Changes in submicrometer particle distributions and light scattering during haze and fog events in a highly polluted environment

The changes in submicrometer atmospheric particle size distributions measured with a Differential Mobility Particle Sizer (DMPS) system during a polluted fog experiment during November, 1994 are presented in this study. Results reveal three modes commonly evident in the size distribution (3 < D_N < 843 nm) measurements; the ultrafine, Aitken and accumulation with respective geometric diameters, (D_gn), of 17, 110 and 400 nm. An additional mode, appears between the ultrafine and Aitken modes (D_gn = 52 nm) in approximately one quarter of the measurements and is linked to several industrial cities upwind of the measurement site. A stabile ultrafine mode appears consistently (84% of measurements) at 16-17 nm throughout the campaign, suggestive of a source, such as a highway in the near vicinity. During fog and haze periods number concentrations for particles less than 25 nm and greater than 400 nm decrease by 78 and 95%, respectively. These changes do not affect the aerosol scattering efficiency significantly. The overall aerosol mass scattering efficiency determined for the Po Valley region is 4.3 ± 0.6 m² g^-1. Closure is achieved for light extinction predicted from droplet distributions and measured with a transmissiometer in 37 of 39 cases during fog periods. Measured and calculated light extinction, b_ext, covary strongly with an R² of 0.92.