Strong light scattering of highly oxygenated organic aerosols impacts significantly on visibility degradation

Abstract. Secondary organic aerosols (SOA) 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 Guangzhou winter (lasted about three 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 (MSE, defined as scattering coefficient per unit aerosol mass) of aerosol components. The more oxidized oxygenated organic aerosol (MOOA) with 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. Organic aerosol hygroscopicity parameter κOA was quantified through hygroscopicity closure. The highest water uptake ability of MOOA among organic aerosol factors was revealed with κMOOA reaching 0.23, thus further enhanced the fractional contribution of MOOA in ambient organic aerosol scattering. Especially, scattering abilities of MOOA was found to be even higher than that 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 Guangzhou winter. 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 degradations. Findings of this study demonstrate that more attentions need 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.