Effects of the physical and optical properties of urban aerosols measured during the CAPITOUL summer campaign on the local direct radiative forcing

Abstract

Measurements of aerosol optical and physical properties, and chemical composition were conducted at Toulouse, an urban site of southwest France, as part of the CAPITOUL experiment. A period of intensive observations was held over 10 days during the summer campaign in June–July, 2004. During this period, PM2.5 concentrations ranged between 1 and 50 µg m−3. Aerosol size distributions exhibit the presence of three distinct modes which could be fitted by lognormal functions. Diurnal variations of aerosol concentration, size distribution, BC concentration, scattering and absorption coefficients, and single scattering albedo show the presence of two maxima corresponding to rush hours due to various urban activities such as vehicle traffic. Over the period of study, the single scattering albedo at 0.52 µm is found to be 0.7 ± 0.1, indicating the dominance of absorbing aerosols over Toulouse. Airborne measurements observed the vertical distribution of aerosols in the atmosphere. These observations clearly show that aerosol is mostly contained within the UBL which contributes significantly to the aerosol optical depth. The GAME radiative transfer model (RTM) has been used to carry out the radiative transfer computations. A synergetic approach was applied to generate spectral values of aerosol parameters required as input for the radiative transfer calculations. Vertical radio soundings and airborne measurements provided in-situ parameters required for the RTM. Simulations indicate that aerosols over Toulouse reduce significantly (−30 W m−2) the solar energy reaching the surface, mainly by absorption of the solar radiation into the atmosphere. At the TOA, aerosol forcing is even found to be positive (+0.8 W m−2). These results confirm that local direct aerosol forcing in urban atmosphere is mainly driven by the absorbing black carbon aerosols. The mean heating rate within the UBL is considerably enhanced, by 4.57 K day−1, due to the presence of absorbing aerosols.