Factors affecting O3 and NO2 photolysis frequencies measured in the eastern Mediterranean during the five-year period 2002-2006

The photolysis frequencies of ozone (O-3; to singlet oxygen, JO(1)D) and nitrogen dioxide (NO2; JNO(2)) were recorded at the remote coastal site Finokalia (35 degrees 20'N, 25 degrees 40'E), on the island of Crete, Greece, during the period 2002-2006. 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 JNO(2) by 5%-14% at a solar zenith angle (sza) of 60 degrees, compared to an aerosol-free atmosphere. It also leads to a similar reduction of JO(1)D by 5%-15% at the 60 degrees sza, for an average total ozone column (300-320 Dobson units (DU)). The effect of regional background AOD (similar to 0.1) is a reduction of JNO(2) and JO(1)D by up to 6% for sza in the range 15 degrees to75 degrees, respectively. During high aerosol loads (AOD 0.5-0.7) 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 JO(1)D at 60 degrees sza as compared with zero AOD conditions. A reduction of Js corresponding to a 24% decrease in the local noon JO(1)D value and a 5% decrease in local noon JNO(2) were found to result in a 12% reduction in the 24 h mean net chemical production of O-3, using a chemical box model.