The Effects of PM_2.5 Concentrations and Relative Humidity on Atmospheric Visibility in Beijing

Abstract

Abstract Atmospheric visibility is often used as a proxy for ambient air quality. However, in addition to the concentrations of particulate matter, visibility is also affected by meteorological conditions. The relative contributions of PM 2.5 concentrations and meteorological conditions to visibility are not yet clear. In this study, the individual contributions of PM 2.5 concentrations and the relative humidity (RH) to the visibility are measured based on observations at 12 stations in Beijing. We find that visibility decreases quickly as PM 2.5 concentrations increase at clean condition and decreases slowly when PM 2.5 concentrations exceed 100 μg/m 3 . The visibility decreases linearly as PM 2.5 concentrations increase when PM 2.5 concentrations lower than 50 μg/m 3 ; however, there tends to be an exponential relationship as PM 2.5 concentrations increase. The PM 2.5 concentrations can explain 50% of the variance in visibility in clean and high‐humidity environments, whereas this fraction decreases to 10–15% when the PM 2.5 concentrations exceed 200 μg/m 3 . In contrast, RH has little effect on visibility under dry conditions. When the RH exceeds 40%, atmospheric visibility tends to display an inversely proportional and exponential relationship with RH under polluted and clean conditions, respectively. Under high‐humidity and highly polluted conditions, up to 40% of the variance in visibility is associated with RH. The PM 2.5 concentrations dominate the variations in visibility under dry or low PM 2.5 concentrations, and the contributions of RH become increasingly important as PM 2.5 concentrations and humidity increase. The difference of aerosol types and weather conditions increase the uncertainties of correlation coefficients between visibility and PM 2.5 /RH. This study emphasizes the atmospheric conditions when employing visibility as a proxy for air pollution.