[Effect of Relative Humidity on Particulate Matter Concentration and Visibility During Winter in Chengdu]

The effect of relative humidity (RH) on particulate matter concentrations and atmosphere visibility were investigated using the continuous on-line observed data of Chengdu city during December 2015, including RH, visibility, the concentrations of particulate matters (PM₁₀, PM_2.5 and PM₁) and gaseous pollutants (SO₂ and NO₂), and the concentrations of SO₄^2- and NO₃^- in PM_2.5. The results showed that the haze process occurred because of the synergistic effects of higher particulate matter concentrations and RH, leading to the reduction of visibility. The average ratio of PM_2.5 to PM₁₀ was 64% and it significantly increased with the increase of RH during observation period, which indicated that the pollution of fine particles during winter in Chengdu was serious, and high RH aggravated the pollution caused by fine particles.Visibility decreased exponentially with the increase of particulate matter concentrations. When RH was higher, visibility was lower at the same concentrations of particulate matter.RH had a strong effect on visibility at lower particle concentrations, while the effect of RH on the visibility decreased, and atmospheric extinction was controlled by PM_2.5 concentrations at higher particle concentrations. With RH increasing from less than 40% to more than 70%, the average sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) increased from 0.27 and 0.11 to 0.40 and 0.19, respectively, indicating that higher RH significantly promoted the formation of secondary sulfate and nitrate. Secondary sulfate and nitrate separately or coordinatively influenced the air quality.