DOAS observations of formaldehyde and its impact on the H[O.sub.x] balance in the tropical Atlantic marine boundary layer

Measurements of formaldehyde (HCHO) were made at the Cape Verde Atmospheric Observatory between November 2006 and June 2007 using the Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) technique. Observations show that typical HCHO mixing ratios ranged between 350 and 550 pptv (with typical 2-[sigmma] uncertainties of ~110 pptv), with several events of high HCHO, the maximum being 1,885± 149 pptv. The observations indicate a lack of strong seasonal or diurnal variations, within the uncertainty of the measurements. A box model is employed to test whether the observations can be explained using known hydrocarbon photochemistry; the model replicates well the typical diurnal profile and monthly mean values. The model results indicate that on average 20% of H[O.sub.2] production and 10% of OH destruction can be attributed to the mean HCHO levels, suggesting that even at these low average mixing ratios HCHO plays an important role in determining the H[O.sub.x] (H[O.sub.2]+OH) balance of the remote marine boundary layer. Keywords Differential optical absorption * Formaldehyde * HOx * Oxidation capacity * Marine boundary layer