Distributions of short-lived iodocarbons and biogenic trace gases in the open ocean and atmosphere in the western North Pacific

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 CH3Cl, CH3Br, CH2ClI, and CH2I2 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 (5–100m) were 114 (56–150) pmolL−1 CH3Cl, 6.9 (4.1–19.4) pmolL−1 CH3Br, 1.7 (0.7–2.9) pmolL−1 CH3I, 1.9 (0.9–4.1) pmolL−1 CH2BrCl, 4.8 (3.2–8.1) pmolL−1 CH2Br2, 1.0 (0.6–1.8) pmolL−1 CHBrCl2, 1.2 (0.7–2.0) pmolL−1 CHBr2Cl, 10.8 (4.7–24.5) pmolL−1 CHBr3, 1.7 (0.7–5.4) pmolL−1 CH2ClI, 3.0 (<0.1–22.2) pmolL−1 CH2I2, and 19.7 (3.8–68.2) pmolL−1 isoprene. The maximum concentration of isoprene was observed in the Oyashio region, where concentrations of chlorophyll a (maximum: 2.94µgL−1) were highest in the present study. However, the peaks of CH3Br, CH2ClI, and CH2I2 were observed in the Tsugaru warm current region, where concentrations of chlorophyll a were not as high (maximum: 0.65µgL−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 CH2I2 (r 2 =0.69 and r 2 =0.71, respectively) and with CH2ClI (r 2 =0.87 and r 2 =0.77, respectively). These results suggest that some species of prasinophytes might contribute to CH2I2 and CH2ClI production. For other compounds, there was no peak in the vertical profile in seawater. In the depth profiles, the peak of CH2ClI was observed above the peak of CH2I2; these profiles suggest that a photochemical reaction could yield CH2ClI from CH2I2 in seawater. The mean mixing ratio and range of CH3Cl, CH3Br, and CH2ClI in the air were measured as 548 (524–609), 12.1 (8.6–19.0), and 0.27 (0.03–0.90) pptv, respectively. CH2I2 was not detected in the atmosphere (<1pptv). The saturation anomaly of CH3Br was positive at all stations (the sea surface temperature varied from 1.7°C to 19°C). The highest mixing ratio of CH2ClI in air was also observed near the station at which the highest concentration of CH2ClI was observed in seawater; the sea-to-air fluxes of CH2ClI and CH2I2 were 3.8 and 1.6nmolm−2 day−1, respectively. These results suggest that the production of CH2ClI and CH2I2 in seawater is an important source of organic iodine compounds in the remote atmosphere. [Copyright &y& Elsevier]