GAW Report No. 205 - IGAC Project

CHAPTER 1 - INTRODUCTIONthese aerosols are difficult to extract. AOD is typically measured at visible wavelengths and is thusmost sensitive to fine-mode aerosol. The information is therefore optimum for air qualityapplications focused on PM 2.5 . Efforts to convert AOD observations to surface PM 2.5 concentrationsrequire additional information on the vertical distributions and speciation of total aerosol, oftenobtained from chemical transport models [Liuet al., 2004]. AOD in the mid-visible is also verysensitive to the surface reflectivity. Historically, the first AOD retrievals were performed solely overoceans, where dark surfaces were more easily characterized. More recently, both multi-spectral(e.g. MODIS) and multi-angle (e.g. MISR) approaches have been applied to account for the impactof surface reflectivity and measure AOD over land as well as ocean [Levy et al., 2010]. Thepresence of clouds is an additional challenge for passive remote sensing observations of aerosols.Active sensors, such as lidars, can profile aerosol and cloud extinction at high vertical resolution. Inrecent years, CALIOP observations have provided detailed observations of aerosol plumes, andoverlaying vertical layers. The drawback of these observations for air quality applications is theirlimited coverage and long repeat times. However, detailed snap shots may provide importantinsight into aerosol distributions and export mechanisms over urban centres (Figure 15).Figure 15 - Aerosol measurements from spaceAll current satellites relevant for air pollution research are in polar low Earth orbit (LEO).This limits the number of passes over a given area to twice per day, but observations can belimited further by cloud cover and by the swath width of satellites dictating a longer time betweenrepeat visits. For air quality monitoring, more frequent observations would be highly desirable. Thiscan be achieved with several LEO satellites or, for the tropics and mid-latitudes, with ageostationary satellite such as the proposed GeoTROPE mission [Burrows et al., 2004]. At presentsuch an instrument is not in orbit, but in a few years the launch of the geostationary Sentinel 418