The diel cycle of carbonyl sulfide in marine surface waters: Field study results and a simple model

Abstract

The concentrations of atmospheric and dissolved carbonyl sulfide (COS) were measured during a Lagrangian study aboard the R/V Meteor in the northeast Atlantic Ocean, April/May 1992, and during a campaign on the research platform Nordsee in the German Bight (southeastern North Sea), September 1992. The arithmetic means and standard deviations of the COS saturation ratios were 1.27 ± 0.58 (northeast Atlantic) and 3.23 ± 0.73 (German Bight). Sea surface COS showed a pronounced diel cycle with highest concentrations in the late afternoon and a mean concentration amplitude of about 2. To account for this diel cycle, we analyze our results using a simple empirical model, which includes a zeroth order photoproduction constant, sea surface UV light intensity, and terms for hydrolysis removal and air-sea exchange. Fitted and observed COS concentrations agreed to within11 % (northeast Atlantic) and 14% (German Bight). The in situ COS photoproduction constants were (0.030 ± 0.008) fmol L−1 s−1 W−1 m2 in the northeast Atlantic (n = 8) and (0.17 ± 0.07) fmol L−1 s−1 W−1 m2 in the German Bight (n = 10). After normalization to the cloud cover corrected UV irradiance at 40‡ latitude, we obtained sea surface COS production rates of (0.034 ± 0.017) nmol L−1 d−1 in the northeast Atlantic and (1.62 ± 0.62) nmol L−1 d−1 in the German Bight. Currently available in situ photoproduction rates show a high degree of correlation with the UV absorbance (r2 = 0.98, n = 4) and fluorescence (r2 = 0.85, n = 4) of chromophoric dissolved organic matter (CDOM). The regional differences between the COS productivity in the northeast Atlantic Ocean and the German Bight is attributed to the distribution pattern of CDOM optical properties.