Abstract. We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transportmodel and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneouschemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model toinclude mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixingratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison tosurface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytimemeasurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very largemissing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a globalmean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global meantropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane,8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %,NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozonesimulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere.
more »
« less
Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem
We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28 × 106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼ 2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼ 15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde.
more »
« less
- Award ID(s):
- 1643217
- PAR ID:
- 10025431
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 16
- Issue:
- 18
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 12239 to 12271
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Reactive chlorine and bromine species emitted from snow and aerosols can significantly alter the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it difficult for most models to include descriptions of halogen snow emissions and to understand the impact on atmospheric chemistry. We investigate the influence of Arctic halogen emissions from snow on boundary layer oxidation processes using a one‐dimensional atmospheric chemistry and transport model (PACT‐1D). To understand the combined impact of snow emissions and boundary layer dynamics on atmospheric chemistry, we model Cl2and Br2primary emissions from snow and include heterogeneous recycling of halogens on both snow and aerosols. We focus on a 2‐day case study from the 2009 Ocean‐Atmosphere‐Sea Ice‐Snowpack campaign at Utqiaġvik, Alaska. The model reproduces both the diurnal cycle and high quantity of Cl2observed, along with the measured concentrations of Br2, BrO, and HOBr. Due to the combined effects of emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is typically confined to the lowest 15 m of the atmosphere, while bromine can impact chemistry up to and above the surface inversion height. Upon including halogen emissions and recycling, the concentration of HO
x (HOx = OH + HO2) at the surface increases by as much as a factor of 30 at mid‐day. The change in HOx due to halogen chemistry, as well as chlorine atoms derived from snow emissions, significantly reduce volatile organic compound lifetimes within a shallow layer near the surface. -
The role of hydroxyl radicals (OH) as a daytime oxidant is well established on a global scale. In specific source regions, such as the marine boundary layer and polluted coastal cities, other daytime oxidants, such as chlorine atoms (Cl) and even bromine atoms (Br), may compete with OH for the oxidation of volatile organic compounds (VOCs) and/or enhance the overall oxidation capacity of the atmosphere. However, the number of studies investigating halogen-initiated secondary organic aerosol (SOA) formation is extremely limited, resulting in large uncertainties in these oxidative aging processes. Here, we characterized the chemical composition and yield of laboratory SOA generated in an oxidation flow reactor (OFR) from the OH and Cl oxidation of n -dodecane ( n -C 12 ) and toluene, and the OH, Cl, and Br oxidation of isoprene and α-pinene. In the OFR, precursors were oxidized using integrated OH, Cl, and Br exposures ranging from 3.1 × 10 10 to 2.3 × 10 12 , 6.1 × 10 9 to 1.3× 10 12 and 3.2 × 10 10 to 9.7 × 10 12 molecules cm −3 s −1 , respectively. Like OH, Cl facilitated multistep SOA oxidative aging over the range of OFR conditions that were studied. In contrast, the extent of Br-initiated SOA oxidative aging was limited. SOA elemental ratios and mass yields obtained in the OFR studies were comparable to those obtained from OH and Cl oxidation of the same precursors in environmental chamber studies. Overall, our results suggest that alkane, aromatic, and terpenoid SOA precursors are characterized by distinct OH- and halogen-initiated SOA yields, and that while Cl may enhance the SOA formation potential in regions influenced by biogenic and anthropogenic emissions, Br may have the opposite effect.more » « less
-
more » « less
ABSTRACT Rationale These experiments were conducted to measure the diversity of organo‐U (IV) and U (III) ions created using multiple‐stage tandem MS and collision‐induced dissociation of halogen‐substituted UVIO2‐phenide complexes [UO2(C6H3FX)]+, X = Cl, Br, or I.
Methods Samples of UO2(O2C‐C6H3FX)2were prepared by digesting UO3with appropriate halogen‐substituted carboxylic acids in deionized water. Solutions for ESI were created by diluting the digested sample in 50:50 H2O/CH3OH. Precursor ions for multiple‐stage tandem MS were generated by electrospray ionization (ESI). Multiple‐stage collision‐induced dissociation (He collision gas) in a linear quadrupole ion trap mass spectrometer was used to prepare species such as [UIVFX(C≡CH)]+and UIIIF(C≡CH)]+for subsequent study of ion‐molecule reactions with adventitious neutrals in the ion trap.
Results Multiple‐stage CID of the [UO2(C6H3FX)]+, X = Cl, Br, or I, complexes caused removal of both “yl” oxo ligands from of the UO22+moiety to create ions such as [UIVFX(C≡CH)]+and [UIIIFX]+. For [UIVFXC≡CH]+and [UIIIFC≡CH]+products, hydrolysis to generate [UIVFX (OH)]+and [UIIIF (OH)]+, with concomitant loss of HC≡CH, was observed. CID of [UO2(C6H3FBr)]+and [UO2(C6H3FI)]+caused reductive elimination of the respective halogen radicals to generate interesting organo‐U (III) species such as [UIIIF(C≡CH)]+and [UIIIC2]+.
Conclusions The use of “preparative” tandem mass spectrometry and a suite of halogen substituted benzoic acid ligands can be used to remove both oxo ligands of UO22+and generate a group of homologous organo‐U (IV) and organo‐U (III) ions for studies of intrinsic reactivity.
-
more » « less
Abstract Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are major carriers of atmospheric bromine and chlorine, respectively, which can catalyze stratospheric ozone depletion. However, in our current understanding, there are missing sources associated with these two species. Here we investigate the effect of copper(II) on CH3Br and CH3Cl production from soil, seawater and model organic compounds: catechol (benzene-1,2-diol) and guaiacol (2-methoxyphenol). We show that copper sulfate (CuSO4) enhances CH3Br and CH3Cl production from soil and seawater, and it may be further amplified in conjunction with hydrogen peroxide (H2O2) or solar radiation. This represents an abiotic production pathway of CH3Br and CH3Cl perturbed by anthropogenic application of copper(II)-based chemicals. Hence, we suggest that the widespread application of copper(II) pesticides in agriculture and the discharge of anthropogenic copper(II) to the oceans may account for part of the missing sources of CH3Br and CH3Cl, and thereby contribute to stratospheric halogen load.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-16-12239-2016
