AEROSOL BACKSCATTERING DETERMINED FROM CHEMICAL AND PHYSICAL-PROPERTIES AND LIDAR OBSERVATIONS OVER THE EAST-COAST OF CANADA

In August to September 1995 a field experiment was conducted over the Gulf of Maine and Bay of Fundy to study the radiative forcing of pollu tion aerosols. The chemical and physical characteristics of two pollut ion cases were studied in detail in contrast to a clean atmosphere cas e. In the pollution cases, NH4++SO4= showed a unimodal distribution wi th a peak at 0.24 mu m diameter. It was dominant among identified chem ical components, including inorganic ions and organic compounds. Howev er, the identified components were only about 1/3 of the aerosol mass as determined from the physical measurements. The unidentified mass, w ith a large accumulation mode, was likely due to unmeasured organic ma tter. In the clean case, sea salt was the dominant species with a bimo dal distribution. The results were used to calculate the direct backsc atter coefficient beta(pi) at 0.532 and 1.064 mu m using the Mie theor y for comparison with LIDAR observations to determine the contribution s by the chemical components. In the dean case, the sea salt aerosols contributed about half of beta(pi). In the pollution cases, NH4++SO4= contributed 20-40% to beta(pi). The unidentified mass had contribution s to beta(pi) of>40% and >70% for the two pollution cases. The LIDAR b eta(pi) results were inverted to derive optical depths over the 300-24 00 m altitude range. Using these optical depths, the direct backscatte red fraction of radiative flux for the pollution aerosols was estimate d to be about 5 times higher than aerosols in the clean atmosphere.