2010 Progress Report - International Joint Commission

Acid Deposition Monitoring, Modelling, Maps and Trends
Airborne pollutants are deposited on the Earth’s
surface by three processes: (1) wet deposition (rain
and snow), (2) dry deposition (particles and gases),
and (3) deposition by cloud water and fog. Wet
deposition is comparatively easy to measure using
precipitation monitors, and the concentration of
sulphate and nitrate in precipitation is regularly used
to assess the changing atmosphere as it responds
Figure 4. 1990 Annual Wet
Sulphate Deposition
to decreasing or increasing sulphur and nitrogen
emissions. In Canada, to facilitate this comparison,
measurements of wet sulphate deposition are typically
corrected to omit the contribution of sea salt sulphate
at near-ocean sites (less than 62 miles, or 100 kilometres
[km], from the coast).
Figures 4 through 6 show the U.S.–Canada spatial
patterns of wet sulphate (sea salt-corrected)
deposition for 1990, 2000 and 2007 (the most recent
data year). Figures 7 through 9 show the patterns
of wet nitrate deposition for the same three years.
Deposition contours are not shown in western Canada
because Canadian scientists judged that the locations
of the contour lines were unacceptably uncertain
due to the paucity of measurement sites in all of the
western provinces. To compensate for the lack of
contours, wet deposition values in western Canada
are shown as coloured circles at the locations of the
federal/provincial/territorial measurement sites.
Commitments
Source: National Atmospheric Chemistry (NAtChem) Database
(www.msc-smc.ec.gc.ca/natchem/index_e.html) and the National
Atmospheric Deposition Program
Figure 5. 2000 Annual Wet
Sulphate Deposition
The three maps indicate that wet sulphate deposition
is consistently highest in eastern North America
around the lower Great Lakes, with a gradient following
a southwest-to-northeast axis running from the
confl uence of the Mississippi and Ohio rivers through
the lower Great Lakes. The patterns for 1990, 2000
and 2007 illustrate that signifi cant reductions occurred
Figure 6. 2007 Annual Wet
Sulphate Deposition
Source: National Atmospheric Chemistry (NAtChem) Database
(www.msc-smc.ec.gc.ca/natchem/index_e.html) and the National
Atmospheric Deposition Program
Source: National Atmospheric Chemistry (NAtChem) Database
(www.msc-smc.ec.gc.ca/natchem/index_e.html) and the National
Atmospheric Deposition Program
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