Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

Abstract

Abstract.Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr 3 ), dibromomethane (CH 2 Br 2 ), methyl iodide (CH 3 I) and diiodomethane (CH 2 I 2 ). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr 3 of up to 44.7 and up to 9.2 pmol L −1 for CH 2 Br 2 in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water, CH 2 Br 2 was often more elevated in greater depth than CHBr 3 , which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH 2 Br 2 indicates an additional source in comparison to CHBr 3 or a slower degradation of CH 2 Br 2 . Concentrations of CH 3 I of up to 12.8 pmol L −1 in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a biological source. CH 2 I 2 was very low in the near surface water with maximum concentrations of only 3.7 pmol L −1 . CH 2 I 2 showed distinct maxima in deeper waters similar to CH 2 Br 2 . For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and one of the main driving factors of their emissions into the atmosphere in the ACTregion. The calculated production rates of the compounds in the mixed layer are 34 ± 65 pmol m −3 h −1 for CHBr 3 , 10 ± 12 pmol m −3 h −1 for CH 2 Br 2 , 21 ± 24 pmol m −3 h −1 for CH 3 I and 384 ± 318 pmol m −3 h −1 for CH 2 I 2 determined from 13 depth profiles.

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).…”

“…Oceanic upwelling occurs along the eddy-rich, shallow region south of Madagascar, which leads to locally enhanced phytoplankton growth (Quartly et al, 2006). It is possible that an upwelling of elevated CH 2 Br 2 concentrations from the deeper ocean could have occurred in a similar way as was observed for the equatorial upwelling in the Atlantic (Hepach et al, 2015). CH 3 I oceanic concentrations range from 0.2 to 16.4 pmol L −1 , with a mean of 3.4 ± 3.1 pmol L −1 .…”

“…The SHIVA calculations show even higher transport efficiencies of 7.9 %, which lead to an entrainment of 48.4 nmol CHBr 3 (Table 5), because of the strong convective activity in that region during the time (Fuhlbrügge et al, 2016b). The MSM18/3 cruise in the equatorial Atlantic (Hepach et al, 2015) has the smallest emissions, entrainment, and a transport efficiency of 0.9 % (Table 5). Overall, the comparison indicates that more CHBr 3 was entrained to the stratosphere from the tropical west Pacific than from the tropical west Indian Ocean during the Asian summer monsoon using available in situ emissions and 6-hourly meteorological fields.…”

“…4. For intercomparison with other ocean basins, we employed exactly the same model setup of transport simulations (including lifetimes) and the same emission calculation method for three previous corresponding cruises in the tropics: the TransBrom campaign in the west Pacific in 2009 (introduction to special issue: Krüger and Quack, 2013), the SHIVA campaign in the South China and Sulu seas in 2011 (Fuhlbrügge et al, 2016a), and the MSM18/3 cruise in the equatorial Atlantic cold tongue (Hepach et al, 2015).…”

Meteorological constraints on oceanic halocarbons above the Peruvian upwelling

“…Recent studies have identified open-ocean upwelling areas in the Atlantic as large source regions for a number of brominated and iodinated oceanic trace gases (Quack et al, 2004(Quack et al, , 2007O'Brien et al, 2009;Raimund et al, 2011;Hepach et al, 2015). Their sources are related to biological and chemical processes in the productive waters of the upwelling.…”