Vertical Characterization and Source Apportionment of Water-Soluble Organic Aerosol with High-resolution Aerosol Mass Spectrometry in Beijing, China

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.