (i.e., hourly) measurements of both gaseous (SO2, HNO3, NH3) and aerosol (sulfate, ammonium, nitrate, chloride and other base cations) components. The website for CASTNET includes program information, data and maps, annual network reports, and quality assurance information (

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Health Effects


Health Canada assessments and research provide health-based guidance for regulatory and non-regulatory actions to improve air quality and human health, including the new national framework for managing air quality called the Air Quality Management System (AQMS), which was developed in collaboration with Environment Canada and the provinces, territories and stakeholders.

Current priorities for addressing ambient air quality include:

  • Implementing the new Canadian Ambient Air Quality Standards
  • Assessing the contribution of different industrial sectors to air pollution and the resulting health risks
  • Improving understanding of the transportation sector’s contribution to air pollution
  • Improving and updating the Air Quality Benefits Assessment Tool (AQBAT) used in the cost-benefit analysis of policies to reduce air pollution
  • Advancing the Air Quality Health Index (AQHI) as an information tool for the general public and vulnerable populations

Some recent highlights of Health Canada’s air quality and health research include:

Canadian Smog Science Assessment

Health Canada and Environment Canada have finalized a comprehensive Canadian Smog Science Assessment covering 2002-2006, to provide credible and relevant scientific information to support actions to improve air quality in Canada. The Highlights and Key Messages were published in April 2012 (see

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Canadian Health and Exposure Research

Canadian Census Cohort – Mortality and Air Pollution Study

The study was initiated in 2009, in collaboration with Statistics Canada, to examine the mortality risk of long-term exposure to air pollution in the Canadian population using long-form census data (1991) on 2.7 million Canadians linked to vital status information up to 2007. Exposure to PM2.5was estimated from ground-based observations and satellite data. The results showed a statistically significant positive association between long-term exposure to PM2.5and mortality. This association was observed at PM2.5concentrations that were lower than have been previously reported (mean = 8.7-µg/m3, inter-quartile range = 6.2-µg/m3).

Some results from this large cohort study have been published in a 2012 article by Crouse et al. entitled “Risk of Mortality Associated with Long-term Exposure to Low Concentrations of Fine Particulate Matter: A Canadian National-level Population-based Cohort Study” (Environmental Health Perspectives 120: 708-71). Additional analyses of the cohort will focus on specific causes of death and will investigate individual communities.

Multi-pollutant Modeling and Monitoring

The management of industrial emissions of air pollutants is an important element of the proposed AQMS. To support this initiative, multi-pollutant modeling and monitoring of emissions from specific industrial sectors are being conducted. These studies will characterize Canadians’ exposure to the range of pollutants emitted from major industrial sources and assess the associated hazards. A major focus is the chemical characterization of PM coming from the different sources. Studies of pulp and paper, aluminum, cement, base-metal smelting, iron and steel, and coal-fired electricity sectors are currently in progress. It is expected that up to 14 major industrial sectors will be addressed in this manner. This information will be used to guide the development of cost-effective actions to reduce industrial emissions.

Industrial Emissions and the Exacerbation of Adverse Health Effects in Asthmatic Children

In 2009, Health Canada initiated a study to examine the impact of industrial emissions on respiratory and cardiovascular health in asthmatic children. A panel of 80 asthmatic children underwent daily tests of pulmonary function, lung inflammation, blood pressure, and heart rate. Preliminary results showed an association between personal exposures to PM2.5 and increased airway inflammation in children with asthma. Results also showed that personal exposure to SO2 (a marker of refinery emissions) was associated with reduction in small airway function in these children. Data analysis comparing refinery emissions and health measures continues.

In Vitro and In Vivo Toxicity Models for Characterization of the Relative Potency of PM

A cytotoxicity assay platform has been developed that reduces the particle mass required for in vitro toxicity bioassays and targeted gene expression analyses. This platform facilitates the assessment of the toxicity of PM samples and supports interpretation of the data in the context of human health risk. This method has been used, for example, to assess the impact of particle composition, size, and aging on particle potency and to assess variability of potency among seasons.

Regression of potency data against elemental composition identified several metals as drivers of toxicity, including zinc, which has been previously implicated in certain adverse health outcomes in toxicological and epidemiological literature. Importantly, the data show that particle potency rankings generated using individual cell lines or assays may differ from one another, indicating that a number of assays and cell lines should be used to assess the cytotoxic potency of particles in an integrated fashion. Regressing in vivo data against in vitro assays showed that subsets of in vitro assays can be predictive of effects observed in vivo.

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Canadian Health and Exposure Tools to Support Risk Management

Air Quality Benefits Assessment Tool

The AQBAT is a computer simulation program similar to the Environmental Benefits Mapping and Analysis Program developed by Health Canada to estimate the human health costs and/or benefits associated with changes in ambient air quality. AQBAT was made publicly available in 2006 and has been applied to federal government policy proposals on air quality as well as by a number of municipal governments and consultants in specific policy contexts. An updated version (AQBAT 2.0) was released in April 2012. The revised version includes a number of improvements, including updated population, air pollution, and baseline incidence data of hospital admissions and mortality; and revised concentration-response functions and valuation parameters for selected outcomes. Life expectancy changes associated with changes in air pollution can also be estimated in AQBAT 2.0.

Canadian Air Quality Health Index

The AQHI is a public information tool, developed jointly by Environment Canada and Health Canada, which helps Canadians protect their health on a daily basis from the negative effects of air pollution. The AQHI is based on epidemiological data that relates air pollution exposures to acute health outcomes. This index employs a linear, non-threshold concentration-response relationship of short-term health risks of the smog mixture using three pollutants (NO2, ground-level ozone, and PM2.5) as a surrogate measure of the more complex mixture in the urban atmosphere. The index is expressed on a 1 to 10+ scale, where higher values represent a greater health risk.

In addition to the scale, corresponding health messages have been developed for general and "at-risk" populations. The current (hourly) and forecasted (today and tomorrow) AQHI values and their associated health messages are publicly available at and on the Weather Network broadcasts and website in locations where the AQHI is available. This information will allow Canadians to make informed choices to protect themselves and those in their care from the short-term health impacts of exposure to air pollution.

The AQHI is now available in 74 communities, representing more than 60 percent of the Canadian population, with additional communities to be added as the AQHI is implemented across the country.

In January 2012, a workshop was held to give stakeholders and scientists an opportunity to discuss the index together. It was agreed that the timing was appropriate for a review of the index. The review has begun with an update of the air pollution and mortality data analysis, particularly for the coefficients of the three current AQHIpollutants, as well as CO and SO2. It will also consider health endpoints other than mortality and other adjustments that may be made in terms of spatial scales, pollutants, instrumentation, and presentation of the index.

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United States

Review of U.S. Ozone, PM, NO2, and SO2 Air Quality Standards

Under the Clean Air Act (CAA), the EPA is required to set NAAQS for widespread pollutants from numerous and diverse sources considered harmful to public health and the environment. The CAAestablished two types of NAAQS.

  • Primary standards set limits with an adequate margin of safety to protect public health, including the health of at-risk populations, such as children, older adults, and persons with pre-existing cardiovascular and respiratory disease such as asthma.
  • Secondary standards set limits to protect public welfare form any known or anticipated adverse effects, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.

The U.S. EPA has set NAAQS for six common pollutants, which are often referred to as “criteria” pollutants. These pollutants are: PM, ozone, SO2, NO2, carbon monoxide (CO), and lead.

The CAA requires U.S. EPA to periodically review (every 5 years) the science upon which the NAAQS are based and the standards themselves. Reviewing the NAAQS is a lengthy undertaking that follows a well-established process.[4] Each review involves a comprehensive review, synthesis, and evaluation of the scientific information (Integrated Science Assessment, ISA), the design and conduct of complex air quality and risk and exposure analyses (Risk and Exposure Assessment, REA), the development of a comprehensive policy assessment providing a transparent staff analysis of the scientific basis for the broadest range of alternative policy options supported by the scientific and technical information (Policy Assessment), and the development of proposed and final rules. The assessments providing the foundation for the Agency’s decisions undergo extensive internal and external scientific peer-review.

Ozone NAAQS review

Exposure to ozone is associated with a wide variety of adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and new onset asthma as well as premature mortality. Children and individuals with lung disease are considered at-risk populations. In addition, repeated exposure to ozone during the growing season damages sensitive vegetation. Cumulative ozone exposure can lead to reduced tree growth, visibly injured leaves, and increased susceptibility to disease, damage from insects, and harsh weather.

On March 12, 2008, the U.S. EPA strengthened the primary and secondary 8-hour standards for ozone by lowering the levels of the standards from 0.08 to 0.075 ppm to improve both public health protection and the protection of sensitive trees and plants.. Final designations for these standards were completed in May 2012 with 46 areas being designated as nonattainment.

The U.S. EPA is in the midst of its next statutorily-mandated review of the ozone standards to ensure that the NAAQS provide appropriate public health and environmental protection. As part of this review, EPA has issued a number of draft documents for external scientific and public review.  Additional information on the current and previous ozone NAAQS reviews can be found at

Particulate Matter NAAQS

An extensive body of scientific evidence shows that exposure to PM causes premature death and is linked to a variety of significant health problems, such as increased hospital admissions and emergency department visits for cardiovascular and respiratory effects, including non-fatal heart attacks. Exposure to PM is also linked to the development of chronic respiratory disease. There are several groups within the general population that are greater risk for experiencing PM-related effects. These at-risk populations include individuals with preexisting heart and lung disease, older adults, children, and those that live in poverty.

In addition, the contribution of particles, especially fine particles, to visibility impairment has been recognized for a long time. Visibility is affected by particles that scatter and absorb light. Particle composition and size as well as relative humidity are important factors for understanding the impacts of particle pollution on visibility impairment. Particles are also associated with a wide range of non-visibility welfare effects including: ecological effects, effects on materials, and climate impacts.

In 2007, the U.S. EPA initiated the current review of the PM NAAQS and on June 14, 2012, the U.S. EPA proposed revisions to the NAAQS which would strengthen the primary and secondary PM2.5 standards to provide requisite protection for public health and welfare. Specifically, the U.S. EPA proposed to lower the level of the primary annual PM2.5 standard to provide increased protection against health effects associated with long- and short-term PM2.5 exposures and to retain the primary 24-hour PM2.5 standard. The Agency proposed to add a distinct secondary standard for PM2.5 to address PM-related visibility impairment and to retain the current secondary PM2.5 and PM10 standards to address non-visibility welfare effects. Also, the U.S. EPA proposed to retain the primary 24-hour standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM10-2.5). Additional information on the proposed rule, including supporting documents, can be found at The U.S. EPA plans to issue a final rule in December 2012.

NO2 NAAQS (Primary Standards)

Exposure to NO2 has been associated with a variety of health effects, including respiratory symptoms, especially among asthmatic children, and respiratory-related emergency department visits and hospital admissions, particularly for children and older adults. On January 22, 2010, based on the results of NO2health effects evidence as assessed in the ISA and estimates of NO2-associated exposures and health risks presented in the REA, the U.S. EPA revised the primary NO2 NAAQS, and established new requirements for the NO2 monitoring network. Specifically, the U.S. EPA promulgated a new 1-hour primary NO2 standard with a level of 100 ppb, retained the existing annual standard with a level of 53 ppb, and established a requirement that a substantial number of NO2 monitors be sited within 50 meters of major roads and in other locations where maximum NO2concentrations are expected to occur. Additional information on the final rule, including supporting documents, can be found at The U.S. EPA has recently started its next periodic review of the primary NO2standards, additional information can be found at

SO2 NAAQS (Primary Standard)

Asthmatics are especially susceptible to the effects of SO2. Short-term exposures of asthmatic individuals to elevated levels of SO2 while exercising at a moderate level may result in breathing difficulties, accompanied by symptoms such as wheezing, chest tightness, or shortness of breath. Studies also provide consistent evidence of an association between short-term SO2 exposures and increased respiratory symptoms in children, especially those with asthma or chronic respiratory symptoms. Short-term exposures to SO2 have also been associated with respiratory-related emergency department visits and hospital admissions, particularly for children and older adults.

On June 2, 2010, based on the results of SO2 health effects evidence assessed in the ISA, and on estimates of SO2-associated exposures and health risks presented in the REA, the U.S. EPA strengthened the primary NAAQS for SO2. The U.S. EPA revised the primary SO2standard by establishing a new 1-hour standard at a level of 75 ppb. The revised standard will improve public health protection, especially for children, older adults, and people with asthma. The U.S. EPA’s evaluation of the scientific information and the risks posed by breathing SO2 indicate that this new 1-hour standard will protect public health by reducing people’s exposure to high short-term (5 minutes to 24 hours) concentrations of SO2. The U.S. EPA revoked the two existing primary standards of 140 ppb evaluated over 24 hours, and 30 ppb evaluated over an entire year because they will not add additional public health protection given a 1-hour standard at 75 ppb. Also, there is little health evidence to suggest an association between long-term exposure to SO2 and health effects. Additional information on the final rule, including supporting documents, can be found at

Oxides of Sulfur and Nitrogen NAAQS (Secondary Standards)

NOX and SOX in the air can damage the leaves of plants, decrease their ability to produce food – photosynthesis – and decrease their growth. In addition to directly affecting plants, NOX and SOX when deposited on land and in estuaries, lakes and streams, can acidify and over-fertilize sensitive ecosystems resulting in a range of harmful deposition-related effects on plants, soils, water quality, and fish and wildlife (e.g., changes in biodiversity and loss of habitat, reduced tree growth, loss of fish species, and harmful algal blooms). On March 20, 2012, the U.S. EPA completed its review of the secondary NOX and SOX standards. This was the first time that the U.S. EPA reviewed the environmental impacts separately from the health impacts of these pollutants. It is also the first time the Agency examined the effects of multiple pollutants in one NAAQS review.

Based on its review of the currently available scientific information, the U.S. EPA retained the current annual NO2 standard set at a level of 0.53 ppm and 2-hour SO2 standard set at a level of 0.5 ppm to address the direct effects on vegetation (e.g., decreased growth and foliar injury). With regard to the deposition-related effects, the final rule recognized that the existing standards do not provide adequate public welfare protection. While there is strong scientific support for developing a multi-pollutant standard to address these deposition-related effects, the U.S. EPA concluded it does not yet have enough information to set such a standard that would adequately protect the diverse ecosystems across the country. Additional information on the final rule, and supporting documentation, can be found at

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U.S. Exposure and Health Research

Clean Air Research Centers

In March of 2011, the U.S. EPA announced the awarding of $32 million to fund four new Clean Air Research Centers at universities conducting cutting-edge air pollution research. The funds will support investigations that focus on the impacts of air pollution mixtures on people’s health, moving the science beyond past studies that concentrated on single pollutants. The work will advance the understanding of the health risks associated with exposure to multiple air pollutants, providing critical insights into real world exposure scenarios.

The research centers will investigate a myriad of health effects, ranging from cardiovascular and pulmonary problems to neurological and inflammation outcomes. The research centers will also study those most susceptible to air pollution, including children, the elderly, people with pre-existing conditions, and people living in communities that present greater health risks associated with air pollution. Each center will receive approximately $8 million over five years. The Clean Air Research Centers are located at:

  • Emory University and Georgia Institute of Technology, Atlanta, Georgia
  • Harvard University, Boston, Massachusetts
  • Michigan State University, East Lansing, Michigan
  • University of Washington, Seattle, Washington

New Insights into Air Pollution and Cardiovascular Health

Recent research results in the U.S. have provided new insights into the association of air pollution and cardiovascular health. A U.S. EPA study of potential health impacts from exposure to emissions from a wildfire in North Carolina used satellite imagery and ER records from the affected and surrounding area to demonstrate, for the first time, an association between smoke from peat fires and an increased number of ER visits for symptoms of heart failure.[5] The study also showed a significant increase in respiratory effects (asthma, pneumonia and acute chronic bronchitis) in the high-smoke areas and discovered that certain groups of people--older adults and those with pre-existing lung and heart problems, for example--were more susceptible to the adverse effects of wildfire smoke. Another U.S. EPA funded study, the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air), is designed to examine the relationship between air pollution exposures and the progression of cardiovascular disease over longer time periods. This ten-year study, which is led by the University of Washington, involves thousands of participants, representing diverse areas of the United States. An early finding from the MESA Air study showed that exposures to fine particles is associated with narrower arteriolar diameter in the retina of middle-aged and older adults.[6] While the clinical significance of the change is yet to be determined, these results demonstrate that exposures to PM may result in measurable cardiovascular effects, which may help explain the development and exacerbation of cardiovascular disease. In addition, a study conducted at Harvard and Brown University found associations between air pollution and an increased risk of ischemic strokes.[7] The study, which was supported by the National Institute of Environmental Health Science and the U.S. EPA, used hourly measurements of fine particles and detailed information from medical records about the timing of initial stroke symptoms, involving more than 1,700 stroke patients in the Boston area over a 10-year period. Finally, in addition to the studies above which focused primarily on particle pollution, a U.S. EPA study has also provided new evidence of associations between ozone and cardiovascular symptoms.[8]

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Ecological Effects

Research and Monitoring of Aquatic Acid Deposition Effects

Precipitation Chemistry

Analyses of trends in North American precipitation and surface water chemistry for the period 1990 to 2008 were recently released as part of a report of the International Cooperative Programme (ICP) Waters program under the United Nations Economic Commission for Europe (UNECE). Canada and the United States contribute to the ICP Waters program as a party to the Convention on Long-range Transboundary Air Pollution.

Levels of sulfate in precipitation presented a decreasing trend at 97 percent of the sites in northeastern North America over the period 1990 to 2008. The decrease was 37 percent on average and was more pronounced during the first decade. The trend can be traced back to the decrease in North American sulfur emissions and the resulting substantial decrease in atmospheric deposition of sulfate. Similarly, significant reductions in NOXemissions in North America led to a 30 percent reduction in average nitrate levels in precipitation. The precipitation data reflected the fact that the NOXemission reductions were not as large as for SO2 and they predominantly occurred during the 1999 to 2008 period.

Of the other parameters that are important in assessing critical loads and exceedances(see critical loads and exceedances), the concentrations of ammonium and base cations (sum of calcium, magnesium, sodium, and potassium ions) did not exhibit a clear trend in North America, while hydrogen ions decreased by 55 percent. Similar to nitrates, the decrease in hydrogen ions, or evolution towards less acidic pH levels, occurred largely in the latter part of the period.

Surface Water Chemistry

Analysis of trends in surface water chemistry for the period 1999 to 2008 provided information on the geographic extent of acidification and recovery of lakes and streams in eastern North America. Figure 31 shows the 96 North American ICP sites that were grouped into six regions (Maine and Atlantic Canada, Vermont and Quebec, Adirondacks, Appalachian Plateau, Virginia Blue Ridge, and Ontario) and analyzed for acidification and or recovery trends. Data from 13 sites in Ontario were added to those covered by the ICP Waters program to improve the data representativeness for that region. The trends observed for 1999 to 2008 were compared to those from 1990 to 1999 to determine if the rate of recovery was changing.

Overall, water chemistry trends at the North American monitoring sites generally showed chemical recovery between 1990 and 2008 corresponding to the observed reductions in acidic deposition. The decreasing trend in sulfate and increasing trends in pH and acid neutralizing capacity showed consistent chemical recovery from acidification across a large number of sites. Some exceptions occurred at a number of sites in Atlantic Canada. The concentration of base cations that is important for aquatic biota and chemical recovery had been decreasing as a result of the decline in sulfate. However, the rate of base cation decrease compared to that of sulfate slowed down after 2000, which also indicates recovery.

Compounding the recovery, levels of dissolved organic carbon (DOC) were shown to be increasing in many of the monitoring sites in North America. DOC affects (among other things) light penetration, primary production, dissolved oxygen concentrations, and is an indicator of natural organic acidity which may impede increases in pH and alkalinity. DOC levels have been rising in many regions around the world and are believed to be influenced by the combination of decreasing sulfur deposition and climatic factors.

Figure 31. Eastern North American Sites Reporting Data to the ICP Waters Database (in green) and the 13 Additional Stations in Ontario (in yellow)

Eastern North American Sites Reporting Data to the ICP Waters Database (in green) and the 13 Additional Stations in Ontario (in yellow)

Source: Skjelkvåle, B.L. and de Wit, H.A. 2011. ICP Waters Report 106/2011: Trends in precipitation chemistry, surface water chemistry and aquatic biota in acidified areas in Europe and North America from 1990 to 2008. Norwegian Institute for Water Research, Report SNO 6218-2011, p. 128

Recovery of Acidified Lakes and Streams in the United States

  • Sulfate ion concentrations in surface waters provide important information on the extent of base cation (i.e., calcium, magnesium, potassium, and sodium) leaching in soils and offer insight on how sulfate concentrations relate to the levels of ambient atmospheric sulfur and atmospheric deposition.
  • Nitrogen is an important nutrient for plant growth and, therefore, most nitrogen inputs by deposition are quickly incorporated into biomass during the growing season as organic nitrogen, with little leaching of nitrate into surface waters during the growing season. As atmospheric nitrogen deposition increases, there is greater potential for increased leaching of nitrate into surface waters.
  • Acid neutralizing capacity (ANC) is a measure of the acid-buffering capacity of water and an important indicator of the sensitivity and the degree of surface water acidification or recovery that occurs over time. Acidification results in a diminishing ability of water in the lake or stream to neutralize strong acids that enter aquatic ecosystems.

Acid rain, resulting from SO2 and NOXemissions, is one of many large-scale anthropogenic effects that negatively affect the health of water bodies (lakes and streams) in the United States and Canada. Surface water chemistry provides direct indicators of the potential effects of acidic deposition on the overall health of aquatic ecosystems.

Two U.S. EPA-administered monitoring programs provide information on the impacts of acidic deposition on otherwise protected aquatic systems: Temporally Integrated Monitoring of Ecosystems (TIME) and Long-term Monitoring (LTM) programs. These programs are designed to track changes in surface water chemistry in the four acid sensitive regions shown in Figure 32: New England, the Adirondack Mountains, the Northern Appalachian Plateau, and the central Appalachians (the Valley and Ridge and Blue Ridge Provinces).

Figure 32. Long-Term Monitoring Program Sites

Long-Term Monitoring Program Sites

Source: U.S. EPA, 2012

Five indicators of aquatic ecosystem response to emission changes are presented: measured ions of sulfate and nitrate, base cations, ANC, and DOC. These indicators provide information regarding the surface water sensitivity to acidification. Trends in these measured chemical receptors allow for the determination of whether the conditions of the water bodies are improving and heading towards recovery or if the conditions are still acidifying.

As seen in Table 5, significant improving (decreasing) trends in sulfate concentrations from 1990 to 2009 are found at nearly all monitoring sites in New England, the Adirondacks, and the Catskill Mountains/Northern Appalachian Plateau. However, in the Central Appalachians only 12 percent of monitored streams showed a decreasing sulfate trend, while 14 percent of monitored streams actually increased, despite decreasing sulfate deposition. The highly weathered soils of the Central Appalachians are able to store large amounts of deposited sulfate, but as long-term sulfate deposition exhausts the soil’s ability to store more sulfate, a decreasing proportion of the deposited sulfate is retained in the soil and an increasing proportion is exported to surface waters.

Surface nitrate concentration trends are decreasing at some of the sites in all four regions, but some sites also indicate flat or slightly increasing nitrate trends. Improving (decreasing) trends for nitrate concentration were noted at 37 percent of all monitored sites, but this improvement may only be partially explained by decreasing deposition. Ecosystem factors, such as vegetation disturbances and soil retention of the deposited nitrogen, are also known to contribute to declining surface water nitrate concentrations.

Reductions in sulfate deposition levels likely result in many of the improving (increasing) ANC trends. From 1990 to 2009, monitoring sites in the Adirondacks (60 percent), and the Catskills/Northern Appalachian Plateau (55 percent) showed the strongest improvement in ANC trends. However, sites in New England (20 percent) and the Central Appalachians (17 percent) had few sites with improving ANC trends. The relatively flat trends in sulfate in the Central Appalachians likely account for why so few sites have improving ANC. In New England, hydrology and declining trends of base cation concentration may delay the onset of recovery. Decreasing base cation levels can balance out reductions of sulfate and nitrate, thereby preventing ANC from increasing. DOC is increasing at only 30 percent of all monitored water bodies. This is likely linked to declines in sulfate concentrations as well as warmer seasonal and annual temperatures.

Table 5 shows that significant improving (decreasing) trends in sullfate concentrations from 1990 to 2009 are found at nearly all monitoring sites in New England, the Adirondacks, and the Catskill Mountains/Northern Appalachian Plateau. In the Central Appalachians only 12 percent of monitored streams showed a decreasing sulfate trend, while 14 percent of monitored streams actually increased, despite decreasing sulfate deposition.

Table 5. Regional Trends in Sulfate, Nitrate, ANC, and DOC at LTM Sites, 1990-2009.
RegionWater bodies CoveredPercentage of Sites with Improving Sulfate TrendPercentage of Sites with Improving Nitrate TrendPercentage of Sites with Improving ANC TrendPercentage of Sites with Improving Base Cations TrendPercentage of Sites with Improving DOC Trend
Adirondack Mountains50 lakes in New York94%48%60%74%48% (29 sites)
Catskills/N. Appalachian Plateau*9 streams in New York and Pennsylvania80%30%55%80%25% (9 sites)
New England26 lakes in Maine and Vermont96%33%20%57%26% (15 sites)
Central Appalachians66 streams in Virginia12%50%17%12%NA


  • Trends are determined by multivariate Mann-Kendall tests.
  • Trends are significant at the 95 percent confidence interval (p < 0.05).
  • DOC was only examined in low-ANC water bodies (ANC less than 25 μeq/L).
  • DOC is not currently measured in Central Appalachian streams.

*Data for streams in N. Appalachian Plateau is only through 2008.

Source: U.S. EPA, 2011

Critical Loads and Exceedances

Improving the Uncertainty Estimate in Critical Loads of Canadian Forest Ecosystems

Critical loads of acidity (sulfur and nitrogen) form the basis of emission reduction policies in Canada. A critical load is developed to protect a specific biological indicator, and is defined as the quantitative estimate of an exposure to one or more pollutants below which the long-term unacceptable effects on specified elements will not occur according to present knowledge and policy.[9] A study was commissioned by the CCME to assess the impact of uncertainties in regional data sets on the probability of exceeding Canadian forest ecosystem critical loads.[10] Uncertainty increases as data are applied on national or continental scales.

In this analysis, the probability of exceeding a critical load was evaluated for the 2002 and 2006 total S+N (sulfur + nitrogen) deposition modeled by A Unified Regional Air quality Modelling System (AURAMS), for two chemical criterion: base cation to aluminum ratios (Bc:Al) of 1 and 10. These two ratios were selected to protect tree roots and soil nutrient pools, respectively. The Bc:Al = 1 ratio is the most commonly used protection limit in Europe and elsewhere, while a Bc:Al = 10 ratio was previously used in Canada for mineral forest soil. The critical loads of acidity were estimated using the Steady-State Mass Balance model.

The analysis showed a significant reduction in 2006 in the area with a high probability of exceedance compared to 2002 (Figure 33). The uncertainty in critical loads averaged 27 to 28 percent under both chemical scenarios across Canada, with greater uncertainty occuring in northern Ontario, central Manitoba and Saskatchewan, and northern British Columbia. Despite uncertainties in regional data sets, the high probabilities of critical loads exceedance in many parts of the country even in the later year of study (2006) support the need for further emission reductions.

Figure 33. Critical Load Exceedance Probability, 2002 and 2006  

Critical Load Exceedance Probability, 2002 and 2006

Note: Under AURAMS 2002 deposition, the high exceedance probability in the northeast and northwest is an artifact caused by model domain boundary parameters.

Source: Environment Canada, 2012

Use of Critical Loads in the U.S.

In the United States, the critical loads approach is not an officially accepted approach to ecosystem protection. Language specifically requiring a critical loads approach does not exist in the CAA. Nevertheless, the critical loads approach is a useful an ecosystem assessment for communicating complex scientific information. Interest in the use of critical loads in the United States has increased in recent years with the advent of the Critical Loads of Atmospheric Deposition Science Committee within NADP in 2010, several recent workshops and meetings on this topic, and several publications exploring greater use of critical loads as a policy-relevant environmental assessment tool.

Drawing on the methods from the peer-reviewed scientific literature, critical loads were calculated for over 2,300 lakes and streams using the Steady-State Water Chemistry model. These critical load estimates represent only lakes and streams where surface water samples have been collected through programs such as National Surface Water Survey, Environmental Monitoring and Assessment Program (EMAP), the TIME program, and the LTM program. The lakes and streams associated with these programs consist of a subset of lakes and streams that are located in areas most affected by acid deposition, but are not intended to represent all lakes in the eastern U.S.

For this particular analysis, the critical load represents the combined deposition loads of sulfur and nitrogen to which a lake or stream could be subjected and still have a calculated ANC of 50 μeq/L or higher. While a critical load can be calculated for any ANC level, this level was chosen because it tends to support healthy aquatic ecosystems and protect most fish and other aquatic organisms, although systems can become episodically acidic and some sensitive species still may be lost. Critical loads of combined total sulfur and nitrogen deposition are expressed in terms of ionic charge balance as milliequivalents per square meter per year.

If pollutant exposure is less than the critical load, adverse ecological effects (e.g., reduced reproductive success, stunted growth, loss of biological diversity) are not anticipated, and recovery is expected over time if an ecosystem has been damaged by past exposure. A critical load exceedance is the measure of pollutant exposure above the critical load. This means pollutant exposure is higher than, or exceeds, the critical load and the ecosystem continues to be exposed to damaging levels of pollutants. In order to assess the extent to which regional lake and stream ecosystems are protected by the emission reductions achieved by the Clean Air Act Amendments (CAAA) so far, this case study compares the amount of deposition systems can receive--the critical load-- to measured deposition for the period before implementation of the CAAA (1989 to 1991) and for a period representing the most recent data (2008 to 2010).

Overall, this critical load analysis shows that emission reductions achieved so far have resulted in improved environmental conditions and increased ecosystem protection in the eastern United States. For the period from 2008 to 2010, 30 percent of the lakes and streams examined received levels of combined sulfur and nitrogen deposition that exceeded the critical load (Figure 34). This is an improvement when compared to the 1989 to 1991 period, during which 55 percent of lakes and streams exceeded the critical load. Areas with the largest concentration of lakes where acid deposition currently is greater than--or exceeds--estimated critical loads include the southern Adirondack mountain region in New York, southern New Hampshire and Vermont, Cape Cod (Massachusetts), and along the Appalachian Mountain spine from Pennsylvania to North Carolina.

Figure 34. Lake and Stream Exceedances of Estimated Critical Loads for Total Nitrogen and Sulfur Deposition, 1989–1991 vs. 2008–2010

Lake and Stream Exceedances of Estimated Critical Loads for Total Nitrogen and Sulfur Deposition, 1989–1991 vs. 2008–2010

Source: U.S. EPA, 2011

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U.S. Atmospheric Science Research


Scientists from the National Aeronautics and Space Administration (NASA) and the U.S. EPA are participating in a five-year collaborative project known as "DISCOVER-AQ"--for Deriving Information on Surface Conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality ( overall goal of the project is to improve the use of satellites to monitor air quality for public health and environmental benefit. The project includes targeted airborne and ground-based observations, which will enable more effective use of current and future satellites to diagnose ground level conditions influencing air quality.

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New Version of the Community Multiscale Air Quality Model

In 2011, U.S. EPA released a new version of the Community Multiscale Air Quality Model (CMAQ) modeling system. The release of CMAQ version 5.0 introduced additional tools for studying air quality and its impacts on climate change. Taking advantage of improved computing power and recent developments in air chemistry and atmospheric science, CMAQ 5.0 combines three individual modules--meteorology, emissions, and chemical transport. Instead of running the models in sequence, as in previous versions, the meteorology and air chemistry-transport models in CMAQ 5.0 now operate together and interact in feedback loops on the fly, providing more accurate forecasts that reflect interactions between pollution and weather. With CMAQ 5.0, scientists can model air quality at the level of individual towns and cities throughout the entire northern hemisphere. The framework combines advances in physical, chemical, mathematical, and computational sciences. On a hemispheric scale, scientists apply CMAQ5.0 to account more accurately for “background pollution” originating from distant locations. This upgrade allows policymakers to understand and use the data to balance local and national air policy standards, and integrate them with international solutions.

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United States–Canada Scientific Cooperation

Air Quality Model Evaluation International Initiative

Scientists in Canada and the U.S. have been participating in an international activity called the Air Quality Model Evaluation International Initiative (AQMEII). This initiative is supported in part by Environment Canada and U.S. EPA with participation from the North American and European modeling communities. The goal of the initiative is to advance regional air quality modeling science through the development of a common model evaluation framework and joint evaluation and analysis of European (EU) and North American (NA) regional air quality models. Phase 1 of AQMEII, which ended in 2011, included annual regional air quality simulations over NA and EU for 2006 that allowed regional AQ models from NA and EU to be compared for common long-term case studies on both continents and promoted the use of different types of model evaluation, including operational, diagnostic, dynamic, and probabilistic. The key findings from AQMEIIPhase 1 are summarized in a series of manuscripts that was published in a special issue of the Air and Waste Management Association’s Environmental Manager (EM) magazine in July 2012. Some of the key findings from AQMEIIPhase 1 included:

  • Lateral boundary conditions exert a large influence on the predictions of limited area models;
  • Surface wind speeds tend to be underestimated over both continents, especially during nighttime and winter and more so over EU than NA;
  • For ozone, mean summertime ozone tends to be overestimated by most models over NA and underestimated by most models over EU; and,
  • For PM, annual average PM10 and PM2.5tend to be underestimated over both continents.

Phase 2 of AQMEIIbegan in 2012 with the overall objective of applying and evaluating coupled meteorology-atmospheric chemistry models over EU and NA, focusing on the evaluation of regional-scale coupled models’ capability to simulate interactions of air quality and climate change.

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Ammonia Workshop

Ammonia science is of interest to policy-makers in both Canada and the U.S. as ammonium sulfate and ammonium nitrate are some of the major constituents of the total mass of fine PM (PM2.5), which has impacts on both human and environmental health. Important policy issues relating to NH3 include the development and implementation of national primary standards for fine particles in the U.S. as well as the secondary standards for NOx and SOXand potential for continued PM Annex negotiations between the two countries. As ambient air quality standards become increasingly stringent and precursor emissions of gaseous precursors continue to decrease, the issue of how much impact NH3 emission reductions will have on ambient PM2.5 levels and attainment of ambient standards is more and more of interest.

A joint United States–Canada workshop on NH3science was held in October 2010 as a follow-up to a 2006 workshop. The purpose of the 2010 workshop was to review the state of NH3 science and to discuss joint collaboration that has occurred since the previous workshop. A further objective of the workshop was to assess whether the state of knowledge was sufficient to make concrete recommendations on NH3emissions actions and in what context; but if not, what gaps still need to be addressed. The workshop was organized around the following topics:  monitoring, processes and surface exchange, emissions, and modeling. A summary of ongoing Canadian and U.S. activities relating to each of these topics was presented at the workshop, followed by discussions that led to the identification of science gaps and potential areas of collaboration. In addition, discussions during the workshop led to the following overall key conclusions:

  • There is a growing body of evidence showing that NH3emissions are influencing PM formation and their long-range transport.
  • The amount of information on what the effect of reducing NH3 emissions would have on ambient PM is also growing.

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Black Carbon Meetings

Black carbon (BC), which is a component of PM as well as a short-lived climate forcing pollutant, was endorsed by the co-chairs of the United States–Canada Air Quality Committee as an area of discussion and exploration under the purview of the Sub-Committee on Scientific and Technical Cooperation. In response, a series of conference calls was organized between decision-makers and scientists in the two countries. The first conference call was held in August 2010, focusing on the policy issues, such as the upcoming U.S. Report to Congress, as well as the work being done on BC under the United Nations Environment Programme (UNEP), the UNECE, and the Arctic Council. The U.S. Black Carbon Report to Congress was completed in March, 2012. Additional information, including the Report, can be found at . The second conference call, which was held in March 2011, was designed to facilitate the exchange of BC-related technical information, particularly in light of the growing international focus on BC.

During the second conference call in March 2011, the U.S. and Canada shared information on the development of the BC emission inventory and additional research work to refine and improve the inventories. Areas that require particular attention and where additional opportunities for collaboration may exist are in improving speciation uncertainty and comparing forest fire emissions. Forest fires are a major source of BC emissions, and these vary from year to year. There are opportunities to further exchange information on how forest fire BC emissions are estimated, how these compare to global estimates, and whether there is a significant temporal trend. The monitoring portion of the second conference call focused on the different networks in the two countries making BC measurements and the methods being used in these networks. The U.S. and Canada have been working together to resolve differences in dry deposition measurements through the collocation of measurement systems at the CAPMoN site in Egbert, Ontario. However, more opportunities exist for comparing monitoring data within each country from the different networks and methods and also between the two countries. Finally, the modeling discussion during the second conference call included an overview of work in the U.S. that is focused on understanding the integrated impacts of reducing BC emissions on air quality and climate change. A point that was emphasized, particularly for decision-makers, was that some strategies to reduce emissions of BC would also reduce emissions of sulfate, which cool the atmosphere, with the overall impact of warming of the atmosphere. However, there is a lot of uncertainty in model results, particularly in how organic carbon is treated. In Canada, the primary scientific needs driving the modeling research are to better understand the role of aerosols and particles in air pollution effects and climate change. Potential areas of collaboration and further information exchange include understanding the role of particle aging and mixing on its radiative effects.

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[4]Information on the NAAQS review process is available at

[5]Rappold AG, Stone SL, Cascio WE, Neas LM, Kilaru VJ, et al. 2011 Peat Bog Wildfire Smoke Exposure in Rural North Carolina Is Associated with Cardiopulmonary Emergency Department Visits Assessed through Syndromic Surveillance. Environ Health Perspect 119(10): doi:10.1289/ehp.1003206.

[6]Adar SD, Klein R, Klein BEK, Szpiro AA, Cotch MF, et al. (2010) Air Pollution and the Microvasculature: A Cross-Sectional Assessment of In Vivo Retinal Images in the Population-Based Multi-Ethnic Study of Atherosclerosis (MESA). PLoS Med 7(11): e1000372. doi:10.1371/journal.pmed.1000372.

[7]Wellenius GA, Burger MR, Coull BA, et al. Ambient Air Pollution and the Risk of Acute Ischemic Stroke. Arch Intern Med. 2012;172(3):229-234. doi:10.1001/archinternmed.2011.732.

[8]Devlin et al. 2012. Controlled Exposure of Healthy Young Volunteers to Ozone Causes Cardiovascular Effects.  Circulation.  Published online June 25, 2012.

[9]Barkman, A., 1997. Applying the critical loads concept: Constraints induced by data uncertainty. Technical Report. Department of Chemical Engineering II, Lund University, Sweden.

[10] Aherne, J. and Wolniewicz, M.B. 2011. Critical loads uncertainty and risk analysis for Canadian forest ecosystems. Final Report, CCME, 19 pp.

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