Methyl and ethyl nitrate saturation anomalies in the Southern Ocean (36 - 65°S, 30 - 70°W)

Abstract

Environmental Context. The alkyl nitrates are a group of organic compounds that are known to be produced naturally in seawater. The sea-to-air flux of alkyl nitrates is believed to contribute significantly to the ‘odd nitrogen’ reservoir of the atmosphere and to play an important role in regulating tropospheric ozone levels in remote marine regions. Here we expand our knowledge of alkyl nitrate concentration distributions and saturation anomalies to Southern Ocean waters. Abstract. We report the first coupled atmosphere and seawater alkyl nitrate measurements for the Southern Ocean in the area bounded by 36–65°S, 30–70°W (November/December, 2004). Methyl and ethyl nitrate concentrations in seawater were 3.1–194.9 and 0.3–71.8 pmol L–1, respectively. Atmospheric mixing ratios ranged from 1.0 to 71.5 ppt for methyl nitrate and 0.6 to 16.6 ppt for ethyl nitrate. No correlations between alkyl nitrate distributions, and sea surface temperature, windspeed or chlorophyll a were observed. However, methyl and ethyl nitrate were well correlated in both the air and seawater, which suggests a common source. Calculations based on these observations estimate median saturation anomalies of –40% (–95 to 220%) for methyl nitrate and –11% (–98 to 174%) for ethyl nitrate. Positive saturation anomalies were spatially patchy, which suggests that some methyl and ethyl nitrate production was taking place in isolated areas of the study region. Overall our negative median saturation anomaly values suggest that during late austral spring (2004) the region of the Southern Ocean in which our measurements were made was not a net source of methyl or ethyl nitrate to the atmosphere. These results reinforce previous findings which suggest that whilst the equatorial ocean is a major source of methyl and ethyl nitrates to the atmosphere, higher latitude waters are generally at equilibrium or under-saturated. More measurements are required to assess how representative our results are of other areas of the Southern Ocean.