Comparison of volatility, hygroscopicity and oxidation state of submicron aerosols over the Pearl River Delta region in China

Volatility and hygroscopicity properties of atmospheric particles with dry sizes of 60 and 145 nm were measured by using a Volatility-Hygroscopicity Tandem Differential Mobility Analyzer (VH-TDMA) at a suburban site over the Pearl River Delta region in China during the late summer of 2016. Specifically, volatility properties of the aerosols were studied by heating the ambient samples step-wise to seven temperatures ranging from 30 to 300℃. In general, particles started to evaporate at the heating temperature of 100℃. After heating the aerosols above 200℃, the probability density function of the volatility growth factor showed an apparent bimodal distribution with a distinct non-volatile mode and a volatile mode, indicating that the particle population was mainly externally mixed. Even at 300℃, around 20% of the aerosol volume still remained in the particle phase (non-volatile material). Black carbon (BC) mass fraction of aerosol mass correlated well (R²≈ 0.5) with the volume fraction remaining (VFR) at 300℃, but could not explain the non-volatile residual alone. On the basis of the comparison analysis between the VFR at different temperatures and the hygroscopic growth factor (HGF) at 90% RH, we observed the non-volatile residual material were hygroscopic (HGF=1.45). These results indicate that the observed non-volatile residual material at 300℃ did not consist solely of black carbon, but some other compounds such as sea salt, low-volatile ammonium or organic polymer.