Factors affecting O 3 and NO 2 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 (NO 2 ; JNO 2 ) were recorded at the remote coastal site Finokalia (35°20′N, 25°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°, compared to an aerosol‐free atmosphere. It also leads to a similar reduction of JO 1 D by 5%–15% at the 60° sza, for an average total ozone column (300–320 Dobson units (DU)). The effect of regional background AOD (∼0.1) is a reduction of JNO 2 and JO 1 D by up to 6% for sza in the range 15° to75°, 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° 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.