Acid neutralization of precipitation in Northern China: Fid-Bau Portal

Acid neutralization of precipitation in Northern China

Wang, Yuesi / Yu, Wenpeng / Pan, Yuepeng / Wu, Dan

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 NO_x 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 further 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 SO₄ ²⁻ 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.

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_x 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.