Hygroscopicity and CCN activity of atmospheric aerosol particles and their relation to organics: Characteristics of urban aerosols in Nagoya, Japan

Abstract

The size‐resolved distributions of hygroscopic growth factor g and the ratios of cloud condensation nuclei (CCN) to condensation nuclei of atmospheric aerosols were investigated in Nagoya, Japan. The average of the distributions of g at 85% relative humidity was bimodal. The size‐resolved mean κ derived from g showed an increasing trend with diameter: 0.17–0.33 at 24–359 nm. The κ values calculated from CCN activation curves were 37% higher than those derived from g. Only 9% of the 37% difference is explained by the difference in the κ of inorganics under subsaturated and supersaturated conditions, suggesting a contribution of organics to the remaining 28% difference. The size‐averaged κ of organics (κorg) was calculated as 0.14 and 0.19 by two different methods. The number fractions of CCN predicted from the hygroscopicity data over the range of 24–359 nm are loosely consistent with those observed if the size‐ and time‐averaged g is applied to all particles (differences: −30% to +10%). This consistency improves if size‐ and time‐resolved g and g distribution are used (differences: −19% to −3%). Whereas the number fractions of CCN predicted from the composition data are greatly underestimated if organics are assumed to be insoluble (differences: −64% to −45%), they are more consistent if κorg of 0.14 or 0.19 is applied (differences: −10% to +14%). The results demonstrate the importance of the dependence of the g of particles on time and particle size and the hygroscopicity of organics for CCN number concentrations in the urban atmosphere.

Hygroscopicity and cloud condensation nucleus activity of forest aerosol particles during summer in Wakayama, Japan

“…Less hygroscopic particles were dominant (>50%) up to 180 nm. In other time sections, the averages of the number‐size distributions and the number fractions were characterized by a large fraction of more hygroscopic particles ( g ≥ 1.25), which contrasts the results at an urban site [ Kawana et al, ] with large N CN and a large fraction of less hygroscopic particles ( g < 1.25). During 2100–0900 JST on NPF event days (Figures g and h), the lowest N CN and largest fraction of more hygroscopic particles were observed among four different time sections.…”

“…The hygroscopic growth factor ( g ) is defined as the ratio of the electrical mobility diameter under a humidified conditions ( d p,wet , 85% RH) to that under dry conditions ( d p,dry , RH < 2%) using the HTDMA. The size‐resolved g distributions of ammonium sulfate (purity: 99.999%, Sigma‐Aldrich) were obtained at the d p,dry of 24.1–359 nm with corrections for the difference in the sizing of DMA1 and DMA2 (<3%) and the width of the transfer functions of the DMAs [ Mochida et al, ; Kawana et al, ]. The mode g values from a lognormal fitting to the g distributions agreed within 2% with those from the literature [ Tang and Munkelwitz , ].…”

“…An HTDMA coupled to a CCN counter (CCNC, Droplet Measurement Technologies, USA) was operated in parallel with a scanning mobility particle sizer (SMPS, model 3080, 3081, and 3772, TSI, USA). The configuration of the HTDMA‐CCNC system and the SMPS was the same as that shown in Figure 1 of Kawana et al []. The inlet for aerosol sampling was placed 7.5 m from the ground.…”

“…The details of the calibration methods for the HTDMA, CCNC, and SMPS, and the data processing procedure were described in Kawana et al []. We provide a brief explanation of the calibration and data processing, and an explanation of the results from the calibration.…”

“…The CCN efficiency spectra, which are functions of the number‐based ratios of CCN to CN versus d p,dry (24.1–359 nm), were obtained with corrections for the presence of multiply‐charged particles under the consideration of the diffusion losses of particles in the tubing and the width of the transfer functions of the DMAs [ Mochida et al, ; Kawana et al, ]. The fitting parameters for the CCN efficiency spectra, F act , d act , and σ in equation [ Rose et al, ], were obtained with the method in Kawana et al []. …”