In this thesis, the troposphere-to-stratosphere transport (TST) of the three halocarbons methyl iodide, bromoform, and dibromomethane, is studied. Halocarbons are carbon based compounds containing halogen atoms, where halogen is the name for group seven of the periodic table of the elements, including e.g. chlorine, bromine and iodine. The three halocarbons studied in this project, are recognized as being contributers to the stratospheric halogen loading (Carpenter et al., 2014b), where they are involved in ozone depletion (Carpenter et al., 2014b; Salawitch et al., 2005). The knowledge about the halo-carbons spatial and temporal variability in emission, loss, and transport processes, are largely based on limited data (Hepach et al., 2016; Quack et al., 2007; Ziska et al., 2013).
It is known that the main atmospheric source for these halocarbons is oceanic (Carpenter et al., 2014b). Although, oceanic measurements of halocarbons are few, high concentra-tions especially over upwelling regions in the tropics and subtropics have been found.
Thus, they are considered important source regions (Hepach et al., 2016; Quack et al., 2007). The intense Peruvian Upwelling is considered one of the most productive oceanic regions in the world (Mann and Lazier, 2013). Therefore, studying halogen transport from this region is of great interest.
2.2.1 Marine Sources
Bromoform and dibromomethane are the primary natural contributors to atmospheric organic bromine (Quack et al., 2007). The source for the bromine-containing source gases is mainly natural and oceanic. For methyl iodide, the oceanic contribution to the atmo-spheric loading is more than 80% (Carpenter et al., 2014b).
The dominant producer of bromocarbons in the open ocean is phytoplankton (Moore et al., 1995b). Other known sources are macro algae and cyanobacteria (Nightingale et al., 1995; Wever and Horst, 2013). Hill and Manley (2009) suggested that a considerable formation pathway for the production of polyhalogenated compounds may be indirect through algal release of hypoiodous acid (HOI) and hypobromous acid HOBr, reacting with dissolved organic matter (DOM). The seaweed and phytoplankton produces H2O2 during photosynthesis and photorespiration (and stress for macro algae), which reacts with bromine in the water, to form HOBr, by enzymatic activity of haloperoxidases (Hep-ach et al., 2016). The HOBr then reacts with DOM, and forms polybromomethanes like bromoform and dibromomethane (Wever and Horst, 2013).
For methyl iodide is produces by non-biological, photochemical degradation of io-dide containing DOM, and biologically by algae and phytoplankton (Carpenter et al., 2014b; Tegtmeier et al., 2013). Methyl iodide may possibly also be formed via bacteria (Hepach et al., 2016)
Hepach et al. (2016) found the Peruvian upwelling region to be only a moderate source region for bromocarbons, but significant source region for iodocarbons, Decem-ber 2012. Previously high concentration of iodocarbons in the tropical oceans have been connected with mainly the photochemical source (Hepach et al., 2016). Stemmler et al.
(2015) used a global three-dimensional ocean biogeochemistry model to simulate bromo-form cycling in the ocean, and it was found to match observations well.
2.2.2 Transport from the Ocean to the Stratosphere
When the ocean is supersaturated with halocarbons, the halocarbons are emitted from the ocean, transported horizontally and vertically mixed in the marine atmospheric bound-ary layer (MABL). The rate at which the halocarbon ocean-to-atmosphere exchange oc-curs depends on the air-sea halocarbon concentration gradient. The air-sea concentration
FIGURE 2.6: Schematic of the oceanic sources and the atmospheric pro-cesses relevant for methyl iodide (CH3I), bromoform (CHBr3) and
dibro-momethane (CH2Br2).
gradient is significantly affected by oceanic upwelling (Fuhlbrügge et al., 2013, 2016a).
When there is coastal oceanic upwelling of cold water to the surface, the air over the ocean cools, resulting in a stable and isolated MABL and high atmospheric halocarbon mixing ratios. The high atmospheric mixing ratios decreases the halocarbon sea-air con-centration gradient, hence the emission to the atmosphere is reduced. Fuhlbrügge et al. (2013, 2016a) found that a strong trade inversion acts as a transport barrier, leading to a near-surface accumulation of halocarbons in the atmosphere. The trade inversion is a temperature inversion that occurs due to large scale subsidence in the descending branches of the Hadley Cell and the Walker Cell, and is found where the cold dry sub-sided air meets the underlaying warm moist air. The coastal emission of oceanic halo-carbons also vary due to the change in amount and types of algae, and with the diurnal and tidal cycles (Carpenter et al., 2014b; Wever and Horst, 2013). Local emission maxima linked to upwelling areas, over the tropical oceans, have been observed (Quack et al., 2007; Wever and Horst, 2013). Stemmler et al. (2015) simulated emissions of bromoform into the atmosphere, using observational-based estimates from Ziska et al. (2013) of near-surface atmospheric bromoform volume mixing ratio (VMR) as upper boundary condi-tion. These were found to be lower than previous estimates by Ziska et al. (2013). This is because seasonality is considered and less bromoform is produced in non-blooming seasons reversing the sea-air flux of bromoform, and also because coastal emissions of bromoform is not represented in this model (Stemmler et al., 2015).
VSLS are defined as substances with atmospheric lifetimes of less than six months (WMO, 2006). The current estimated atmospheric lifetimes at 10 km altitude are 17 days for bromoform, 150 days for dibromomethane, and 3.5 days for methyl iodide (Carpen-ter et al., 2014b). Hence, all three compounds belong to the VSLS category. Especially short-lived VSLS, like methyl iodide and bromoform, need to be emitted close to deep, convective systems to be able to reach the Stratosphere (Tegtmeier et al., 2013). Their impact on stratospheric halogen loading is still uncertain due to limited observations (Carpenter et al., 2014b). Several studies have found indications that methyl iodide and bromoform reaches the stratosphere over tropical regions, i.e., Fiehn et al. (2017), Hos-saini et al. (2015), Saiz-Lopez et al. (2015), and Tegtmeier et al. (2013) in despite of the stable MABL and the trade inversion.
The VSLS sources to the stratosphere halogen loading is typically separated between
SGI and PGI (Ko and Poulet, 2003), where the SGI is the direct injection of the VSLS sources, and the PGI is the injection of halogens from the atmospheric degraded VSLS products. In this thesis, only the SGI of VSLS has been focused on.
2.2.3 Atmospheric Removal and Ozone Depletion
The main sink of methyl iodide in the troposphere is photolysis (Carpenter et al., 2014b).
The tropospheric sinks of bromocarbons are OH oxidation and photolysis (Carpenter et al., 2014b). Photolysis is the most important removal for bromoform, and a major sink process is oxidation by OH radicals for dibromomethane (Carpenter et al., 2014b).
Other sinks of atmospheric VSLS are uptake by the oceans and soil microbial degradation (Carpenter et al., 2014b).
When bromine and iodine containing halocarbons are degraded in the atmosphere, they form reactive halogen radicals. This takes place both in the troposphere and strato-sphere, and they therefore differ from chlorofluorocarbons (CFC’s) which can only be broken down in the Stratosphere by ultra-violet radiation (Wever and Horst, 2013). One of the halogen radical’s most important reaction is ozone depletion. Ozone in the tropo-sphere is an active greenhouse gas, and it is also toxic for humans. The halocarbon emis-sions from the sea can lower tropospheric ozone, which contribute to reducing global warming, and inproving air quality. The VSLS source gases and product gases which reaches the stratosphere will take part in catalytic ozone depletion there (Wever and Horst, 2013). Thus, VSLS contribute to increased transmission of harmful ultraviolet light through the ozone layer.
PGI of bromine containing VSLS makes a non-negligible contribution to the strato-spheric bromine loading, with bromoform and dibromomethane as the most important sources. The PGI of brominated VSLS can range from 1.1 to 4.3 ppt. By including bromine containing VSLS in modeling studies, the modeled O3trends have been closer to obser-vations. The PGI of idodine containing VSLS to the stratosphere is still uncertain. It has been estimated to be less than 0.15 ppt, and is suggested to be a minor sink for O3 (Hossaini et al., 2012).