The Effects of PM2.5 Concentrations and Relative Humidity on Atmospheric Visibility in Beijing

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 PM2.5 concentrations and meteorological conditions to visibility are not yet clear. In this study, the individual contributions of PM2.5 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 PM2.5 concentrations increase at clean condition and decreases slowly when PM2.5 concentrations exceed 100g/m(3). The visibility decreases linearly as PM2.5 concentrations increase when PM2.5 concentrations lower than 50g/m(3); however, there tends to be an exponential relationship as PM2.5 concentrations increase. The PM2.5 concentrations can explain 50% of the variance in visibility in clean and high-humidity environments, whereas this fraction decreases to 10-15% when the PM2.5 concentrations exceed 200g/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 PM2.5 concentrations dominate the variations in visibility under dry or low PM2.5 concentrations, and the contributions of RH become increasingly important as PM2.5 concentrations and humidity increase. The difference of aerosol types and weather conditions increase the uncertainties of correlation coefficients between visibility and PM2.5/RH. This study emphasizes the atmospheric conditions when employing visibility as a proxy for air pollution. Plain Language Summary Atmospheric horizontal visibility provides the public with an intuitive grasp of air quality. People commonly think that the lower the atmospheric visibility is, the more severe is the ambient air pollution. However, in addition to the particulate matter concentrations, visibility is also affected by other meteorological factors, especially the relative humidity. Our study found that the variation in visibility is dominated by the PM2.5 concentrations only under dry or low PM2.5 concentrations conditions, in which the visibility can be considered as a proxy of air pollution. However, as the PM2.5 concentrations and humidity increase, visibility decreases gradually. Compared to PM2.5 concentrations, the contributions of the RH to visibility become increasingly important in low visibility conditions, and visibility does not exactly reflect the variation in ambient air quality.