Variations in aerosol optical properties and their sensitivity to elemental carbon: insights from year–round observations in Suzhou, China

Abstract

Current research predominantly emphasizes elemental carbon (EC) light absorption, while ignoring its relationship with aerosol hygroscopic scattering. In this study, concentrations and optical properties of aerosol components were measured during a full–year monitoring campaign at an urban site in Suzhou. Results from a multiple linear regression model suggested that secondary organic carbon was a primary contributor to high mass absorption efficiency of EC in summer. Through the estimation of aerosol scattering coefficients under both dry and ambient atmospheric conditions, it was found that hygroscopic growth accounted for more than 35.0 % of the total aerosol scattering coefficient. Hygroscopic growth of nitrate and sulfate enhanced their annual mean scattering contributions by 42.1 % and 45.2 %, respectively. A negative correlation between EC concentration and the hygroscopic growth factor (f(RH)) was observed under varying relative humidity (RH) conditions. Associated with the decrease in f(RH), reductions in PM_2.5 scattering coefficients (14.0 ± 2.2, 29.4 ± 5.2, and 24.5 ± 8.2 Mm⁻¹) were linked to EC concentration increases of 0.37 ± 0.1, 0.40 ± 0.1, and 0.21 ± 0.1 μg/m³ under low, medium, and high RH conditions, respectively. An increase in EC concentration by 0.19–0.37 μg/m³ elevated the PM_2.5 absorption coefficient by 2.66–5.41 Mm⁻¹, and reduced the scattering coefficient by 10.53–17.91 Mm⁻¹. Collectively, increased EC concentrations reduced aerosol single scattering albedo (SSA), particularly under high RH conditions. This study reveals that EC not only reduces aerosol extinction coefficients but also shifts aerosol radiative forcing in a positive direction by suppressing hygroscopic scattering.