Simultaneous HTDMA and HR-ToF-AMS measurements at the HKUST supersite in Hong Kong in 2011

This paper presents the results of simultaneous aerosol hygroscopicity and composition measurements using a Humidified Tandem Differential Mobility Analyzer (HTDMA) and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) at the HKUST Supersite, located at a coastal suburban site in Hong Kong. Growth factors (GFs) at 90% relative humidity and the size-resolved composition of ambient aerosols at dry electrical mobility diameters of 75, 100, 150, and 200 nm were measured in May, September, and November 2011. Bimodal GF distributions with a dominant more hygroscopic (MH) mode and a small fraction of the nonhygroscopic or less hygroscopic mode (number fraction < 0.2) were observed throughout the study. The average MH mode GF (1.44–1.52) over the second half of September was significantly lower than that in other periods (1.53–1.64) due to the high organic loading under the influence of the continental airstream. The average ensemble mean growth factor (GF*) of maritime aerosols was higher (1.53–1.59) than that of continental aerosols (1.45–1.48). Closure between the aerosol hygroscopicity and chemical composition was also evaluated. Over 90% of measured data and predictions based on Extended Aerosol Inorganics Model with a constant GF of the organic fraction (GF_org) of 1.18 are within 10% closure. Approximations for GF_org using the fraction of m/z 44 in organic mass spectra, the oxygen-to-carbon atomic ratio, and PMF-resolved organic factors from HR-ToF-AMS measurements did not yield better closure results, likely because of the overall dominance of sulfate over the whole study period. Finally, GF_org of 1 to 1.5 (with κ_org up to 0.29) was found to best fit the additional water content unexplained by the inorganic species.