The enhancement of PM2.5 mass and water-soluble ions of biosmoke transported from Southeast Asia over the Mountain Lulin site in Taiwan

Biomass burning (BB) in Southeast Asia (Indochina and southern China) occurs frequently in March and April every year. The burning plume is ordinarily transported eastward by the prevailing westerly, further affecting downstream air quality in East Asia. In this study, atmospheric aerosols were collected at the downstream Lulin Atmospheric Background Station (LABS, 2862 m a.s.l., central Taiwan) from April 2003 to April 2009. Results show that monthly means of PM2.5 were highest during the BB period, especially in March. The PM2.5 mean for BB activity was 17.5 mu g m(-3), while the daily PM2.5 mean can sometimes be above 40 mu g m(-3). The background PM2.5 level in free troposphere of the West Pacific was at 3.7 +/- 1.8 mu g m(-3). This mean is roughly the same regardless of the air masses moving from China, Pacific Ocean, and South China Sea toward LABS. In addition, the highest PM2.5 level occurred in 2004, making it the most active year of BB for the whole observation period. Greater amounts of nitrate and potassium ions were observed in the PM2.5 collected during the BB period compared to the non-BB (NBB) period. Linear regression analysis on PM2.5 water-soluble ions shows a moderate correlation (R-2 = 0.59) between non-sea-salt potassium and nitrate ions during the BB period. Furthermore, for all trajectory source origins, ammonium ion had the best correlation (R-2 = 0.84) with non-sea-salt sulfate when the air masses were influenced by anthropogenic sources during the NBB period. The enhancement ratios of nitrate ion during the BB period could reach 6.7 and 9.7 relative to air masses from the BB source region and from the pristine area during the NBB period, respectively. During the study period, ammonia gas was found to be insufficient to neutralize sulfuric and nitric gases. Therefore, most aerosols were more acidic than basic. Our long-term observation of atmospheric aerosols with inter-annual variability is valuable in providing data for verifying BB source inventory and model performance in East Asia. (C) 2011 Elsevier Ltd. All rights reserved.