Effect of Secondary Organic Aerosol Formation Pathways on Human Health

LU QI, Ka Yuen Cheung, Chuan Ping Lee, Dongyu S. Wang, Zhiyu Li, Weikang Ran, Yufang Hao, Xinmei Huang, Gang Chen, Qingqing Wang, Weiqi Xu, Tianqu Cui, Kun Li, Vishal Verma, Yuemei Han, Qiyuan Wang, Zifa Wang, Yele Sun, Kaspar R. Daellenbach, Urs Baltensperger, Robin Modini, Imad El Haddad, Junji Cao, André S. H. Prévôt, Jay G. Slowik, Paul Scherrer Institute

     Abstract Number: 339
     Working Group: Urban Aerosols

Abstract
Atmospheric aerosols are among the largest global environmental risks to public health and responsible for considerable uncertainty in the climate system. Much of this uncertainty is due to secondary organic aerosol (SOA), which consists of thousands of multifunctional, oxygenated species that are difficult to measure in situ. In consequence, source apportionment studies typically treat SOA as a single bulk quantity (or as a linear combination of non-source-specific vectors), and the effects of specific SOA sources and on human health and climate effect remain poorly constrained.

Here we utilize extractive electrospray ionization coupled to time-of-flight and orbitrap mass spectrometers (EESI-TOF and EESI-Orbitrap) to elucidate wintertime SOA sources and formation pathways in Beijing, China. High-RH conditions with widespread cloud formation are typical of severe haze events in Beijing, yielding high aerosol liquid water content (LWC) and thereby favoring aqueous-phase reactions. We identify two factors related to aqueous SOA production, one of which (AqSOA) is characterized by small, highly oxygenated molecules and depends mainly on LWC. The second aqueous factor (AqSOA_AMN) is related to both LWC and NH₃⁺/NH₄⁺, and shows enhancements from N-containing molecules consistent with imidazoles, which are known to be formed from aqueous reactions of glyoxal in the presence of NH₄⁺- containing seed. The AqSOA_AMN factor also exhibits far higher oxidative potential (OP) on a per mass basis than either AqSOA, or aged biomass burning. Further, during pollution events conducive to its formation, AqSOA_AMN has considerable effects on both the SOA-derived and total OP. This highlights the implications of different SOA pathways, including NH₃/NH₄⁺- mediated aqueous chemistry, for human health. More generally, we demonstrate that the SOA contributions to oxidation potential (OP) are both considerable and strongly source-dependent and highlight the shortcomings of treating SOA as a bulk or non-source-specific quantity.