Tropospheric OH and Cl levels deduced from non-methane hydrocarbon measurements in a marine site

In situ continuous hourly measurements of C-2 C-8 non- methane hydrocarbons ( NMHCS) have been performed from March to October 2006 at two coastal locations ( natural and rural) on the island of Crete, in the Eastern Mediterranean. Well defined diel variations were observed for several short lived NMHCS ( including ethene, propene, n- butane, n- pentane, n- hexane, 2- methyl- pentane). The daytime concentration of hydroxyl ( OH) radicals estimated from these experimental data varied from 1.3 x 10(6) to similar to 4.0 x 10(6) radical cm(-3), in good agreement with box-model simulations. In addition the relative variability of various hydrocarbon pairs ( at least 7) was used to derive the tropospheric levels of Cl atoms. The Cl atom concentration has been estimated to range between 0.6 x 10(4) and 4.7 x 10(4) atom cm(-3), in good agreement with gaseous hydrochloric acid ( HCl) observations in the area. Such levels of Cl atoms can be of considerable importance for the oxidation capacity of the troposphere on a regional scale.