Primary and secondary aerosols in Beijing in winter: sources, variations and processes

Abstract

Abstract. Winter has the worst air pollution of the year in the megacity of Beijing. Despite extensive winter studies in recent years, our knowledge of the sources, formation mechanisms and evolution of aerosol particles is not complete. Here we have a comprehensive characterization of the sources, variations and processes of submicron aerosols that were measured by an Aerodyne high-resolution aerosol mass spectrometer from 17 December 2013 to 17 January 2014 along with offline filter analysis by gas chromatography/mass spectrometry. Our results suggest that submicron aerosols composition was generally similar across the winter of different years and was mainly composed of organics (60 %), sulfate (15 %) and nitrate (11 %). Positive matrix factorization of high- and unit-mass resolution spectra identified four primary organic aerosol (POA) factors from traffic, cooking, biomass burning (BBOA) and coal combustion (CCOA) emissions as well as two secondary OA (SOA) factors. POA dominated OA, on average accounting for 56 %, with CCOA being the largest contributor (20 %). Both CCOA and BBOA showed distinct polycyclic aromatic hydrocarbons (PAHs) spectral signatures, indicating that PAHs in winter were mainly from coal combustion (66 %) and biomass burning emissions (18 %). BBOA was highly correlated with levoglucosan, a tracer compound for biomass burning (r2 = 0.93), and made a considerable contribution to OA in winter (9 %). An aqueous-phase-processed SOA (aq-OOA) that was strongly correlated with particle liquid water content, sulfate and S-containing ions (e.g. CH2SO2+) was identified. On average aq-OOA contributed 12 % to the total OA and played a dominant role in increasing oxidation degrees of OA at high RH levels (> 50 %). Our results illustrate that aqueous-phase processing can enhance SOA production and oxidation states of OA as well in winter. Further episode analyses highlighted the significant impacts of meteorological parameters on aerosol composition, size distributions, oxidation states of OA and evolutionary processes of secondary aerosols.

Fine particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

“…The average O/C ratio calculated based on the AMS PM1 measurements and updated calibrations by 5 Canagaratna et al (2015) is 0.4 ± 0.1 (Fig. S11), similar to values reported previously in winter Beijing Sun et al, 2016). The effect of liquid-liquid phase separation on particle pH remains unclear.…”

“…Synoptic weather patterns are dominated by the Siberian high-pressure system (Jia et al, 2015), under which the northerly winds bring dry and clean air into this region with ambient RH often dropping to as low as about 20% (when aerosol particles are most likely solid). When the northerly winds slacken, often occurring during 25 the NCP winter haze events, the atmospheric conditions are characterized by stagnant inversion, weak southerly winds, and rapid accumulation of both air pollutants and water vapor, and the ambient RH often reaches 80-90% (when aerosol particles are most likely liquid) (Zheng et al, 2015;Tie et al, 2017;Sun et al, 2016;Gao et al, 2016;Yin et al, 2017). Recent field measurements of PM1 in winter Beijing by Liu et al (2017b) suggested that the phase state of particles was sensitive to ambient RH and that there existed a gradual transition from semisolid at RH of about 20% to liquid at RH of 30 about 60%.…”

“…20 An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (referred to as the AMS) was used to measure sizeresolved non-refractory submicron aerosol (NR-PM1) species (DeCarlo et al, 2006). The detailed operations and calibrations of the AMS have been described elsewhere (Sun et al, 2016;Xu et al, 2015b). Briefly, aerosol particles were drawn into the sampling chamber at a flow rate of 10 L min −1 , of which ~ 0.1 L min −1 was isokinetically sampled into the AMS after being 25 dried with a silica gel dryer.…”

“…The AMS was calibrated for ionization efficiency using pure NH4NO3 particles following standard protocols (Jayne et al, 2000). A constant collection efficiency of 0.5 (Sun et al, 2016), and the default relative ionization efficiencies, except for ammonium that was determined from pure NH4NO3, were applied to all of the species for mass quantifications. The overall uncertainties for each species were estimated following Bahreini et al (2009) with details provided in the Supplement, Sect.…”

Production of N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub> in summer in urban Beijing, China

“…A more detailed description of the operations and calibrations of this HR-AMS can be found in Xu et al (2015) and Sun et al (2016). Other collocated measurements in two containers at ground level included gaseous species of O 3 (TEI 49C UV absorption analyzer), NO (TEI 42i TL NO analyzer), and NO 2 (CAPS NO 2 monitor, Aerodyne Research Inc.) and size-resolved particle number concentrations (11-550 nm) by a scanning mobility particle sizer (SMPS) equipped with a long differential mobility analyzer (DMA, TSI, 3081A) and a condensation particle counter (CPC, TSI, 3772).…”