Factors affecting O3 and NO2 photolysis frequencies measured in the eastern Mediterranean during the five-year period 20022006

The photolysis frequencies of ozone (O3; to singlet oxygen, JO1D) and nitrogen dioxide (NO2; JNO2) were recorded at the remote coastal site Finokalia (35°20N, 25°40E), on the island of Crete, Greece, during the period 20022006. We present a study of their main climatological aspects and a quantification of the effect of aerosol and total ozone column on these frequencies. The 5-yr mean Aerosol Optical Depth (AOD) at 380 nm in the area is equal to 0.27 ± 0.13 and reduces JNO2 by 5%14% at a solar zenith angle (sza) of 60°, compared to an aerosol-free atmosphere. It also leads to a similar reduction of JO1D by 5%15% at the 60° sza, for an average total ozone column (300320 Dobson units (DU)). The effect of regional background AOD (0.1) is a reduction of JNO2 and JO1D by up to 6% for sza in the range 15° to75°, respectively. During high aerosol loads (AOD 0.50.7) the percentage reduction of Js was found to be as much as 30%40% at high sza. The day-to-day variability of total ozone column over the area, of the order of 20 DU, was found to result in a 12% change in JO1D at 60° sza as compared with zero AOD conditions. A reduction of Js corresponding to a 24% decrease in the local noon JO1D value and a 5% decrease in local noon JNO2 were found to result in a 12% reduction in the 24 h mean net chemical production of O3, using a chemical box model.