Drivers of diel and regional variations of halocarbon emissions from the tropical North East Atlantic

Abstract

Abstract. Methyl iodide (CH 3 I), bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ), which are produced naturally in the oceans, take part in ozone chemistry both in the troposphere and the stratosphere. The significance of oceanic upwelling regions for emissions of these trace gases in the global context is still uncertain although they have been identified as important source regions. To better quantify the role of upwelling areas in current and future climate, this paper analyzes major factors that influenced halocarbon emissions from the tropical North East Atlantic including the Mauritanian upwelling during the DRIVE expedition. Diel and regional variability of oceanic and atmospheric CH 3 I, CHBr 3 and CH 2 Br 2 was determined along with biological and physical parameters at six 24 h-stations. Low oceanic concentrations of CH 3 I from 0.1-5.4 pmol L −1 were equally distributed throughout the investigation area. CHBr 3 and CH 2 Br 2 from 1.0 to 42.4 pmol L −1 and to 9.4 pmol L −1 , respectively were measured with maximum concentrations close to the Mauritanian coast. Atmospheric CH 3 I, CHBr 3 , and CH 2 Br 2 of up to 3.3, 8.9, and 3.1 ppt, respectively were detected above the upwelling, as well as up to 1.8, 12.8, and 2.2 ppt at the Cape Verdean coast. While diel variability in CH 3 I emissions could be mainly ascribed to oceanic nonbiological production, no main driver was identified for its emissions over the entire study region. In contrast, biological parameters showed the greatest influence on the regional distribution of sea-to-air fluxes of bromocarbons. The diel impact of wind speed on bromocarbon emissions increased with decreasing distance to the coast. The height of the marine atmospheric boundary layer (MABL) influenced halocarbon emissions via its influence on atmospheric mixing ratios. Oceanic and atmospheric halocarbons correlated well in the study region, and in combination with high oceanic CH 3 I, CHBr 3 and CH 2 Br 2 concentrations, local hot spots of atmospheric halocarbons could solely be explained by marine sources. This conclusion is in contrast to previous studies that hypothesized elevated atmospheric halocarbons above the eastern tropical Atlantic to be mainly originated from the West-African continent.

Delivery of halogenated very short-lived substances from the west Indian Ocean to the stratosphere during the Asian summer monsoon

“…We compare with cruises and open ocean estimates, since OASIS mainly covered open ocean regions and only small coastal areas close to Madagascar, the British Indian Ocean Territory, and the Maldives. The average CHBr 3 emission during the OASIS campaign (910 pmol m −2 h −1 ) was larger than during most campaigns in tropical regions: 1.5 times larger than during TransBrom in the subtropical and tropical west Pacific (Tegtmeier et al, 2012), 1.2 times larger than during DRIVE in the tropical northeast Atlantic (Hepach et al, 2014), and 1.5 times larger than during MSM18/3 in the Atlantic equatorial upwelling (Hepach et al, 2015). Only the SHIVA campaign in the South China and Sulu seas yielded larger CHBr 3 emissions of 1486 pmol m −2 h −1 because of very high oceanic concentrations close to the coast (Fuhlbrügge et al, 2016b).…”

Meteorological constraints on oceanic halocarbons above the Peruvian upwelling

“…the Mauritanian upwelling (Carpenter et al, 2010;Fuhlbrügge et al, 2013;Hepach et al, 2014). While dibromomethane concentrations are comparable, methyl iodide concentrations are almost 8 times higher than in the Mauritanian upwelling (Fig.…”

“…The analysis has a precision of 10 % (1σ ) determined from duplicate samples. The approach is described in detail by Hepach et al (2014).…”

“…Details on deriving the air-sea concentration gradient and emissions are further described in Hepach et al (2014) and references therein.…”

“…Meteorological conditions strongly influenced the atmospheric mixing ratio of the marine compounds bromoform (CHBr 3 ), dibromomethane (CH 2 Br 2 ) and also methyl iodide (CH 3 I) (e.g. Fuhlbrügge et al, 2013;Hepach et al, 2014). The combination of a pronounced low MABL above cold upwelling waters with high concentrations and emissions of the com-pounds causes elevated atmospheric mixing ratios.…”