U.S.-Canada Air Quality Agreement: Transboundary PM Science

Assessment

Report to the Air Quality Committee

June, 2004

Transboundary PM Science Assessment

Background:• Requirement of the 1998 Joint Work Plan for PM

• Product of three bi-national workshops between 1999-2003

• Tailored to address the information needs of the policy community

• Reviewed externally by both Canadian and U.S. reviewers; comments

addressed prior to the June meeting

Purpose:• Synthesize current atmospheric science of PM2.5 in keeping with

information needs of Subcommittee 1, summarized in seven

Assessment objectives.

PM Assessment Objectives

1. Identify whether or not there is a fine PM problem in the border regions with a focus on health, visibility and environmental endpoints;

2. Identify the extent of the problem;

3. Describe the PM issue in terms of geographic regions;

4. Identify PM precursors of concern on a regional or sub-regional basis;

5. Describe sources (or source regions) of PM and PM precursors in the context of geographic regions;

6. Describe emissions of PM precursors, the spatial distribution of emissions and the transport characteristics of these emissions and,

7. Identify the current and proposed emission reductions scenarios on fine PM levels in North America.

Assessment Status

• Science document was completed and externally peer reviewed by parties in both Canada and the United States

• Reviewer comments have been addressed to the extent possible at this time

• A final document will be made available publicly after technical edit, layout and translation.

Air Quality Standards

• Canada:– 98th percentile, 24 hour average, averaged

over 3 consecutive years of 30 µg/m3, to be achieved by 2010

• U.S:– Annual: 15 µg/m3 based upon a 3 year

average of the annual arithmetic mean– 24-hour: 65 µg/m3 based on 98th percentile, 24

hour average, averaged over 3 years

Ambient Air Quality

• Levels of PM2.5 exceed both Canadian and U.S. air quality standards in several regions.

• 2000-2003 mean PM2.5 levels, many sites over 15 µg/m3, particularly in the northeast

Ambient Air Quality

• 98th percentile values over 30 µg/m3 (yellow dots) over a majority of the northeastern U.S. (some in Canada)

• Also elevated concentrations in the northwestern U.S.

Ambient Air Quality – United States

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92 93 94 95 96 97 98 99 00 01

Year

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g/m

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Washington D.C. Site 9 Eastern Sites 23 Western Sites

PM2.5 trend 1992-2001

Mean/98th percentile W-E 2000-20020

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ug

/m3

Washington MontanaND & Minn Michigan Ohio New York VT Maine

• 3-year annual mean displayed as dot, 3-year average 98th percentile identified by star.

• West to East (longitude) order

• 200k distance used in lieu of 300k to reduce dataset

Ambient Air Quality – Canada

YEAR

PM

2.5 (

µg/m

³)

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19841985

19861987

19881989

19901991

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75%25%

Median

98th

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M 2.5 C

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µg/m

³)

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Urban SitesRural Sites

B.C. AlbertaSask.Man.

Ontario Quebec N.B.Nfld.

PM2.5 trend 1984-2002

98th percentile W-E 2001

Ambient Air Quality - Characterization

• Urban PM2.5 speciation graphs (EPA and NAPS speciation sites): eastern regions dominated by carbon compounds and sulfate; west dominated by carbon compounds and nitrate.

• Both nitrate and sulfate are associated with particle ammonium.

Emissions

Base case emissions of SO2 and NOx are

concentrated in the industrial Midwest, northeastern United States, and southern Ontario

Emissions of NH3 are concentrated further west in the central Midwest region.

Model Applications

• Three models were used to evaluate the impact of current and proposed commitments on air quality in the transboundary region.

• REMSAD: v.7.06, 5 scenarios, full year runs for PM and PM constituents

• AURAMS: v.0.30a, 5 scenarios, 2 two-week episodic evaluations (summer & winter) for PM and PM constituents

REMSAD and AURAMS scenarios: 1996 base case, 2010 base and control, 2020 base and control

• CMAQ: for the northwestern domain, summer and winter simulations run for PM and ozone, turning off either U.S. or Canadian emissions

Model Applications – REMSAD

Predicted reductions in annual PM2.5 levels up to 2.3 µg/m3, sulfate reductions up to 1.4 µg/m3 2020 Control Case vs. 2020 Base Case

Model Applications – REMSAD

Predicted reductions in annual nitrate levels up to 0.6 µg/m3, ammonium reductions up to 0.5 µg/m3 2020 Control Case vs. 2020 Base Case

Model Applications – AURAMS

• Total PM2.5 reductions of up to 2.4 µg/m3

• Episodic evaluation indicates SO4 and NH4 reductions up to 1.5 µg/m3

• NO3 substitution may result in increased NO3 levels in some regions (up to 1.5 µg/m3), and possibly increasing total PM2.5 concentrations in some areas in the winter

Winter episode

Model Applications – AURAMS

• Summer episode evaluation indicates reductions in total PM2.5 of up to 6 µg/m3, driven by reductions in SO4 and NH4.

Summer episode

Model Applications - CMAQ

• Ambient concentrations are predicted to both increase and decrease in various regions of western North America, correlated to increases in primary PM, marine and agricultural emissions forecasts for 2010 and 2020

Summer episodeBase case

Source-Receptor Analyses

• A wide variety of source-receptor analyses were carried out in both countries, in order to quantify the relationship between sources and measured ambient PM and PM precursor levels

• These methods include:– Observational receptor-oriented analyses

– Positive Matrix Factorization (PMF)

– Principal Component Analysis (PCA)

• Space-based observations can also aid in the observation of PM episodes in North America

Source-Receptor Analyses

Emissions of PM and PM precursors from the eastern U.S. and southern Canada have an impact on PM2.5 levels in both countries.

PM2.5 3.8 µg/m3 PM2.5 20.3 µg/m3

Simple Trajectory Analysis

Source-Receptor Analyses

More complex trajectory analysis indicates that air transported from the Ohio River Valley was most often associated with high PM concentrations. Observation-based findings are consistent with the spatial distribution of emissions.

Quantitative Transport Bias Analysis

Source-Receptor Analyses

• Regions of influence identified, and indicate the highest probability of three types of coal-related sources influencing PM2.5 in the border region

• Correlates well with high density emission areas for SO2 and NOx

Incremental Probability Technique

Source-Receptor Analyses

Model applications in combination with source-receptor analyses indicate that there is evidence of transboundary transport of both NH3 and NH4.

Incremental Probability Technique

Source-Receptor Analyses

• Regional transport of sulfate implies regional transport of PM2.5, while carbon components are more local in nature

Urban Excess Analysis

Natural Contributions of PM

Wild and prescribed forest fires can impact air quality on both an episodic and long-term average basis over wide regions

Quebec, Canada forest fires July, 2002

Satellite Observations of Aerosol Optical Depth

Composites of aerosol optical depth and cloud optical depth superimposed over continuous PM2.5 (bars) for July 6th, 2002. Hourly PM2.5 concentration is indicated by the height of the bar, while bar color represents the 24 hour running average mass concentration color coded to the US EPA Air Quality Index The yellow to red colors of aerosol optical depth show elevated aerosol concentrations, correlated strongly with PM2.5 levels.

Co-benefits of Emissions Reductions

• Implementation of existing and proposed controls will have a positive effect on other air quality issues, including ground-level ozone, sulfate and nitrate deposition.

• Ozone levels are predicted to decrease by a maximum of 16 ppb over much of the northeastern U.S, based on AURAMS output.

Co-benefits of Emission Reductions

• Reductions predicted up to 6 kg/ha SO4 and 4 kg/ha NO3 wet deposition, based on REMSAD output

• Widespread exceedence of forest critical loads in eastern Canada

Co-benefits of Emission Reductions

Sulfate is a key contributor to annual visibility reduction in the U.S.

Visibility is reduced(particularly in the warm season), in a large portion of the border region

Outcome of External Peer Review

• Outcome of the peer review was generally positive, with a few minor issues to resolve.

• The key issues include: – Representativeness of measurement sites– Uncertainties in emissions inventory information and

process parameterization for air quality models – Incomplete model evaluation for AURAMS, CMAQ

and REMSAD– Emphasized the need for precise quantification of

transboundary flow of PM and PM precursors• Have addressed the reviewer comments where

possible and the resulting revisions did not change the conclusions of the report.

Conclusions

• A variety of analytical techniques were used in both countries to provide a weight of evidence approach on PM in the border region. These include observations, model applications and source-receptor analyses.

• There is evidence illustrating transboundary flow between the two countries that would support further policy discussions.

• PM2.5 and its constituents (SO4, NO3, NH4) are of concern in the transboundary region, although the concentrations of these pollutants vary geographically.

• Sources contributing to PM2.5 in the border region include industrial point source emissions, vehicle emissions, and road dust.

• The formation of PM from precursors including SO2, NOx and NH3 is also significant.

Conclusions

• Transport of PM and PM precursors from the U.S. to Canada appears to be greater than transport from Canada to the U.S.

• The largest predicted air quality improvements occur in the areas of the largest predicted emission reductions; however, improvements are seen regionwide.

• The primary areas of concern for transport are in the east; additional areas of concern are the Canadian Prairie/U.S. Northern Plains, where visibility is an issue, and the west coast.

• In the west, PM appears to be primarily a sub-regional problem.

• Reductions in PM and PM precursors have co-benefits on other air quality issues, including ground level ozone, acid deposition and visibility.