A Summary of the Special Acid Rain 2015 Issue of Atmospheric Environment

Douglas Burns1, Julian Aherne2, David Gay3 and Christopher Lehmann4

The International Acid Rain Conference was held on Oct. 19 – 23, 2015 in Rochester, New York and attended by 350 scientists from across the globe. A special Acid Rain 2015 issue of the journal Atmospheric Environment will be published based on the conference that is expected to include more than 30 papers representing authors and studies from North America, Europe, Asia, and Africa. The papers are divided about equally between studies of patterns and trends in atmospheric deposition and studies of the effects of atmospheric deposition on ecosystems. The special issue will highlight some common themes and summarize recent advances in scientific understanding of spatial and temporal trends in deposition and patterns of ecosystem responses. Sulfate (SO42-) concentrations in precipitation and rates of deposition have declined globally in recent decades at all sites where long-term and continuous measurements are available, including in Asia. These decreases in SO42- are largely attributed to policies that have reduced emissions from coal-fired electric generation facilities and other major sources. Concentrations and deposition of nitrogen (N) species are more varied with decreasing, unchanging, and increasing trends reported at various locations over the past 2-3 decades. Nitrate (NO3-) concentrations generally show more reported declines than do those of reduced N species such as ammonia (NH3) and ammonium (NH4+). The fraction of total N in atmospheric deposition that consists of reduced N is increasing at many sites around the globe, and locations in the US and Asia report increasing trends in reduced N concentrations. These patterns can be attributed to increases in reduced N emissions in some cases, but declines in the formation of particulate NH4+ species resulting from decreases in SO2 and NOx emissions are likely contributing to increased NH3 concentrations. Despite several atmospheric deposition monitoring programs in parts of Europe, Asia, and North America, large geographic gaps still exist in many developing regions, and there is a widespread lack of measurements of some key species such as organic carbon, dust, and hydroperoxides as highlighted by papers in the special issue.

Surface waters in regions such as the Adirondacks in the northeastern US, southeastern Canada, and southern Norway are showing increases in pH and ANC as well as decreases in toxic inorganic monomeric aluminum in response to long-term decreases in S and N deposition. However, the recovery of surface water chemistry has not matched that of deposition, which is a result, in part, of long-term losses of base cations from soils that are inherently slow to recover. Recovery from surface water acidification is predicted to eventually promote a parallel recovery in sensitive aquatic ecosystems, but thus far, fish communities in North America and Europe have shown little or only marginal improvements. In Asia, little surface water acidification occurs, but soils in forested regions of China are experiencing ongoing acidification. Ecosystem effects of N deposition that include enhanced NO3- leaching and decreased biodiversity of plant communities remain a concern across North America, Europe, and Asia. However, some encouraging case studies exist such as the likely contribution of decreasing atmospheric N deposition to decreasing N loads to the Chesapeake Bay. Critical loads studies are becoming more widespread across the globe as a means of informing the success of air pollutant policies at achieving ecosystem effects goals as reflected by several papers in the special issue.


1U.S. Geological Survey, daburns@usgs.gov
2Trent University, jaherne@trentu.ca
3National Atmospheric Deposition Program, dgay@illinois.edu
4NADP Central Analytical Laboratory, clehmann@illinois.edu