Acid neutralization of precipitation in Northern China

There is an increasing concern over the impact of human-related emissions on the acid precipitation in China. However, few measurements have been conducted so far to clarify the acid-neutralization of precipitation on a regional scale. Under a network of 10 sites across Northern China operated during a 3-year period from December 2007 to November 2010, a total of 1118 rain and snow samples were collected. Of this total, 28% was acid precipitation with pH <5.6. Out of these acid samples, 53% were found heavily acidic with pH value below 5.0, indicating significantly high levels of acidification of precipitation. Most of the acidity of precipitation was caused by H2SO4 and HNO3, their relative contribution being 72% and 28%, respectively. However, the contribution of HNO3 to precipitation acidity will be enhanced due to the increasing NOx and stable SO2 emissions in future. Neutralization factors for K+, NH4+, Ca2+, Na+ and Mg2+ were estimated as 0.06, 0.71, 0.72, 0.15, and 0.13, respectively. The application of multiple regression analysis farther quantified higher NH4+ and Ca2+ contribution to the neutralization process, but the dominant neutralizing agent varied from site to site. The neutralization was less pronounced in the rural than urban areas, probably due to different levels of alkaline species, which strongly buffered the acidity Presence of high concentrations of basic ions was mainly responsible for high pH of precipitation with annual volume-weighted mean (VWM) values larger than 5.6 at several sites. It was estimated that in the absence of buffering ions, for the given concentration of SO42- and NO3-, the annual VWM pH of precipitation would have been recorded around 3.5 across Northern China. This feature suggested that emissions of particles and gaseous NH3 played very important role in controlling the spatial variations of pH of precipitation in the target areas. Implications: Acid precipitation has long been recognized as a serious environmental problem in East Asia, especially in China. The present study identifies that acidity of precipitation in Northern China generated by H2SO4 and HNO3 was primarily neutralized by Ca2+ and NH4+. However, in the future, acidity due to HNO3 may increase as a result of the increasing trends of NO and stable trends of SO2 emissions in megacities. The reducing primary emissions of particles and gaseous ammonia may offer more clear-sky. days in urban areas, but might also lead to enhanced acid precipitation in Northern China in the near future.