Modeling of photolysis rates over Europe: impact on chemical gaseous species and aerosols
Authors: Real E. , Sartelet K. 
Published: July 2010

Abstract. This paper evaluates the impact of photolysis rate calculation on European air composition and air quality monitoring. In particular, the impact of cloud parametrisation and the impact of aerosols on photolysis rates are analysed. Photolysis rates are simulated using the Fast-JX photolysis scheme and gas and aerosol concentrations over Europe are simulated with the regional model Polair3D of the Polyphemus platform. The photolysis scheme is first use to update the clear sky tabulation used in the previous Polair3D version. Important differences in photolysis rates are simulated, mainly due to updated cross-sections in the Fast-JX scheme. In the previous Polair3D version, clouds were taken into account by multiplying the clear-sky photolysis rates using a correction factor. In a second stage, the impact of clouds is taken into account more accurately by simulating them directly in the photolysis scheme. Differences in photolysis rates inside clouds are as high as differences between simulations with and without clouds. Outside clouds, the differences are small. The largest difference in gas concentrations is simulated for OH with a mean increase of its tropospheric burden of 4 to 5%. To take into account the impact of aerosols on photolysis rates, Polair3D and Fast-JX are coupled. Photolysis rates are updated every hour. Large impact on photolysis rates is observed at the ground, decreasing with altitude. The aerosol species that impact the most photolysis rates is dust especially in South Europe. Strong impact is also observed over anthropogenic emission regions (Paris, The Po and the Ruhr Valley) where mainly nitrate and sulphate reduced the incoming radiation. Differences in photolysis rates lead to changes in gas concentrations, with the largest impact simulated for OH and NO concentrations. At the ground, monthly mean concentrations of both species are reduced over Europe by around 10 to 14% and their tropospheric burden by around 10%. The decrease in OH leads to an increase of the life-time of several species such as VOC. For example, isoprene ground concentrations increase in average by around 10%. NO2 concentrations are not strongly impacted and O3 concentrations are mostly reduced at the ground with a monthly mean decrease of about 3%. O3 peaks are systematically decreased because of the NO2 photolysis rate decrease. Not only gas are impacted but also secondary aerosols, due to changes in gas precursors concentrations. Monthly mean concentrations of nitrate, ammonium, sulphate and secondary organic aerosol at the ground are modified by up to 4% but PM10 and PM2.5 only by 1 to 2%. However monthly mean local differences in PM10 and PM2.5 concentrations can reach 8% over regions with strong production of secondary aerosols such as the Po valley. In terms of air quality monitoring, ground concentrations of O3, NO2 and PM10 are compared with measurements from the EMEP stations. Statistics are usually better for simulation taking into account aerosol impact on photolysis rates, but changes are small. On the other hand, the systematic O3 peak reduction leads to large differences in the exceedances of the European O3 threshold as calculated by the model. The number of exceedances of the information and the alert threshold is divided by 2 when the aerosol impact on photochemistry is simulated. This shows the importance of taking into account aerosols impact on photolysis rates in air quality studies.