10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Insights into PM2.5 Chemical Composition and Sources in Beijing Using an Extractive Electrospray Ionisation Long-Time-Of-Flight Mass Spectrometer (EESI-LTOF)
YANDONG TONG, Veronika Pospisilova, Lu Qi, Giulia Stefenelli, Varun Kumar, Urs Baltensperger, Junji Cao, Rujin Huang, Andre S.H. Prévôt, Jay G. Slowik, Paul Scherrer Institute
Abstract Number: 1414 Working Group: Air Quality in Megacities: from Sources to Control
Abstract Air pollution in megacities is of increasing importance due to rapid economic growth and urbanisation. Severe air pollution affects many fast-growing megacities in developing countries every year, including Beijing. Serious haze events in Beijing are mostly characterised by fine particulate matter (PM2.5), which can either be directly emitted (primary) or formed by reactions of emitted gaseous precursors (secondary). Organic aerosol (OA) is a major component of atmospheric fine particulate matter and is responsible for much of the uncertainty in PM2.5 sources, climate impacts, and health effects. Sources of primary organic aerosols (POA) are reasonably well understood, whereas many challenges remain in identifying and quantifying secondary organic aerosol (SOA) sources, because atmospheric ageing yields secondary compounds with similar functional groups, regardless of source, and existing online measurement techniques destroy important chemical information by thermal decomposition and/or ionisation-induced fragmentation.
We present results from an intensive online campaign in Beijing from the beginning of October to mid-December using a novel extractive electrospray ionisation long-time-of-flight mass spectrometer (EESI-LTOF) and a long-time-of-flight aerosol mass spectrometer (L-TOF-AMS). The EESI-LTOF enables rapid, real-time measurement of OA with soft ionisation by extracting aerosol into primary electrospray droplets (100 ppm NaI in 50:50 water:acetonitrile), resulting in [M]Na+ adducts without thermal decomposition or ionisation-induced fragmentation. Although the EESI-LTOF shows an advantageous combination of near-molecular chemical information and high time resolution without sacrificing a linear response to mass, quantitative analysis is not yet completely established. Therefore, these measurements are complemented with an Aerodyne L-TOF-AMS equipped with a PM2.5 aerodynamic lens, as well as measurement of organic gas species, black carbon and total carbon.
The campaign lasted for ten weeks, covering late autumn, heating season and transition period in between. During the 10-week campaign, 14 episodes with 24h-mean concentration higher than 50 μg/m3 and five episodes with 24h-mean concentration higher than 150 μg/m3 were observed. Distinctive difference is found between late autumn and heating season in winter from both L-TOF-AMS and EESI-LTOF. During the event in heating season (from 30 Nov to 3 Dec), aerosol composition was composed of organics (55%), nitrate (25%) and sulphate (10%), whereas in late autumn during the event (from 3 to 7 Nov), nitrate was the major component (40%), followed by organics (33%) and ammonium (15%). The different composition indicates different sources and processes between two periods. EESI-LTOF spectra suggest that biomass burning activities were enhanced relative to coal burning in the late autumn non-heating compared to the winter heating period. L-TOF-AMS and EESI-LTOF mass spectra are further investigated using positive matrix factorisation (PMF) as implemented by the multilinear engine (ME-2) with Source Finder (SoFi) interface. These results provide a quantitative description of the major primary and secondary sources influencing OA, as well as the effects of the recent change in fuel from coal to natural gas implemented in northern China in 2017.