A Global Assessment of Precipitation Chemistry and Deposition

Robert Vet1, Richard S. Artz2, Silvina Carou3, Mike Shaw4, Chul-Un Ro5, Wenche Aas6, Alex Baker7, Van C. Bowersox8, Frank Dentener9, Corinne Galy-Lacaux10, Amy Hou11, Jacobus J. Pienaar12, Robert Gillett13, M. Christina Forti14, Sergey Gromov15, Hiroshi Hara16, Tamara Khodzher17, Natalie M. Mahowald18, Slobodan Nickovic19, P. S. P. Rao20 and Neville W. Reid21

A Global Assessment of Precipitation Chemistry and Deposition of Sulfur, Nitrogen, Sea Salt, Base Cations, Organic Acids, Acidity and pH, and Phosphorus was recently published as a Special Issue of Atmospheric Environment (Volume 93, August 2014).  The Assessment was written under the direction of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) Scientific Advisory Group for Precipitation Chemistry (SAG-PC) and addressed three major questions: (1) what do measurements and model estimates of precipitation chemistry and wet, dry and total deposition of the above chemical species show globally and regionally? (2) has wet deposition of major ions changed since 2000 (and, where information and data are available, since 1990) and (3) what are the major gaps and uncertainties in our knowledge? To that end, regionally-representative measurements for two 3-year-averaging periods, 2000-2002 and 2005-2007, were compiled worldwide. Data from the 2000-2002 averaging period were combined with 2001 ensemble-mean modeling results from 21 global chemical transport models produced in Phase 1 of the Coordinated Model Studies Activities of the Task Force on Hemispheric Transport of Air Pollution (TF HTAP). The measurement data and modeling results were used to generate global and regional maps of concentrations and deposition. A major product of the assessment was a database of quality assured data gathered from regional and national monitoring networks worldwide, including NADP. The database is available for download from the World Data Centre for Precipitation Chemistry (http://wdcpc.org/). The assessment concludes that global concentrations and deposition of sulfur and nitrogen are reasonably well characterized with levels generally highest near emission sources and more than an order of magnitude lower in areas largely free of anthropogenic influences. In many parts of the world, wet deposition of reduced nitrogen exceeds that of oxidized nitrogen and is increasing. Sulfur and nitrogen concentrations and deposition in North America and Europe have declined significantly, in line with emission reduction policies. Major regions of the world, including South America, the more remote areas of North America, much of Asia, Africa, Oceania, polar regions, and all of the oceans, are inadequately sampled, particularly for phosphorus, organic forms of nitrogen, and weak acids including carbonates and organic acids. Measurement-based inferential estimates of dry deposition are limited to sulfur and some nitrogen species in only a few regions of the world, and methods are highly uncertain. The assessment concludes with recommendations that address major gaps and uncertainties in global ion concentration and deposition measurements.


1Environment Canada, robert.vet@ec.gc.ca