Abstract
In this study, we used ground instruments, namely, a visibility meter, an integrating nephelometer, an aethalometer, a lidar, and a weather monitor, to measure the scattering enhancement factor, f(RH), which quantifies the effect of ambient relative humidity (RH) on aerosol light-scattering, and to generate a model of its annual variation in the city of Chiba, Japan. First, the f(RH) values were calculated with chemical analysis data. Second, visibility-meter and aethalometer data were used to model the monthly trends of f(RH) at 550 nm. The f(RH) values were higher during summer than during the other three seasons, which can be attributed to the general pattern of the regional climatology as well as the loading of different particle types into the lower troposphere. Third, the f(RH) values at 532 nm were obtained from lidar and aethalometer measurements. Low and constant f(RH) values were observed during November, whereas higher and increasing f(RH) values were observed during May. Also, dust events during March 2015 showed decreasing f(RH) with increasing RH .
Highlights
Relative humidity (RH) strongly affects the scattering properties of aerosols in the troposphere
Aerosol scattering coefficients are evaluated through the Mie scattering calculation
The scattering enhancement factor is computed as the ratio of the scattering coefficients to the scattering coefficient at low relative humidity (RH)
Summary
Relative humidity (RH) strongly affects the scattering properties of aerosols in the troposphere. The increase of ambient RH in the atmosphere increases particle sizes and their scattering coefficients (Tang, 1996). One of the optical parameters that is used to characterize changes in scattering coefficients with RH is the scattering enhancement factor, f(RH). This is defined as the ratio between the average scattering coefficient at a particular RH and the scattering coefficient at a dry RH. Aerosol sampling and optical measurements reported in previous works have revealed that as RH increases, the aerosol mass extinction efficiency and scattering enhancement factor increase (Bagtasa et al, 2007; Zieger et al, 2013).
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