2.1 Sources of tropospheric halogens
Table 1 lists the global sources and sinks of tropospheric gas-phase inorganic chlorine (Cly), bromine (Bry), and iodine (Iy) in GEOS-Chem (see Table 1 for definitions of Cly, Bry, and Iy). SSA emissions are from Jaeglé et al. (2011). Open-fire emissions of HCl are obtained by applying the emission factors from Andreae (2019) for different vegetation types to the GFED4 (Global Fire Emissions Database version 4) biomass burned inventory (van der Werf et al., 2017). The resulting global source of 0.5 Tg Cl a−1 is much smaller than in Wang et al. (2019), who used older emission factors from Lobert et al. (1999). Organohalogen gases can produce halogen radicals by oxidation and photolysis. Emissions of CH3Cl, CH2Cl2, CHCl3, and CHBr3 are implicitly treated in the model by specifying latitudinal and monthly surface air boundary conditions from CMIP6 (historical greenhouse gas concentrations for climate modeling) (Meinshausen et al., 2017). Emissions of other bromocarbons (CH3Br, CH2Br2) and iodocarbons (CH3I, CH2I2, CH2ICl, CH2IBr) are from Bell et al. (2002), Liang et al. (2010), and Ordóñez et al. (2012).
We do not include continental emissions of inorganic chlorine from anthropogenic sources (fuel combustion, waste incineration, etc.) and dust because they are highly uncertain and most likely negligible from a global perspective. The only available global emission inventory for anthropogenic HCl and Cl− is that of McCulloch et al. (1999) at 6.7 Tg Cl a−1 for 1990s, but we previously found this inventory to be too high by an order of magnitude in comparison to regional inventories and atmospheric observations (Wang et al., 2019). Analysis of deposition data by Haskins et al. (2020) finds that anthropogenic chlorine emissions have decreased by 95 % in the US since 1998, further indicating that the McCulloch et al. (1999) inventory is outdated. Our previous model comparisons to aerosol Cl− observations indicate that anthropogenic chlorine sources are important in China (Wang et al., 2020) but not in the US, where the observed Cl− concentrations can be attributed to long-range transport of SSA plus some dust influence in the southwest (Wang et al., 2019). Zhai et al. (2021), who include anthropogenic HCl emissions using the observed HCl:SO2 ratio (Lee et al., 2018), also find that anthropogenic sources of chlorine are very small over North America and western Europe. Because of this neglect of anthropogenic sources, our model results may underestimate chlorine concentrations in continental source regions.
The main global source of tropospheric Cly is mobilization of Cl− from SSA. A total of 50 TgCl-a-1 (2.4 % of SSA emissions) is mobilized to Cly in the model by acid displacement and other heterogeneous reactions. This number is smaller than our previous estimate in Wang et al. (2019) (64 TgCl-a-1), mainly due to slower ClNO2 generation from the N2O5 + Cl− reaction (Sect. 2.3). Organochlorines provide a tropospheric source of 3.3 TgCl-a-1 as Cl atoms from photolysis and oxidation. Transport from the stratosphere adds 0.14 Tg Cl a−1 to tropospheric Cly. The source of Iy is estimated to be 2.7 Tg I a−1, mostly from the inorganic iodine (HOI, I2) formed from the ocean surface reaction of O3 with iodide (I−), based on Carpenter et al. (2013) and MacDonald et al. (2014) and as described by Sherwen et al. (2016b).
In GEOS-Chem versions before 12.9, SSA debromination was not included despite being known to be an important source for Bry (Sander et al., 2003). This is because SSA debromination generated excessive BrO concentrations in comparison to observations, which then drove excessive ozone depletion (Schmidt et al., 2016; Zhu et al., 2019). Revision of HOBr reactive uptake as a source of bromine radicals effectively corrects this problem (Sect. 2.2), allowing us to include mechanistically the SSA debromination source. This provides the main global source of tropospheric Bry (20 Tg Br a−1), mostly through the HOBr, HOCl, and HOI + Br− heterogeneous reactions. Bromocarbon gases contribute only 0.54 Gg Br a−1 to Bry but still dominate the Bry source in the free troposphere.