Space Observation of Aerosols from Satellite Over China During Pollution Episodes: Status and Perspectives

High pollution events frequently occurred in China during the last years, as industrial activity, traffic, and urban heating has been increasing sources of pollutants (gases and particles). High particle concentrations, well above the recommended threshold values have been measured in large urban areas of eastern and southern China. A major contributor to summertime pollution episodes is dust. Main dust sources are located in the western part of China but mineral surface active in dry periods may contribute to aerosol sources over eastern urban areas in winter. To allow a comprehensive regional survey, ground-based measurements have been developed and satellite radiometric observations (as for example MODIS, MISR or OMI) have been extensively used to derive pollution index and particulate matter (PM) concentration estimates from optical properties over the column (aerosol optical depth -AOD- and size information). Satellites have revealed that pollution was extending over large areas at the regional scale, and that they could be used as first estimates of pollution with good accuracy on average. However, the dispersion due to the nature of the particles in space and time, and the sensitivity of their size to relative humidity has been a limitation in this observational approach. To better characterize and predict pollution events, chemical transport models are now used as integration tools to account for aerosol formation and composition, meteorological and physical constraints on links between microphysical and optical particle properties. Matching observed and modeled parameters and understanding biases is however a prerequisite for that. Radiometric satellite AODs have been first used in models, but it has further been shown that the integration of active sensor profiles allowed significant information to be added on the vertical to account for moisture impact, using data from ground-based lidar (acronym of light detection and ranging, also identified as an optical radar) networks and more widely from the space lidar CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) onboard CALIPSO (Cloud and Aerosol Lidar and Infrared Pathfinder Space Observations). CALIOP is providing data on aerosol type and distribution which are further expected to bridge the gap between in situ and radiometric remote sensing space observations, better constrain models, and help deriving more accurate retrieval of particle extinction near the surface. Although CALIPSO satellite track and repeat cycle do not allow to revisit the same place within days, the duration and extent of pollution episodes over eastern China is such that CALIOP measurements offer a way to constrain regional models to better analyze such episodes, accounting for a better categorization of aerosols. After discussing the context of observations and analyses, a presentation of satellite analyses is presented in the light of examples on severe winter regional pollution episodes.