Aerosol characteristics for the Po Valley (northern Italy) are analyzed using 7-year time series of automated optical measurements recorded at two sites located at the extreme ends of the valley, one in the northwest area and the other in the northern Adriatic coastal region (Acqua Alta Oceanographic Tower). The annual cycles of aerosol optical thickness τa at 440 nm are very similar, with an overall daily average of 0.35--0.39, minima in winter and maxima in spring or summer. The ¿ngstr¿m exponent is remarkably constant in time and across sites, averaging 1.55 ¿ 0.35. Conversely, the diurnal variability is very different. The site of Ispra shows an increase in aerosol load during the day (amplitude of ~10%), typical of an environment near urban/industrial sources, whereas the coastal Adriatic site exhibits a strong decrease from morning to afternoon (amplitude of ~20%). Both trends are mirrored by the variability in precipitable water vapor.