Relative Humidity-Dependent HTDMA Measurements of Ambient Aerosols at the HKUST Supersite in Hong Kong, China

Abstract

The hygroscopic tandem differential mobility analyzer (HTDMA) has been frequently used to measure the hygroscopic properties of atmospheric aerosols at a fixed high relative humidity (RH) of about 90%. To evaluate if such measurements could be used to determine the hygroscopicity of aerosols at lower RH, simultaneous hygroscopic growth factor (GF) and sizeresolved composition measurements were made with an HTDMA and a high-resolution aerosol mass spectrometer (HR-AMS), respectively, at a coastal site in Hong Kong from January to June and in August 2012. A total of 58 cycles of dehydration (decreasing RH) and hydration (increasing RH) of 100 nm and 200 nm particles with organic-to-inorganic mass ratio ranging from 0.19 to 1.97 were measured at RH D 10-93%. The Kappa (k) equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) k at individual RHs (k RH ) and (ii) best-fit k covering the range of RHs measured (k f ) for the morehygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, k at 90% RH or above (k >90 ) fell between 0.18 and 0.48, and was within 15% of k f in 83% of the datasets. Regression analysis between k >90 or k f and AMS mass fractions showed that k was positively correlated with sulfate but negatively correlated with organic and nitrate. In most cases, k RH increased as RH decreased and the average increase in k was 45% from 90% RH to 40% RH, but these differences yielded insignificant changes in the GF-RH curves. The Zdanovskii-Stokes-Robinson (ZSR) estimated k were mostly within 20% of k >90 and k f . GF predictions using the empirical correlation of k with AMS mass fractions or the ZSR estimated k were within 10% of additional measurements and hence k >90 is useful for predicting GF at lower RHs.

Aerosol optical properties in the southeastern United States in summer – Part 1: Hygroscopic growth

“…The value of κ chem,OA < 0.1 is broadly consistent with most previous work measuring atmospheric aerosol hygroscopicity in the subsaturated regime (Cheung et al, 2015;Hersey et al, 2013;Mikhailov et al, 2013;Nguyen et al, 2014;Sihto et al, 2011). Many chemistry-climate models use κ-Köhler theory to predict the hygroscopic growth and ambient radiative properties of the aerosol (e.g., Liu et al, 2012).…”

“…As particles grow due to water uptake as RH increases, the extinction cross section can change nonlinearly, and can even decrease (e.g., Bohren and Huffman, 1983). However, as pointed out earlier (Chylek, 1978;Pinnick et al, 1980), for a physically realistic, polydisperse aerosol composed of particles predominantly smaller than the wavelength of light (but larger than Rayleigh scatterers), σ ext is roughly proportional to integrated particle volume or mass. This proportionality results because the extinction efficiency α(D p , n) for visible light can be approximated as a linear function of particle diameter over the relatively broad size range of a polydisperse accumulation-mode atmospheric aerosol (i.e., α is proportional to D p , Fig.…”

“…Cerully et al (2015) show that, in the southeastern US, κ chem,OA is not simply related to oxidation level, but to additional parameters including OA volatility. Values of κ chem for atmospheric aerosols are commonly determined experimentally using measurements of droplet activation diameter in the supersaturated regime (e.g., Cerully et al, 2015;Chang et al, 2010;Dusek et al, 2010;Gunthe et al, 2009;Levin et al, 2012;Sihto et al, 2011) or using hygroscopic growth measurements in the subsaturated regime (Cappa et al, 2011;Cheung et al, 2015;Hersey et al, 2013;Malm et al, 2000;Mikhailov et al, 2013;Nguyen et al, 2014;Sihto et al, 2011). Atmospheric variability in these measurements is compounded by potential measurement artifacts (Good et al, 2010b).…”

Hygroscopicity of organic surrogate compounds from biomass burning and their effect on the efflorescence of ammonium sulfate in mixed aerosol particles

“…6b, derived from laboratory HTDMA measurements in the range from 90 to 40 % RH. A similar trend of an increase in κ with a decrease in RH has also been observed by Cheung et al (2015). Their observation is based on ambient particle measurement with a HTDMA in Hong Kong, therefore probing particles of more complex compositions in the field campaign.…”

Comment on “The effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer”

“…Values of the hygroscopicity parameter (κ OA , Petters and Kreidenweis, 2007) obtained from laboratory and field studies for oxidized OA are ~0.05-0.3 (e.g. Gunthe et al, 2009;Lambe et al, 2011;Brock et al, 2016;Cheung et al, 2015;. Values of κ OA needed for hygroscopicity closure and assuming that AMS OA mass concentrations were erroneously high have been estimated with the method described in Downloaded by [University of Sussex Library] at 16:41 27 June 2016 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 21 the appendix.…”