Hourly measurements of organic molecular markers in urban Shanghai, China: Observation of enhanced formation of secondary organic aerosol during particulate matter episodic periods

摘要

Field campaigns rarely measure individual polar organic compounds online, leaving unexplored their potential in tracking the formation dynamics of secondary organic aerosol (SOA). In a three-week-long field campaign in urban Shanghai, we deployed a commercial Thermal desorption Aerosol Gas chromatograph system (TAG) that integrates sampling, in-situ derivatization, and thermal desorption gas chromatography-mass spectrometric analysis, producing hourly measurement of polar organics including a set of biogenic and anthmpogenic SOA tracers. The abundance and variations of these SOA tracers were examined in relation to five PM2.5 episodes, varying from 4 to 64 h in duration, during the field campaign from 9 November to 3 December 2018. The episodes were associated with stagnant air parcels. In comparison with the non-episodic hours, the episodic hours showed distinct chemical characteristics of a large mass increment of nitrate (an average of 4.7 fold) and secondary organic compounds (similar to 3-6.4 fold), exceeding those of primary pollutants (1.6-1.9 fold). These results clearly indicate the significant contributions of secondary inorganic and organic production processes to forming PM2.5 episodes. The SOA concentration estimated by the set of TAG-measured SOA tracers reached an average concentration of 2.6 mu g/m(3), accounting for a significant portion of OA in view of that the total organic matter (OM) in PM1 measured by an Aerosol Mass Spectrometer was 7.9 mu g/m(3). Among them the SOA attributed to monoaromatic compounds (e.g., toluene) accounted for a noticeable portion of the total OM, implying that control strategies targeting local anthropogenic emissions would be effective in reducing the severity of episodic PM pollution. Examining the ratio of two monoterpene-derived SOA products, we found evidence of less-aged SOA in urban Shanghai, and this result was collaborated by the O/C value (0.4) of bulk OA. This work demonstrates hourly SOA tracer measurements by TAG are uniquely specific on identification of major SOA precursors for episodic events and observing the evolution of SOA.