PM2.5 episodes as observed in the speciation trends network

Urban PM2.5 speciation data from the 2000-2002 Speciation Trends Network were analyzed to gain an understanding of the various species contributing to regional PM2.5 episodes. The seasonal differences of these episodes and their relationship to meteorology were also studied. The results of this analysis revealed that summertime PM2.5 episodes are dominated by high concentration of acidic sulfate particles and some elevated organic aerosols. There is clear evidence that simultaneously elevated organics coincide with high concentration of acidic sulfate particles in many areas in the summer PM2.5 episode and they peaked on a day. when solar radiation was obviously not very strong. This suggests that in summer, in addition to the conventional explanation of SOA formation through photochemical oxidation and partitioning, some other mechanism such as the presence of acidic particles could also play a role in enhancing the SOA production under heterogeneous reactions. In the cold seasons, however, nitrate and organic particles are the major contributors to the observed PM2.5 episodes with organic particles peaking in the fall and nitrates peaking in the winter. Given the weak solar radiations, cold temperatures, and an obvious decoupling of high organic aerosol concentrations with acidic particles in the winter, it suggests that the observed wintertime organic aerosols are essentially primary in nature. Meteorologically, regional PM2.5 episodes are predominantly triggered by stagnant high pressure systems with most of the high values occurred on the backside of the high pressure system or ahead of a quasi-stationary front where temperatures and relative humidity are high. The fact that the regional PM2.5 episodes can occur in almost any season and their major components are essentially secondary aerosols suggests that meteorology and aerosol thermodynamics are at least as important as the photochemistry in determining the high airborne PM2.5 concentrations. The dominance of nitrate episodes in the cold seasons reflects the fact that the wintertime ammonia emissions are generally high enough to neutralize most of the acids produced in the major industrial source regions. This is supported by the ammonium availability calculations. Published by Elsevier Ltd.