Modelling of long-range transport of Southeast Asia biomass-burning aerosols to Taiwan and their radiative forcings over East Asia

Biomass burning produces aerosols and air pollutants during springtime in Southeast Asia. At the Lulin Atmospheric Background Station (LABS) (elevation 2862 m) in central Taiwan, the concentrations of carbon monoxide (CO), ozone (O-3) and particulate matter with a diameter less than 10 mm (PM10) were found to be 135-200 ppb, 40-56 ppb and 13-26 mu g/m(3), respectively, in the springtime (February-April) between 2006 and 2009, which are 2-3 times higher than those in other seasons. Simulation results indicate that higher concentrations during springtime are related to biomass-burning plumes transported from the Indochinese peninsula of Southeast Asia. The spatial distribution of high aerosol optical depth (AOD) was identified by satellite measurement and Aerosol Robotic Network (AERONET) ground observation, and could be reasonably captured by the WRF-Chem model during the study period of 15-18 March 2008. Simulated AOD reached as high as 0.8-1.2 in Indochina situated between 10-22 degrees N and 95-107 degrees E. According to the simulation results, 34% of the AOD was attributed to organic carbon over Indochina, while the contribution of black carbon to AOD was about 4%. During the study period, biomass-burning aerosols over Indochina have a net negative effect (-26.85 W.m(-2)) at ground surface, a positive effect (22.11 W.m(-2)) in the atmosphere and a negative forcing (-4.74 W.m(-2)) at the top of atmosphere. Under the influence of biomass-burning aerosol plume transported by strong wind, there is a NE-SW zone stretching from southern China to Taiwan with reduction in shortwave radiation of about 20 W.m(-2) at ground surface. Such significant reduction in radiation attributed to biomass-burning aerosols and their impact on the regional climate in East Asia merit attention.