Long-term, Seasonal and Diurnal Variation of PM2.5 in Washington, DC Area

Monitoring PM2.5 (particulate matter with aerodynamic diameter d≤2.5μm) mass concentration and retrieving the source of fine particles are recognized as important due to the negative impacts of fine particles on people’s health. In this study, 16 years (from 1999 to 2015) of PM2.5 data from EPA monitoring sites in the Washington, DC area were analyzed relative to gaseous pollutants and meteorological conditions. In the Washington, DC area, the annual mean PM2.5 was found significantly decreasing since 1999. Significant long-term decreasing concentration was also found for SO2 and NO2, and a slightly decreasing trend in the annual mean for VOC was found. Pronounced seasonal variation of PM2.5 concentrations showed that PM2.5 concentrations were generally higher in the summer months and lower in the spring and fall months in Washington, DC area. However, both SO2 and NO2 concentrations were found higher in the winter months while seasonal variation of SO2 showed a bimodal pattern with another peak in summer months. Through the composition analysis, sulfate was found dominating the fine particles in the summer months, and the mass fraction of sulfate was largely decreased during the winter time. The variation of component of nitrate in fine particles was opposite to that of SO2. It is implied that the higher PM2.5 concentrations in summer time may be due to the higher SO2 concentrations. Diurnal variation of PM2.5 concentrations showed a negative correlation between PM2.5 concentration and boundary layer height, and a positive correlation between PM2.5 and vehicle emissions during rush hour. Besides pollutant emissions, meteorological conditions are also important to PM2.5 concentrations due to their impacts on mixing, deposition, transportation and formation of aerosol particles. The relationship between surface temperature, relative humidity, pressure, wind speed, wind direction and PM2.5 are also analyzed respectively.