Convective transport of particulate matter in an Apennine valley: 1. Sun-photometric measurements of aerosol optical depth time-variations at various altitudes

Multispectral measurements of direct solar irradiance were simultaneously taken employing various sun-photometers at five stations located at different altitudes along the western slope of the Leo valley, in the Apennines (Italy), on a number of clear-sky autumn days in 1981 and 1982. Using precise estimates of the calibration constants of the various sun-photometers, obtained through a rigorous intercomparison procedure, the measurements were examined following the most up-to-date criteria of the sun-photometry method to determine daily homogeneous sets of aerosol optical depths at three window-wavelengths, at all five stations. The values of aerosol optical depth obtained from the 1982 measurements were corrected for the extinction effects produced by the stratospheric cloud of El Chichón volcanic aerosols, following a procedure based on the multimodal aerosol extinction model proposed by Pueschel et al. (Pueschel, R.F., Kinne, S.A., Russell, P.B., Snetsinger, K.G., 1993. Effects of the 1991 Pinatubo volcanic eruption on the physical and radiative properties of stratospheric aerosols. Keevallik, S. Kärner, O. (Eds.), IRS ʹ92: Current Problems in Atmospheric Radiation. Proceedings of the International Radiation Symposium, A. Deepak Publ., Hampton, Va., 183–186.) for 6-month-old volcanic particles (see Appendix). sults show that the tropospheric aerosol optical depth at 500-nm wavelength usually assumed low values in the early morning, most generally decreasing from 0.08 to 0.01 with the station altitude, and considerably increased during the morning, especially at the lower stations, as a result of the vertical transport of particulate matter due to the upslope breezes. The corresponding daily percentage increases varied between 60% and 370%, presenting a median value of 175% and quartiles of 140% and 230%. The time-variations of the volume aerosol extinction coefficient at the 500-nm wavelength were evaluated within the four atmospheric layers defined by the station altitudes, presenting early morning values mainly varying from 0.02 to 0.20 km−1 in the first layer, from 0.02 to 0.11 km−1 in both second and third layers and from 0.01 to 0.03 km−1 in the fourth. Therefore, greater increases were found to occur within the three lower layers situated below 1500 m height, with hourly mean percentage variations ranging between 10% and 420%, with a median value of 230% and quartiles of 70% and 270%.