Abstract. Secondary organic aerosols (SOAs) account for a large fraction of atmospheric aerosol mass and play significant roles in visibility impairment by scattering solar radiation. However, comprehensive evaluations of SOA scattering abilities under ambient relative humidity (RH) conditions on the basis of field measurements are still lacking due to the difficulty of simultaneously direct quantifications of SOA scattering efficiency in dry state and SOA water uptake abilities. In this study, field measurements of aerosol chemical and physical properties were conducted in winter in Guangzhou (lasting about 3 months) using a humidified nephelometer system and aerosol chemical speciation monitor. A modified multilinear regression model was proposed to retrieve dry-state mass scattering efficiencies (MSEs, defined as scattering coefficient per unit aerosol mass) of aerosol components. The more oxidized oxygenated organic aerosol (MOOA) with an O/C ratio of 1.17 was identified as the most efficient light scattering aerosol component. On average, 34 % mass contribution of MOOA to total submicron organic aerosol mass contributed 51 % of dry-state organic aerosol scattering. The overall organic aerosol hygroscopicity parameter κOA was quantified directly through hygroscopicity closure, and hygroscopicity parameters of SOA components were further retrieved using a multilinear regression model by assuming hydrophobic properties of primary organic aerosols. The highest water uptake ability of MOOA among organic aerosol factors was revealed with κMOOA reaching 0.23, thus further enhancing the fractional contribution of MOOA in ambient organic aerosol scattering. In particular, the scattering abilities of MOOA were found to be even higher than those of ammonium nitrate under RH of <70 %, which was identified as the most efficient inorganic scattering aerosol component, demonstrating that MOOA had the strongest scattering abilities in ambient air (average RH of 57 %) during winter in Guangzhou. During the observation period, secondary aerosols contributed dominantly to visibility degradation (∼70 %), with substantial contributions from MOOA (16 % on average), demonstrating significant impacts of MOOA on visibility degradation. The findings of this study demonstrate that more attention needs to be paid to SOA property changes in future visibility improvement investigations. Also, more comprehensive studies on MOOA physical properties and chemical formation are needed to better parameterize its radiative effects in models and implement targeted control strategies on MOOA precursors for visibility improvement.

Cloud condensation nuclei (CCN) play significant roles in aerosol-cloud interactions. Accurate prediction of CCN number concentrations in climate and regional models is crucial for climate and weather prediction as well as assessment of aerosol indirect radiative effects (Pruppacher & Klett, 1997;Seinfeld & Pandis, 1998;Twomey, 1963). The CCN activity of aerosols, namely the fraction of particles activated into cloud droplets, can be directly measured by the continuous-flow streamwise thermal-gradient CCN counter (CCNC). The most commonly used CCNC was developed and commercialized by Droplet Measurement Technologies (DMT) (

Abstract. Emission controls have substantially brought down aerosol pollution in China, however, aerosol mass reductions have slowed down in recent years in the Pearl River Delta (PRD) region, where secondary organic aerosol (SOA) formation poses a major challenge for air quality improvement. In this study, we characterized the roles of SOA in haze formation in urban Guangzhou City of the PRD using year-long aerosol mass spectrometer measurements for the first time and discussed possible pathways of SOA formations. On average, organic aerosols (OA) contribute dominantly (50 %) to non-refractory submicron aerosol mass (NR-PM1). The average mass concentration of SOA (including by less and more oxidized OA, LOOA and MOOA) contributed most to NR-PM1, reaching about 1.7 times that of primary organic aerosols (POA, including hydrocarbon-like and cooking-related OA) and accounting for 32 % of NR-PM1, even more than sulfate (22 %) and nitrate (16 %). Seasonal variations of NR-PM1 revealed that haze formation mechanisms differed much among distinct seasons. Sulfate mattered more than nitrate in fall, while nitrate was more important than sulfate in spring and winter, with SOA contributing significantly to haze formations in all seasons. Daytime SOA formation was weak in winter under low oxidant level and air relative humidity, whereas prominent daytime SOA formation was observed in fall, spring and summer almost on daily basis, suggesting for important roles of photochemistry in SOA formations. Further analysis showed that the coordination of gas-phase photochemistry and subsequent aqueous-phase reactions likely played significant roles in quick daytime SOA formations. Obvious nighttime SOA formations were also frequently observed in spring, fall and winter, and it was found that daytime and nighttime SOA formations together had resulted in the highest SOA concentrations in these seasons and contributed substantially to severe haze formations. Simultaneous increases of nitrate with SOA after sunset suggested the important roles of NO3 radical chemistry in nighttime SOA formations, and confirmed by continuous increase of NO+/NO2+ fragment ratio after sunset. Findings of this study have promoted our understanding in haze pollution characteristics of the PRD and laid down future directions on investigations of SOA formation mechanisms in urban areas of southern China that share similar emission sources and meteorological conditions.