Abstract
Estimates of the particulate matter optical thickness at 0.55 μm wavelength are obtained from measurements of attenuation of solar radiation through hazy atmospheres at three stations in the Po Valley. Simultaneous observations of visibility at ground level are used to obtain careful evaluations of the volume extinction coefficient produced by the particulate matter at 0.55 μm wavelength. The ratio between the optical thickness and the volume extinction coefficient is taken to measure the scale height H for particulate extinction. Values of H between 0.5 and 1.3 km are found in cold atmospheres characterized by temperature inversions whereas values of more than 2 km are found on summer days in the presence of strong convective motions due to intense ground heating by the sun. These marked variations of height H with season are closely related to the evolving features of the boundary layer. The daily time-patterns of H increase throughout the morning in an approximately linear fashion with average rates ranging between 30 and 180 m h−1 and show almost constant values in the afternoon.
