Aerosol properties over two urban sites in South Spain during an extended stagnation episode in winter season

摘要

Columnar and ground level aerosol properties as well as mass concentrations of some gaseous pollutants were measured at two urban sites (Granada and Malaga, South Spain) before, during and after an extended stagnation episode from 7 to 13 February 2011. This long lasting event was associated with a very strong and persistent blocking high-pressure system over the Iberian Peninsula, together with very intense and persistent temperature inversions near the ground level. The columnar aerosol load at Granada showed a significant increase during this stagnation episode as indicated by aerosol optical depth at 440 nm, reaching values four times higher (0.6) than before and after the event. A significant increase in aerosol load at night time was also evidenced by star photometer measurements. Similarly, pronounced enhancement in columnar aerosol load was observed at Malaga, indicating the regional extension of this event. Analysis of ground level measurements obtained at Granada showed a significant increase in aerosol scattering coefficients and aerosol number concentrations during the stagnation episode. Furthermore, the analysis of aerosol size distribution measurements has evidenced the large contribution of fine particles at ground level as well as in the atmospheric column during the stagnation period. The fine mode radius measured at Granada showed a large displacement towards larger sizes together with a pronounced increase in the geometric standard deviation of the fine mode during the high pollution event in the morning hours on 9 February. This was attributed to the growth of aerosol particles due to coagulation and condensation processes as a result of the high fine aerosol load next to the surface favoured by the high pressure system and thermal inversion on that day. This increases in the radius and width of the fine mode results in more efficient scattering in the 440-1020 nm spectral range which, in combination with nearly constant and low imaginary refractive index (0.002) leads to high single scattering albedo with neutral spectral dependency. During the persistent temperature inversion episode the daily European PM_(10) mass concentration limit (50 μg m~3) was exceeded in Granada for most days and in Malaga for three consecutive days. Nevertheless, the mass concentrations of NO_2, CO and O_3 were below the European thresholds with a noticeable decrease in O3 mass concentration at both sites.