Model evaluation and scale issues in chemical and optical aerosol properties over the greater Milan area (Italy), for June 2001

We evaluate first the mesoscale Transport of Atmospheric Pollutants Model TAPOM) performance in calculated aerosol surface concentrations and aerosol optical depth (AOD) values for the greater Milan area in Italy during June 2001. A comparison of the aerosol concentrations and AOD calculations is made with several data sets. Satellite data from both the MISR and MODIS instruments on board the Terra platform are used, as well as sun photometer data from both the AERONET and the Swiss CHARM networks and EMEP measurement data. Calculated monthly mean of SO4-, and NO3- at the EMEP Ispra station are overestimated by a factor 1.5 and 2 respectively. EC corresponds well with measurements when compared to the year 2000. OC is underestimated by a factor 1.5. Calculated AOD values are in general in good agreement with sun photometer and satellite data. Differences between model AOD and observations for dry clear sky days are smaller than for other days when, cirrus and Saharan dust are observed. We find for some clear sky days a good agreement between calculated and observed AOD spatial distribution. AOD retrievals by remote sensing instruments are not always consistent with each other. Significant differences are found between satellite AOD retrieval for the same area and time frame. Therefore it makes it difficult to use remote sensing data for model comparison. Secondly, we used the model to study scale issues in aerosol modelling at three horizontal resolutions (5 x 5 km, 10 x 10 km and 20 x 20 krn) through calculations of AOD and other aerosol properties for the same area and period. A finer model resolution shows a more detailed AOD distribution pattern following the domain's orography than the coarser resolution, and AOD values of the finer resolutions correspond better with the observations than the coarser resolution. Thirdly, we study the role of the boundary conditions on aerosol concentrations and AOD calculations for Ispra. A sensitivity analysis showed that when the boundary conditions are set to zero, SO4=, NH4+ and NO3- are underestimated by a factor 2.5, 2 and 8 when compared to measurements, leading to a strong underestimation of the AOD values at all times when compared to AERONET AOD. This indicates that the boundary conditions are very important for urban scale modelling. (C) 2007 Elsevier B.V. All rights reserved.