Long-term trends for nitrate and sulfate ions in snowcover on Hokkaido, northern Japan

Abstract

On the island of Hokkaido in northern Japan, a survey of air pollutants in snowpack has been carried out every four years from 1988 to 2012. In this report Hokkaido was divided into four areas, and we determined the concentration and deposition of non-sea salt (nss) sulfate, ammonium and nitrate ions in each. Transported air pollution was found to have accumulated in the snowcover, especially on the Sea of Japan side (JS). The amount of nss-sulfate decreased continuously after 1988, but nitrate increased after 2000, especially in JS, from 3.7 meq m-2 (2000) to 5.3 meq m-2 (2012). Nitrogen deposition (nitrogen in ammonium and nitrate) was estimated at 8.9 meq m-2 (1.2 kg N ha-1) in JS in 2012. The nitrate/ nss-sulfate concentration ratio shows a clear increasing trend. Previous studies have found that sulfur emission in China decreased after 2006. The present result appears to reflect the change in winter emissions in northern/northeastern China.