Estimation of particulate matter from satellite-and ground-based observations over Hyderabad, India

Long-term trends in surface-level particulate matter of dynamic diameter ≤2 µm PM2 in regard to air quality observations over Greater Hyderabad Region GHR, India are estimated by the synergy of ground-based measurements and satellite observations during the period 2001–2013 satellite and July 2009–Dec 2013 ground-based. Terra Moderate Resolution Imaging Spectroradiometer MODIS-derived aerosol optical thickness AOT MODIS-AOTs was validated against that measured from Microtops-II Sunphotometer MTS AOTs MTS-AOTs and then utilized to estimate surface-level PM2 concentrations over GHR using regression analysis between MODIS-AOTs, MTS-AOTs, and measured PM2. In general, the MODIS-estimated PM2 concentrations fell within the uncertainty of the measurements, thus allowing the estimate of PM2 from MODIS, although in some cases they differed significantly due to vertical heterogeneity in aerosol distribution and the presence of distinct elevated aerosol layers of different origin and characteristics. Furthermore, significant spatial and temporal heterogeneity in the AOT and PM2 estimates is observed in urban environments, especially during the pre-monsoon and monsoon seasons, which reduces the accuracy of the PM2 estimates from MODIS. The estimates of PM2 using MTS or MODIS-AOT exhibit a root mean square deference RMSD of about 8–16% against measured PM2 on a seasonal basis. Furthermore, a tendency of increasing PM2 concentrations is observed, which however is difficult to quantify for urban areas due to uncertainties in PM2 estimations and gaps in the data set. Examination of surface and columnar aerosol concentrations, along with meteorological parameters from radiosonde observations on certain days, reveals that changes in local emissions and boundary-layer dynamics, and the presence or arrival of distinct aerosol plumes aloft, are major concerns in the accurate estimation of PM2 from MODIS, while the large spatial distribution of aerosol and pollutants in the urban environment makes such estimates a considerable challenge.