TROPOSPHERIC AEROSOL SIZE DISTRIBUTIONS FROM 1982 TO 1988 OVER EASTERN NORTH-AMERICA

The size distribution of a portion of the low- (> 800 mb) and middle- (500-800 mb) troposphere aerosol has been measured from 1982 until 1988 at several eastern North America locations between 42-degrees-N and 85-degrees-N. Particles between about 0.17 and 35-mu-m were measured using light scattering instrumentation (Particle Measuring Systems ASASP-100X and FSSP-100) mounted under a wing of a Twin Otter aircraft. Away from immediate sources, the lower-troposphere particle volumes are dominated by particles in two size ranges: 0.2-0.5-mu-m and 5-30-mu-m. The middle-troposphere number distribution is typically dominated by particles in the 0.17-0.5-mu-m diameter range, in some cases to the point that the distributions are effectively nearly monodisperse. An enhancement in the middle troposphere particles between 0.7-mu-m and 2.0-mu-m appears in many cases when the total number concentration of particles is low (i.e. < 100 scm-3). This may be due to the transport of dust particles, primary marine particles, or possibly the intrusion of stratospheric aerosol. Through the examination of the variability of particle number concentration at each location, the typical minimum concentration of greater-than-or-equal-to 0.17-mu-m aerosol particles in eastern North America is estimated to be 50-100 scm-3 for 950-800 mb and 0-25 scm-3 for 800-500 mb. Aerosol mass estimates assuming spheres of density 2.0 g cm-3 are 1.4-2.8-mu-g sm-3 and 0-0.7-mu-g sm-3, respectively.