Abstract

Biogenic trace gases, especially halomethanes, which are important with respect to atmospheric chemistry, are released from the ocean and carry halogens to the troposphere and stratosphere. The concentrations of 10 halocarbons and isoprene in seawater were measured during the spring of 2007 in the western North Pacific Ocean (37–43° N, 143–146° E). Sea–air fluxes of CH 3Cl, CH 3Br, CH 2ClI, and CH 2I 2 were also estimated based upon the atmospheric as well as oceanic measurement of these species. Temperature–salinity scatter diagram analyses divided the sampling stations into the Oyashio region, Tsugaru warm current region, and Kuroshio region. Mean (range) concentrations of the gases in the water columns (5–100 m) were 114 (56–150) pmol L − 1 CH 3Cl, 6.9 (4.1–19.4) pmol L − 1 CH 3Br, 1.7 (0.7–2.9) pmol L − 1 CH 3I, 1.9 (0.9–4.1) pmol L − 1 CH 2BrCl, 4.8 (3.2–8.1) pmol L − 1 CH 2Br 2, 1.0 (0.6–1.8) pmol L − 1 CHBrCl 2, 1.2 (0.7–2.0) pmol L − 1 CHBr 2Cl, 10.8 (4.7–24.5) pmol L − 1 CHBr 3, 1.7 (0.7–5.4) pmol L − 1 CH 2ClI, 3.0 (< 0.1–22.2) pmol L − 1 CH 2I 2, and 19.7 (3.8–68.2) pmol L − 1 isoprene. The maximum concentration of isoprene was observed in the Oyashio region, where concentrations of chlorophyll a (maximum: 2.94 µg L − 1 ) were highest in the present study. However, the peaks of CH 3Br, CH 2ClI, and CH 2I 2 were observed in the Tsugaru warm current region, where concentrations of chlorophyll a were not as high (maximum: 0.65 µg L − 1 ). The results of chlorophyll a size fractionation showed a high occurrence of halomethanes in the stations dominated by pico-sized phytoplankton. These results indicate the importance of picoplankton as a possible source of halocarbon production. Chlorophyll b and prasinoxanthin had a statistically significant positive correlation with CH 2I 2 ( r 2 = 0.69 and r 2 = 0.71, respectively) and with CH 2ClI ( r 2 = 0.87 and r 2 = 0.77, respectively). These results suggest that some species of prasinophytes might contribute to CH 2I 2 and CH 2ClI production. For other compounds, there was no peak in the vertical profile in seawater. In the depth profiles, the peak of CH 2ClI was observed above the peak of CH 2I 2; these profiles suggest that a photochemical reaction could yield CH 2ClI from CH 2I 2 in seawater. The mean mixing ratio and range of CH 3Cl, CH 3Br, and CH 2ClI in the air were measured as 548 (524–609), 12.1 (8.6–19.0), and 0.27 (0.03–0.90) pptv, respectively. CH 2I 2 was not detected in the atmosphere (< 1 pptv). The saturation anomaly of CH 3Br was positive at all stations (the sea surface temperature varied from 1.7 °C to 19 °C). The highest mixing ratio of CH 2ClI in air was also observed near the station at which the highest concentration of CH 2ClI was observed in seawater; the sea-to-air fluxes of CH 2ClI and CH 2I 2 were 3.8 and 1.6 nmol m − 2 day − 1 , respectively. These results suggest that the production of CH 2ClI and CH 2I 2 in seawater is an important source of organic iodine compounds in the remote atmosphere.

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