Hygroscopicity of aerosol particles and CCN activity of nearly hydrophobic particles in the urban atmosphere over Japan during summer

Abstract

We investigated the hygroscopicity of 150 nm particles and the number‐size distributions and the cloud condensation nuclei (CCN) activity of nearly hydrophobic particles in aerosols over Nagoya, Japan, during summer. We analyzed the correlations between the number concentrations of particles in specific hygroscopic growth factor (g) ranges and the mass concentrations of chemical components. This analysis suggests the association of nearly hydrophobic particles with hydrocarbon‐like organic aerosol, elemental carbon and semivolatile oxygenated organic aerosol (SV‐OOA), that of less hygroscopic particles with SV‐OOA and nitrate and that of more hygroscopic particles with low‐volatile oxygenated organic aerosol (LV‐OOA) and sulfate. The hygroscopicity parameter (κ) of organics was derived based on the g distributions and chemical composition of 150 nm particles. The κ of the organics correlated positively with the fraction of the total organic mass spectral signal at m/z 44 and the volume fraction of the LV‐OOA to the organics, indicating that organics with highly oxygenated structures including carboxylic acid groups contribute to the water uptake. The number‐size distributions of the nearly hydrophobic particles with g around 1.0 and 1.1 correlated with the mass concentrations of chemical components. The results show that the chemical composition of the particles with g around 1.0 was different between the Aitken mode and the accumulation mode size ranges. An analysis for a parameter Fmax of the curves fitted to the CCN efficiency spectra of the particles with g around 1.0 suggests that the coating by organics associated with SV‐OOA elevated the CCN activity of these particles.

Secondary aerosol formation promotes water uptake by organic-rich wildfire haze particles in equatorial Asia

“…A more significant discrepancy between κ org and κ HTDMA was observed at night, whereas a larger difference between κ OOA and κ HTDMA occurred during the day, which was likely due to the greatly inhibited oxidation processes of organic compounds in the evening in contrast to the enhanced situations during the day. This correlation could be visually observed in the corresponding diurnal patterns of both mean GF and OA factors, as the fraction of OOA was demonstrated as a moderately good indicator of the hygroscopicity of organics (Ogawa et al, 2016).…”

“…The mean normalized particle size distribution after humidification at 85 % RH has spanned a few different modes, reflecting mixing states of ambient wildfire haze particles observed at Singapore (Bougiatioti et al, 2016;Ogawa et al, 2016). For particles in a specific hygroscopic mode, i, with g 1,i < GF < g 2,i , the number fraction of this mode (nf i ) was derived from the measured probability den-…”

“…More hygroscopic particles (κ ≥ 0.2; g ≥ 1.27): aerosol particles contain inorganic salts as well as some more hygroscopic organic species such as multifunctional organic acids (Peng et al, 2001;Carrico et al, 2008;Duplissy et al, 2011;Ogawa et al, 2016).…”

“…Almost all the sulfate and nitrate were neutralized by ammonia, and these three most abundant inorganic constituents were combined and assumed as ammonium sulfate (i.e., ε SNA = ε SO 4 + ε NO 3 + ε NH 4 ≈ ε AS ). Thus, the value of κ SNA was considered to approximate the κ value of ammonium sulfate under the condition when sulfate dominates inorganics (Gunthe et al, 2009;Chang et al, 2010;Ogawa et al, 2016). The elemental carbon is known as non-hygroscopic (i.e., κ EC ≈ 0).…”

“…The density of oxygenated organics was assumed to be 1.50 g cm −3 (as summarized in Table 1), which is a typical value for carboxylic and multifunctional organic acids (Saxena et al, 1995;Peng et al, 2001;Gysel et al, 2004;Carrico et al, 2010;Ogawa et al, 2016). According to the reported average density for common dry smoke particles (i.e., 1.20-1.40 g cm −3 ; Reid et al, 2005), and considering the dominance of non-refractory organic material in fresh Indonesian peatland burning particles , the densities of PBOA and non-peat BBOA were assumed to be 1.10 and 1.20 g cm −3 , respectively.…”