title: Impact of relative humidity on visibility degradation during a haze event: A case study.
abstract: Light scattering of aerosols depends on ambient relative humidity (RH) since hygroscopic particles absorb significant water at high RH, and this results in low visibility. This paper used custom-made parallel nephelometers (PNEPs) to measure aerosol light scattering enhancement factor ƒ(RH), and utilized data including visibility, PM2.5, black carbon, water-soluble ions mass concentrations and surface meteorological parameters, in conjunction with background weather conditions, to analyze a haze event in Guangzhou during 8th-15th Dec. 2013. Unfavorable weather conditions, such as high RH and low wind speed, were observed during the haze event. The hourly average mass concentration of PM2.5 was 127μg/m(3), with concentration of 192.4μg/m(3) on 9th and 196μg/m(3) on 13th. The ƒ(RH) did not exhibit significant changes during this haze process, with value of ƒ(80%)=1.58±0.07. Although the mass fraction of water-soluble ions to PM2.5 decreased after 12th Dec., the aerosol hygroscopicity might not have changed significantly since the mass fraction of nitrate became more dominant, which has stronger ability to take up water. The best-fitted parameterized function for ƒ(RH) is ƒ(RH)=0.731+0.1375∗(1-RH/100)(-1)+0.00719∗(1-RH/100)(-2). Combining the fixed parameterization of ƒ(RH) above, the visibility was calculated with the measured light scattering and absorption coefficient of particles and gas under dry condition, as well as ambient RH. The predicted visibility range agrees well with the measurements without precipitation. Using ISORROPIA II model, the calculated aerosol liquid water content (ALWC) at ambient RH varied consistently with the PM2.5 under lower RH, while it was more influenced by high RH. This work also show that high RH accompanied with precipitation will enhance aerosol hygroscopic growth effect, leading to further visibility degradation, even if PM2.5 mass decreased due to precipitation.
authors.affiliation: Guangdong Ecological Meteorology Center, Guangzhou 510080, China.
authors.forename: Hua
authors.initials: H
authors.lastname: Deng
authors.affiliation: Guangdong Ecological Meteorology Center, Guangzhou 510080, China; Key Laboratory of Regional Numerical Weather Prediction, Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, Guangzhou 510080, China. Electronic address: hbtan@grmc.gov.cn.
authors.forename: Haobo
authors.initials: H
authors.lastname: Tan
authors.affiliation: Key Laboratory of Regional Numerical Weather Prediction, Guangzhou Institute of Tropical and Marine Meteorology, China Meteorological Administration, Guangzhou 510080, China.
authors.forename: Fei
authors.initials: F
authors.lastname: Li
authors.affiliation: Sun Yat-Sen University, Guangzhou 510275, China.
authors.forename: Mingfu
authors.initials: M
authors.lastname: Cai
authors.affiliation: Hong Kong Observatory, Hong Kong, China.
authors.forename: P W
authors.initials: PW
authors.lastname: Chan
authors.affiliation: Sun Yat-Sen University, Guangzhou 510275, China.
authors.forename: Hanbing
authors.initials: H
authors.lastname: Xu
authors.affiliation: Guangzhou Central Meteorological Observatory, Guangzhou 510080, China.
authors.forename: Xiaoying
authors.initials: X
authors.lastname: Huang
authors.affiliation: Jinan University, Guangzhou, 510632, China.
authors.forename: Dui
authors.initials: D
authors.lastname: Wu
keywords: Aerosol; Hygroscopic enhancement factor; Visibility