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 Sul fate Clouds and Radiation-Atlantic (SCAR-A) experiment in 1993. Airbor ne in situ methods directly measure aerosol characteristics such as si ze distribution and scattering coefficient at a particular altitude an d infer the total column optical properties, such as optical thickness . Ground-based remote sensing is sensitive to the aerosol optical prop erties of the entire column and infers the physical properties from in version of sky radiance. Comparison of optical thickness measurements are encouraging but inconclusive because of measured profiles which ex tend no higher than 2 km. By comparing aerosol volume size distributio ns we find that the two systems are in agreement in the radius size ra nge 0.05-2 mu m, after the stratospheric aerosol mode is removed from the remote sensing data. At larger aerosol sizes both systems suffer f rom greater uncertainty, and the larger aerosols themselves are less s patially uniform because of their short lifetimes. The combination of factors makes the comparison at larger radii impossible. The disadvant ages of the in situ systems are that there is a measuring efficiency f or each device which is dependent on aerosol size and that airborne in situ measurements are rare events in time and space. Also, in situ in struments dry the aerosol before measurement. Automatic remote sensing procedures measure the total column ambient aerosol unaffected by dry ing or sampling issues, and these instruments can be installed globall y to make observations many times per day. However, the disadvantages to remote sensing are that the inferred physical properties are depend ent on the assumptions and numerical limitations of the inversion proc edures. The favorable comparison between the two types of measurement systems suggests that these drawbacks are manageable in both cases.