A STUDY OF REGIONAL AEROSOL RADIATIVE PROPERTIES AND EFFECTS ON ULTRAVIOLET-B RADIATION

A field experiment was conducted in western North Carolina to investig ate the relationship between aerosol optical properties and atmospheri c transmission. Two research measurement sites in close horizontal pro ximity but at different altitudes were established to measure the tran smission of UV radiation through a slab of atmosphere. An identical se t of radiation sensing instruments, including a broadband UV-B radiome ter, a direct Sun pyrheliometer, a shadowband radiometer, and a spectr al photometer, was placed at both sites, a mountaintop site (Mount Gib bes 35.78 degrees N, 82.29 degrees W, 2004 m elevation) and a valley s ite;(Black Mountain, North Carolina 35.66 degrees N, 82.38 degrees N, 951 m elevation). Aerosol size distribution sampling equipment was loc ated at the valley site. Broadband solar pseudo-optical depth and aero sol optical depths at 415 nm, 500 nm, and 673 nm were measured for the lowest 1-km layer of the troposphere. The measurements exhibited vari ations based on an air mass source region as determined by back trajec tory analysis. Broadband UV-B transmission through the layer also disp layed variations relating to air mass source region. Spectral UV trans mission revealed a dependence upon wavelength, with decreased transmis sion in the UV-B region (300-320 mm) versus UV-A region (320-363.5 mm) . UV-B transmission was found to be negatively correlated with aerosol optical depth. Empirical relations were developed to allow prediction of solar noon UV-B transmission if aerosol optical depth at two visib le wavelengths (415 and 500 nm) is known. A new method was developed f or determining aerosol optical properties from the radiation and aeros ol size distribution measurements. The aerosol albedo of single scatte r was found to range from 0.75 to 0.93 and the asymmetry factor ranged from 0.63 to 0.76 at 312 nm, which is close to the peak response of h uman skin to UV radiation.