Urban/industrial aerosol: Ground-based Sun/sky radiometer and airborne in situ measurements

Both airborne in situ and ground‐based remote sensing methods are used to measure the properties of urban/industrial aerosols during the Sulfate Clouds and Radiation—Atlantic (SCAR‐A) experiment in 1993. Airborne in situ methods directly measure aerosol characteristics such as size distribution and scattering coefficient at a particular altitude and infer the total column optical properties, such as optical thickness. Ground‐based remote sensing is sensitive to the aerosol optical properties of the entire column and infers the physical properties from inversion of sky radiance. Comparison of optical thickness measurements are encouraging but inconclusive because of measured profiles which extend no higher than 2 km. By comparing aerosol volume size distributions we find that the two systems are in agreement in the radius size range 0.05–2 μm, after the stratospheric aerosol mode is removed from the remote sensing data. At larger aerosol sizes both systems suffer from greater uncertainty, and the larger aerosols themselves are less spatially uniform because of their short lifetimes. The combination of factors makes the comparison at larger radii impossible. The disadvantages of the in situ systems are that there is a measuring efficiency for each device which is dependent on aerosol size and that airborne in situ measurements are rare events in time and space. Also, in situ instruments dry the aerosol before measurement. Automatic remote sensing procedures measure the total column ambient aerosol unaffected by drying or sampling issues, and these instruments can be installed globally to make observations many times per day. However, the disadvantages to remote sensing are that the inferred physical properties are dependent on the assumptions and numerical limitations of the inversion procedures. The favorable comparison between the two types of measurement systems suggests that these drawbacks are manageable in both cases.