Empirical relationship between particulate matter and aerosol optical depth over Northern Tien-Shan, Central Asia

Measurements were obtained at two sites in northern Tien-Shan in Central Asia during a 1-year period beginning July 2008 to examine the statistical relationship between aerosol optical depth (AOD) and of fine [PM2.5, particles less than 2.5 μm aerodynamic diameter (AD)] and coarse (PMCoarse, particles between 2.5 and 10 μm AD) mass concentrations and composition. The measurements represent the first extended particulate matter measurements in the northern Tien-Shan region of Central Asia. A sun photometer (Microtops II) was used to measure AOD from the surface, which is a widely used aerosol monitoring technique that is used in the AERONET network. In parallel, less routine measurements of continuous hourly PM2.5 data were obtained with the TEOM/FDMS whereas daily average PM2.5 and PM10 were obtained using URG-3000ABC samplers. Daily samples were collected on an every-other-day basis throughout the year. Since clouds interfere with the AOD measurement, a cloud screening procedure based on LIDAR measurements was applied to the AOD data and cloud impacted days were removed from the AOD data set. Depending on the season, the correlation coefficient (r) varied from 0.56 up to 0.87. Higher correlation coefficients between PM2.5 mass and AOD were observed during the spring and autumn periods and appeared to result from the transport of Asian dust (desert crustal material) particles from outside the area. One of the main source areas was the Taklimakan desert located in northwestern China. Linear regression results between AOD and PM2.5 are presented that allow for an estimate of PM2.5 mass concentrations at the surface based on the AOD data, which can be used to help interpret AOD measurements made in Central Asia and potentially other regions of the world.