Direct links between hygroscopicity and mixing state of ambient aerosols: Estimating particle hygroscopicity from their single particle mass spectra

Hygroscopicity plays a crucial role in determining aerosol optical properties and aging processes in the atmosphere. We investigated submicron aerosol hygroscopicity and composition by connecting an aerosol time-of-flight mass spectrometer (ATOFMS) to the downstream of a hygroscopic tandem differential mobility analyzer (HTDMA), to simultaneously characterize hygroscopicities and chemical compositions of ambient aerosols in Shanghai, China. Major particle types, including biomass burning, EC, Dust/Ash, organics particles, cooking particles and sea salt, were shown to have distinct hygroscopicity distributions. It is also found that particles with stronger hygroscopicities were more likely to have higher effective densities. Based on the measured hygroscopicity-composition relations, we developed a statistical method to estimate ambient particle hygroscopicity just from their mass spectra. This method was applied to another ambient ATOFMS dataset sampled from September 12nd to 28th, 2012 in Shanghai, and it is found that ambient particles were present in three major hygroscopicity modes, whose growth factors at relative humidity 85 % peaked at 1.05, 1.42 and 1.60, respectively. The temporal variations of the estimated particle hygroscopicity were consistent with the back-trajectory analysis and atmospheric visibility observations. These hygroscopicity estimation results with single particle mass spectra analysis can provide critical information on particulate water content, particle source apportionment and aging processes.