Size-resolved characterization of organic aerosol in the North China Plain: new insights from high resolution spectral analysis

Organic aerosol (OA), a large fraction of fine particles, has a large impact on climate radiative forcing and human health, and the impact depends strongly on size distributions. Here we conducted size-resolved OA measurements using a high-resolution aerosol mass spectrometer at urban and rural sites in the North China Plain (NCP) in summer and winter. Our results showed substantially different size distributions of OA with the diameters peaking at similar to 550 nm in summer, and 420 nm and 350 nm at urban and rural sites, respectively, during wintertime. Positive matrix factorization (PMF) of size-resolved high-resolution mass spectra of OA resolved various OA factors at urban and rural sites. In particular, we found that the mass spectra of the same type of secondary OA (SOA) from bulk PMF analysis can be largely different across different sizes. Biomass burning OA (BBOA) and fossil-fuel-related OA (FFOA) showed broad size distributions peaking at 350 nm in winter at the rural site, where primary OA (POA = BBOA + FFOA) dominated OA across different sizes. Comparatively, secondary OA (SOA) in the NCP peaked at similar to 400-500 nm during wintertime, and similar to 500-650 nm in summer. SOA played an enhanced role during more severely polluted days with peak diameters shifting to larger sizes, while the changes in POA size distributions were small. The size-resolved oxygen-to-carbon (O/C) ratios were also determined and linked with the hygroscopicity parameter of OA (kappa (OA)). The results showed that kappa (OA) increased substantially with particle size, with higher values in summer in Beijing (0.28 +/- 0.021) than those during wintertime (0.17 +/- 0.019 and 0.12 +/- 0.018). The size-resolved kappa (OA) would benefit a better prediction of cloud condensation nuclei than bulk kappa (OA) in future studies.