Water-soluble organic aerosol (WSOA) constitutes a large fraction of organic aerosol (OA) and plays an important role in affecting aerosol hygroscopicity and cloud condensation nuclei formation. Here, we characterized WSOA in fine particles that were collected at ground level and at 260 m on a meteorological tower in Beijing in winter using offline aerosol mass spectrometry (AMS) and compared them with the simultaneous real-time online AMS measurements. Our results showed large increases in WSOA from nonheating season to heating season by 43% and 53% at ground level and at 260 m, respectively. WSOA was highly oxidized in winter with a higher oxygen-to-carbon ratio (O/C = 0.69) at 260 m than at ground level (0.63). On average, WSOA contributed 47% and 52% to the total OA at ground level and at 260 m, respectively. Positive matrix factorization analysis further showed that primary OA was the dominant source of WSOA (52−61%) with coal combustion (CCOA) and biomass burning OA (BBOA) being two major contributors. The vertical differences in water-soluble CCOA and BBOA, however, were very different with ubiquitously higher CCOA at 260 m than at ground level. Comparatively, more than 50% of secondary OA in winter was found to be water-insoluble, highlighting the influences of fossil emissions. The observed high water-solubilities of CCOA (42−53%) and BBOA (61−78%) may have significant implications in severe haze formation in winter in Beijing through a positive feedback mechanism between substantial increases in primary emissions during heating season and the subsequent changes in aerosol liquid water content, radiative forcing, and boundary layer, which is not yet considered in chemical-transport models.

Biomass burning (BB) aerosols, primary biological aerosol particles, and secondary organic aerosols, are critical components of particulate matter in the atmosphere. In this study, saccharides, lignin/resin acids, biogenic secondary organic aerosols tracers, aromatic acids, and hydroxyacids/polyacids were measured in total suspended particles collected from April 2012 to March 2013 at an urban site in Beijing using gas chromatography/mass spectrometry. The concentration of levoglucosan was elevated during 18-20 June. The high ratio of levoglucosan to mannosan and low ratio of galactosan to mannosan in June demonstrated the influence of wheat straw combustion in the North China Plain via long-rang transport. Sucrose, glucose, and fructose were more abundant in April, the spring bloom season. The maximum trehalose content (298 ng/m 3 ) occurred during a dust storm (27-29 April) with the highest stable carbon isotope ratio (δ 13 C, À21.2‰) of total carbon. Fungal spore tracers (arabitol and mannitol) peaked in summer. The largest concentrations of the oxidization products of isoprene and α/β-pinene also occurred in summer. However, the β-caryophyllene tracer was much more abundant on long-lasting haze days in winter and was positively related to levoglucosan. The contributions of biogenic primary sources (plant debris, fungal spores, and BB) to organic carbon were estimated to be in range of 2.64-35.2% (11.8%), with BB being the predominant source. Biogenic secondary sources (isoprene, α-pinene, and β-caryophyllene) and anthropogenic secondary source (naphthalene) accounted for 0.23-10.9% (3.36%) of organic carbon with large contributions in summer and winter, respectively.