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Potential impacts of global climate and emissions changes on regional air quality over southern (western and eastern) Canada and northern Mexico are examined by comparing future summers' (i.e., 2049–2051) average regional O 3 and PM 2.5 concentrations with historic concentrations (i.e., 2000–2002 summers). Air quality modeling was conducted using CMAQ and meteorology downscaled from the GISS-GCM using MM5. Emissions for North America are found using US EPA, Mexican and Canadian inventories and projected emissions following CAIR and IPCC A1B emissions scenario. Higher temperatures for all sub-regions and regional changes in mixing height, insolation and precipitation are forecast in the 2049-2051 period. Future emissions are calculated to be lower over both Canadian sub-regions, but higher over northern Mexico. Global climate change, alone, is predicted to affect PM 2.5 concentrations more than O 3 for the projections used in this study: average daily maximum eight (8) hour O 3 (M8hO 3 ) concentrations are estimated to be slightly different in all examined sub-regions while average PM 2.5 concentrations are estimated to be higher over both Canadian sub-regions (8% over western and 3% over eastern) but 11% lower over northern Mexico. More days are forecast where M8hO 3 concentrations are over 75 ppb in all examined sub-regions but the number of days where PM 2.5 concentration will be over 15 μg/m 3 is projected higher only over western Canada. Climate change combined with the projected emissions lead to greater change in pollutant concentrations: average M8hO 3 concentrations are simulated to be 6% lower over western Canada and 8% lower over eastern Canada while average PM 2.5 concentrations are simulated to be 5% lower over western Canada and 11% lower over eastern Canada. Although future emissions over northern Mexico are projected higher, pollutant concentrations are simulated to be lower due to US emissions reductions. Global climate change combined with the projected emissions will decrease average M8hO 3 4% and PM 2.5 17% over northern Mexico. Significant reductions in the number of days where M8hO 3 concentrations are over 75 ppb and PM 2.5 concentration over 15 μg/m 3 are also projected with a significant reduction in peak values.


Introduction
Global climate and emissions changes are critical factors for future air quality.Although climate change impacts on regional air quality have been examined to some degree EGU (e.g.Mickley et al., 2004;Hogrefe et al., 2004;Knowlton et al., 2004;Murazaki and Hess, 2006;Langner et al., 2005) and have been summarized by Tagaris et al. (2007) there are limited studies examining the effect of long term emission changes on air quality.Dentener et al. (2006) recently compared the global atmospheric environment for the years 2000 and 2030 using global atmospheric chemistry models and different emissions scenarios.Different emissions scenarios result in different global and regional ozone levels, while climate change alone seems to play a minor role.Tagaris et al. (2007) examined the impacts of global climate and emissions changes on regional ozone and fine particulate matter concentrations over the United States.They found that the impacts of climate change alone on regional air quality over US are small compared to the impacts from emission control-related reductions, although increases in pollutant concentrations due to stagnation events are found.Most of the aforementioned studies focus on the US.However, it is equally important to investigate the impact of the climate and emissions changes to the border US regions, given that a large part of Mexican and Canadian population lives there and will both affect and be affected by pollutant transport.These border regions are some of the most dynamic regions of North America in economic, environmental, demographic and cultural terms.Extending the study by Tagaris et al. (2007), the impacts of global climate and emissions change on regional air quality over northern Mexico and southern Canada are assessed.Future O 3 and PM 2.5 concentrations for northern Mexico and southern Canada are compared to historic ones under two different cases: i) the impacts of changes on regional air quality by climate change alone are examined by keeping emissions sources, activity levels and controls constant, and ii) the future pollutant concentrations are estimated based on changes in both climate and emissions using the IPCC A1B emission scenarios (IPCC, 2000) and planned controls.This is the first study examining the impacts of climate and emissions changes in these regions and how changes in future US air quality will affect the neighbor countries.Introduction

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Methods
Following the same methodology as described in details by Tagaris et al. (2007), and summarized below, we use the Goddard Institute of Space Studies (GISS) II' (Rind et al., 1999) global results downscaled using the Penn State/NCAR Mesoscale Model (MM5) (Grell et al., 1994), forecast North American emissions and the Community Multiscale Air Quality model (CMAQ) (Binkowski and Roselle 2003) to simulate historic and future air quality.The primary difference between this study and the former is that improved emissions became available for Canada and Mexico.
The Environment Canada's 2000 inventory has been used for area and mobile Canadian sources (http://www.epa.gov/ttn/chief/net/canada.html).For point sources, the 2002 inventory that the New York State Department of Environmental Conservation compiled using the Canadian National Pollution Release Inventory (NPRI) was scaled using Environment Canada's state level summary.For Mexico, the US EPA's 1999 Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study Emissions Inventory was updated with the Mexico National Emissions Inventory (NEI) (http://www.epa.gov/ttn/chief/net/mexico.html).The 2001 Clean Air Interstate Rule (CAIR) emission inventory is used for the U.S. for the early 21st century, as well as the basis for projected emissions up to 2020 (Woo et al., 2006, 2007  1 ).

EGU
Meteorological fields are derived from the GISS GCM II', which was applied at a horizontal resolution of 4 • latitude by 5 • longitude to simulate current and future climate at global scale (Mickley et al., 2004).The simulation followed the IPCC-A1B emission scenario (IPCC, 2000) for greenhouse gases.Leung and Gustafson (2005) downscaled the GISS simulations for 1995-2005 and 2045-2055 using the Penn State/NCAR Mesoscale Model (MM5) to the regional scale; no data assimilation has been used.Although there are uncertainties in using regionally downscaled climate in air quality simulations, this approach is necessary in air quality models that employ higher resolution meteorological fields produced by regional instead of global climate models (Gustafson and Leung, 2007).CMAQ with SAPRC-99 chemical mechanism is used for the regional air quality modeling.O 3 and PM 2.5 concentrations for three historic (2000-2002) summer (June-July-August) episodes are compared to three future (2049-2051) summer episodes.Regional concentrations are predicted for northern Mexico and western and eastern Canada (Fig. 1).To quantify the net impact of climate change and the impact of climate change combined with projected emissions, both the historic period and future cases are examined.Future cases are: i) using the 2001 emissions inventory for historic and future years to quantify the impact of climate change on air quality, and ii) using future forecast emissions along with forecast climate to simulate future pollutant levels over northern Mexico and western and eastern Canada allowing the quantification of both impacts on future air quality.

Meteorology
Statistics and spatial distributions for forecast temperature, mixing height, insolation and precipitation for northern Mexico and western and eastern Canada (Table 1 and Fig. 2a) show higher average temperatures.Northern Mexico is simulated to be the sub-region with the greatest average temperature increase (2.6 K).The average tem-Introduction

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perature is calculated 1.7 K and 1.5 K higher in western and eastern Canada, respectively.Locally changes up to 4 K in the northern Mexico and up to 3 K for Canada are forecast.The mixing heights are simulated to be higher in most of the northern Mexico (average around +30 m).Maximum increases (around 200 m) are forecast near the US border where the maximum temperature increase is also estimated.For both the Canadian sub-regions mixing height is calculated to be lower in the majority of the domain (average around −30 m) except the central part where a small increase is estimated.The average insolation at the earth's surface decreases by 10 Wm −2 in Canada and increases by 3 Wm −2 in Mexico.Insolation is simulated to be lower in most of the Canadian sub-regions except the central part, while regional changes are expected in the northern Mexico.Regional changes in daily precipitation are forecast with more precipitation in northern Mexico where the average daily change is up to 6 mm locally.Little change is expected for both Canadian sub-regions.Fewer rainy days are estimated for the majority of the northern Mexican and western Canadian sub-regions in contrast to eastern Canada for which more rainy days are predicted (Fig. 2b).All the mentioned local changes in climatic conditions will affect the future local pollutant concentrations.

Emissions
Control strategies applied on anthropogenic Canadian sources result in significantly lower NO x , SO 2 and NH 3 emissions in both Canadian sub-regions (Table 2).NO x emissions are projected to be 32% and 50% lower in western and eastern Canada respectively while SO 2 emissions are projected to be 64% and 74% lower in both areas, respectively.NH 3 emissions are projected to be 30% and 60% lower in western and eastern Canada respectively.Emissions reduction of anthropogenic VOCs combined with the higher biogenic emissions in the warmer climate results in a small change in VOC emissions: 6% higher in the western Canada and 10% higher in the eastern Canada.
For the case where only climatic changes are considered, although the emission Introduction

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inventory is kept the same, emissions are not, since some pollutant emissions (e.g., biogenic and mobile) depend on meteorology.A minor increase in NO x emissions in both Canadian sub-regions is calculated but VOC emissions will be higher in the future (up to 19% in western Canada) due to climate change alone (Table 2).For Mexico, the growth of the industrial sector leads to significantly higher emissions (Table 2).NO x , SO 2 , VOCs and NH 3 emissions are projected to be 99%, 88%, 24% and 220% higher in the future summers in the northern Mexico.For the case where only climatic changes are considered a minor increase in NO x emissions is calculated.VOC emissions are projected to be much higher in the future due to climate change alone (around 24%), but slightly lower when emissions projection is used caused by the higher projected VOC emissions by human activities.Spatial distribution plots of emissions rate changes for the historic and future summers are presented in auxiliary materials.http://www.atmos-chem-phys-discuss.net/8/3405/2008/ acpd-8-3405-2008-supplement.pdf3.3 Air quality

Ozone
The impact of climate change alone and the combined effect of climate and emissions changes on M8hO 3 are illustrated in Fig. 3a.Under the impact of climate change alone the average M8hO 3 concentrations are estimated to be 0.1 ppb higher (0.1%) over western Canada, 0.6 ppb lower (2%) over eastern Canada and 0.5 ppb higher (1%) over northern Mexico (Fig. 3, Table 3).Global climate change combined with the projected emissions are calculated to reduce the atmospheric pollutant concentrations.Average M8hO 3 concentrations are estimated to be 3 ppb lower (6%) over western Canada, 3 ppb lower (8%) over eastern Canada and 2 ppb lower (4%) over northern Mexico (Fig. 3, Table 3).Interestingly, although future emissions over northern Mexico are projected higher, pollutant concentrations are forecast to be lower.This is caused by the large reduction in US emissions which affect pollutant concentrations  (2050s) shown by their Cumulative Distribution Functions (CDFs) (Fig. 4).Significant reductions are expected for the concentrations above 50 ppb, especially over eastern Canada.The same trend is found for M8hO 3 concentrations over northern Mexico with significant reductions in concentrations above 60 ppb.
Here, boundary conditions for both historic and future periods are kept the same due to uncertainties in future global changes.Setting varying boundary conditions affect our ability to isolate the impacts of regional climate and emissions changes.Further, calculations were repeated excluding five grid cells deep of the outer perimeter of modeling domain (Giorgi and Bates, 1989), with negligible change.Regional average concentrations are similar since the winds typically come from the west well away from the land.
Over Canadian sub-regions, typical M8hO 3 concentrations are calculated to be between 30 and 50 ppb (Fig. 5a).Climate change alone is simulated to increase M8hO 3 concentrations up to 1 ppb in the center of Canada but a reduction of up to 2 ppb is estimated for the rest of Canada (Fig. 5c).Emission controls are expected to reduce M8hO 3 concentrations up to 5 ppb in both Canadian sub-regions (Fig. 5d).The combined effect of climate change and emissions changes is also found to reduce M8hO 3 concentrations (up to 5 ppb) in both Canadian sub-regions (Fig. 5b).Over northern Mexico, the highest forecast M8hO 3 concentrations are calculated between 50 and 60 ppb near the US border (Fig. 5a).Climate change alone is simulated to increase M8hO 3 concentrations up to 4 ppb in the east but to decrease it up to 3 ppb in the west (Fig. 5c).Emissions changes are expected to reduce M8hO 3 concentrations up to 5 ppb near the US border while it is expected an increase up to 5 ppb on the west coast due to emission increases (Fig. 5d).The combined effect of climate change and emission changes are found to reduce M8hO 3 concentrations up to 5 ppb in the majority of the region expect the east part where an increase up to 2 ppb is predicted.(Fig. 5b).Introduction

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Full Global climate change alone has a significant effect on future summer PM 2.5 concentrations over western and eastern Canada and northern Mexico as compared to O 3 , as changes in temperature and precipitation impact gas phase partitioning and wet deposition of particulate matter.Average PM 2.5 concentrations are estimated to be 0.3 µg/m 3 higher (8%) over western Canada, 0.1 µg/m 3 higher (3%) over eastern Canada and 0.4 µg/m 3 lower (11%) over northern Mexico (Fig. 3, Table 3).These changes come mainly in SO = 4 and OC over Canada (western Canada: SO = 4 0.1 µg/m 3 higher (12%), OC 0.1 µg/m 3 higher (7%), eastern Canada: SO = 4 0.01 µg/m 3 higher (0.6%), OC 0.06 µg/m 3 higher (6%)) and from SO = 4 over northern Mexico (0.3 µg/m 3 lower (14%)).PM 2.5 composition will be slightly different due to climate change alone (Fig. 6, Table 4).Global climate change combined with the projected emission changes is simulated to reduce the atmospheric pollutant concentrations.Average PM 2.5 concentrations are estimated to be 0.2 µg/m 3 lower (5%) over western Canada, 0.3 µg/m 3 lower (11%) over eastern Canada (Fig. 3, Table 3).PM 2.5 composition is calculated to be significantly modified setting OC as the dominant component followed by sulfate (Table 4).Over northern Mexico, average PM 2.5 concentrations are estimated to be 0.6 µg/m 3 lower (17%) (Fig. 3, Table 3).No significant change in PM 2.5 composition is expected with sulfate to be the dominant component (about 50%) (Table 4).Although there is no change in the lower PM 2.5 concentrations (i.e., below 7 µg/m 3 ) there are significant reductions in the higher levels in eastern Canada and northern Mexico when climate change and emissions projection are considered (Fig. 7).Spatial distribution plots for average PM 2.5 concentrations for historic years and the changes caused by climate and emission projection are presented in Fig. 8.Over the majority of both Canadian sub-regions average PM 2.5 concentrations are calculated between 2.5 and 5.0 µg/m 3 (Fig. 8a).Climate change alone is simulated to increase PM 2.5 concentrations up to 0.5 µg/m 3 in the majority of Canadian sub-regions except the east where a decrease up to 0.5 µg/m 3 is estimated (Fig. 8c).Emissions pro-Introduction

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jection is expected to reduce PM 2.5 concentrations up to 1 µg/m 3 in the major part of both Canadian sub-regions, but there are small areas with reductions up to 2 µg/m 3 (Fig. 8d).The combined effect of climate change and emissions projection is estimated to reduce PM 2.5 concentrations up to 1 µg/m 3 in both Canadian sub-regions but there are small areas where increase up to 1 µg/m 3 is projected (Fig. 8b).Over Northern Mexico average PM 2.5 concentrations are simulated higher in the northeast part with average concentrations up to 7.5 µg/m 3 (Fig. 8a).Climate change alone is calculated to decrease PM 2.5 concentrations up to 1 µg/m 3 in the central part (Fig. 8c).Emissions projection is expected to reduce PM 2.5 concentrations up to 1 µg/m 3 near US borders while an increase up to 1 µg/m 3 is expected in the west coast (Fig. 8d).The combined effect of climate change and emissions projection is estimate to reduce PM 2.5 concentrations up to 2 µg/m 3 in the north eastern region close to US borders while small increases are expected in the west (Fig. 8b).
Comparing the effects caused by climate and emission changes between the subregions examined here and the US sub-regions (Tagaris et al., 2007) it is revealed that climate change alone is not expected to significantly modify summer M8hO 3 concentrations over Canadian, Mexican and US sub-regions.The effect of climate change on PM 2.5 concentrations is expected to reduce summer concentrations over US and Mexico, they are more important over the Plains, Midwest and Southeast US sub-regions and Northern Mexico where significant reductions are expected in PM 2.5 levels, but over both Canadian sub-regions small increases are forecast.The combined effect of climate change and projected emissions changes are simulated to reduce M8hO 3 and PM 2.5 concentrations over Canadian and Mexican sub-regions, but this reduction is much lower compared to the reduction simulated for the US sub-regions due to the projected greater emissions reductions in the latter.Full

Figure 1 :
Figure 1: Modeling domain and regions examined

Fig. 2b .
Fig. 2b.Spatial distribution plots for the change in rainy days between the three historic and future summers.
Figures are presented in auxiliary materials).Both Canadian sub-regions are simulated to have lower future M8hO 3 concentrations due to emissions reduction

Table 1 .
Global climate change impacts on air quality over western and eastern Canada and northern Mexico are simulated to change future summer PM 2.5 concentrations but have little impact on O 3 levels.Global climate change combined with projected emission changes is simulated to reduce pollutants concentrations in all examined sub-regions.One of the most important findings of this study is that although future emissions over northern Mexico are projected to be higher, future pollutant concentrations are not as reductions in the US provide benefits to the south.PM 2.5 composition is calculated to be slightly different due to climate change alone but when projected emissions are considered, it is calculated to be significantly changed over Canadian sub-regions setting OC as the dominant component followed by sulfate.Over northern Mexico sulfate is simulated to continue to be the dominant PM 2.5 component.Introduction Regional average climatic parameters for the three historic and future summers.

Table 2 .
Regional average emissions rates (tons/day) for historic and future summers using emissions projections (Future) and no emissions projection (Future np*) and the relative change (%) based on the historic emissions.*np: 2001 emission inventory and 2050 meteorology.

Table 3 .
Regional average M8hO 3 and PM 2.5 concentrations and PM 2.5 composition for the historic and future summers using emissions projection (Future) and no emissions projection (Future np*) and the relative change (%) based on the historic emissions.*np: 2001 emission inventory and 2050 meteorology.

Table 4 .
Regional average PM 2.5 composition (%) for the historic and future summers using emissions projection (Future) and no emissions projection (Future np*).*np: 2001 emission inventory and 2050 meteorology.