Modeling direct and indirect effect of long range transport on atmospheric PM2.5 levels

The adverse effect of long range transport (LRT) of pollutants on local air quality has been long recognized. Previous studies only cover LRT effect without distinguishing direct and indirect effect, that is LRT of precursors directly forming PM2.5 in local areas (direct effect) and/or transported precursors interacting with local precursors in forming PM2.5 (indirect effect). The present study aims to simulate effects of East Asia emissions in PM2.5 levels in Taiwan by quantifying the direct and indirect effects. The modeling system included simulations of 4 cases: (A) Base: consider emission from both Taiwan and East Asia; (B) Zero Emission from East Asia: consider Taiwan emission only without considering emission from East Asia; (C) Zero Emission from Taiwan: consider East Asia emission only without emission from Taiwan; and (D) Zero Emission (background). The results of the CMAQ v4.6 model (with Mesoscale Model ver. 5) indicate that the contributions to annual PM2.5 average of 30 μg m−3 in Taiwan are 60, 27, 9, 3%, respectively, from Taiwanʹs own contribution, direct LRT, indirect LRT and background. The primary PM2.5 is almost from Taiwanʹs contribution whereas only 45% secondary PM2.5 (SPM2.5) is from Taiwan and the majority (53%) from LRT contribution (direct LRT 35% with indirect LRT 19%). As for composition in SPM2.5, SO 4 2 − accounts for 43% with the major contribution from direct LRT (57%), followed by NH 4 + (32%). NO 3 − is mostly from Taiwanʹs contribution (57%) with indirect LRT contribution of 36%. Indirect contribution of NH 4 + is 24% in the form of NH4NO3. Clearly Taiwan cannot control direct contribution ( SO 4 2 − ), but control of NO 3 − and NH 4 + may reduce PM2.5 levels. To meet the proposed annual average of 15 μg m−3, it is certainly a challenging task in that 30% emission should be reduced in Taiwan, preferably the reduction of NOX and primary PM.