Modeling the observed tropospheric BrO background: Importance of multiphase chemistry and implications for ozone, OH, and mercury

Schmidt, Johan A.; Jacob, Daniel J.; Horowitz, Hannah Marie; Hu, Li; Sherwen, Tomás; Evans, M. J.; Liang, Qing; Suleiman, R. M.; Oram, D. E.; Breton, Michael Le; Percival, Carl J.; Wang, Siyuan; Dix, B.; Volkamer, Rainer

doi:10.1002/2015jd024229

Cited by 148 publications

(364 citation statements)

References 80 publications

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“…10, BRCHEM). The underestimation of the absolute OM concentrations by all models besides ECHMERIT is in line with the findings of Schmidt et al (2017), who find that current models strongly underestimate bromine concentrations in this area.…”

Section: North Americasupporting

confidence: 90%

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

et al. 2017

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Abstract. Atmospheric chemistry and transport of mercury play a key role in the global mercury cycle. However, there are still considerable knowledge gaps concerning the fate of mercury in the atmosphere. This is the second part of a model intercomparison study investigating the impact of atmospheric chemistry and emissions on mercury in the atmosphere. While the first study focused on ground-based observations of mercury concentration and deposition, here we investigate the vertical and interhemispheric distribution and speciation of mercury from the planetary boundary layer to the lower stratosphere. So far, there have been few model studies investigating the vertical distribution of mercury, mostly focusing on single aircraft campaigns. Here, we present a first comprehensive analysis based on various aircraft observations in Europe, North America, and on intercontinental flights.The investigated models proved to be able to reproduce the distribution of total and elemental mercury concentrations in the troposphere including interhemispheric trends. One key aspect of the study is the investigation of mercury oxidation in the troposphere. We found that different chemistry schemes were better at reproducing observed oxidized mercury patterns depending on altitude. High concentrations of oxidized mercury in the upper troposphere could be reproduced with oxidation by bromine while elevated concentrations in the lower troposphere were better reproduced by OH and ozone chemistry. However, the results were not always conclusive as the physical and chemical parameterizations in the chemistry transport models also proved to have a substantial impact on model results.

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Section: North Americasupporting

confidence: 90%

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

et al. 2017

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“…The standard v9-02 model uses Br concentration fields from the GEOS-Chem tropospheric bromine simulation by Parrella et al (2012) and features other model updates relative to Holmes et al (2010), including the gas-particle Hg II equilibrium partitioning described above, a corrected washout algorithm , and improvements to the 2-D surface ocean model (Soerensen et al, 2010). In this work we update the Br concentration fields to a more recent version of GEOS-Chem (Schmidt et al, 2016), as discussed further in Sect. 3.2.…”

Section: General Descriptionmentioning

confidence: 99%

“…Here we further expand and update the chemistry using the mechanism described in Table 1. Br and Cl radical concentrations are taken from a new GEOS-Chem simulation of tropospheric oxidant-aerosol chemistry by Schmidt et al (2016). Unlike Parrella et al (2012), this simulation does not include a bromine radical source from sea salt debromination as recent evidence suggests that BrO concentrations in the marine boundary layer (MBL) are generally sub-ppt (Gómez Martín et al, 2013;Wang et al, 2015).…”

Section: Atmospheric Chemistrymentioning

confidence: 99%

“…The Hg chemical mechanism in the current standard version of GEOS-Chem (www.geos-chem.org) is based on . Our updated mechanism includes more recent information and is applied here with an improved GEOS-Chem simulation of halogen chemistry (Schmidt et al, 2016). As part of this work, we also introduce a new coupling of GEOSChem to a global 3-D ocean model (Y. X. to better interpret observed seasonal variations of atmospheric Hg in the context of both atmospheric chemistry and oceanic drivers of air-sea exchange .…”

Section: Introductionmentioning

confidence: 99%

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A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

et al. 2017

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Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg 0 . Oxidation to water-soluble Hg II plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg 0 / Hg II redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphereocean Hg 0 / Hg II cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg 0 oxidant and that second-stage HgBr oxidation is mainly by the NO 2 and HO 2 radicals. The resulting chemical lifetime of tropospheric Hg 0 against oxidation is 2.7 months, shorter than in previous models. Fast Hg II atmospheric reduction must occur in order to match the ∼ 6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM ≡ Hg 0 + Hg II (g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase Hg II -organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere.The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM-ozone relationship indicative of fast Hg 0 oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0-30 %). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO 2 and HO 2 radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast Hg II reduction in the presence of high organic aerosol concentrations. We find that 80 % of Hg II deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for Hg II reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO 2 and NO 2 for second-stage HgBr oxidation.

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“…Halogens from inorganic (sea-salt) and organic (photodissociation of biogenic compounds) sources, particularly of marine origin, are thought to be important for the tropospheric ozone budget, as they take part in efficient ozone loss catalytic cycles (Read et al, 2008;Saiz-Lopez and von Glasow, 2012;Wang et al, 2015). Studies with single models have shown that this chemistry may have a notable impact on the ozone budget and associated radiative forcing in the troposphere Sarwar et al, 2015;Schmidt et a., 2016;Sherwen et al, 2016aSherwen et al, , 2016b, and the inclusion of tropospheric bromine chemistry has been suggested as a way of partially accounting for the low preindustrial ozone levels suggested by the measurements (Parrella et al, 2012) (although these measurements are uncertain; see TOAR-Observations).…”

Section: Chemistrymentioning

confidence: 99%

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

Young

Naik

Fiore

et al. 2018

Elementa: Science of the Anthropocene

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The goal of the Tropospheric Ozone Assessment Report (TOAR) is to provide the research community with an up-to-date scientific assessment of tropospheric ozone, from the surface to the tropopause. While a suite of observations provides significant information on the spatial and temporal distribution of tropospheric ozone, observational gaps make it necessary to use global atmospheric chemistry models to synthesize our understanding of the processes and variables that control tropospheric ozone abundance and its variability. Models facilitate the interpretation of the observations and allow us to make projections of future tropospheric ozone and trace gas distributions for different anthropogenic or natural perturbations. This paper assesses the skill of current-generation global atmospheric chemistry models in simulating the observed present-day tropospheric ozone distribution, variability, and trends. Drawing upon the results of recent international multi-model intercomparisons and using a range of model evaluation techniques, we demonstrate that global chemistry models are broadly skillful in capturing the spatio-temporal variations of tropospheric ozone over the seasonal cycle, for extreme pollution episodes, and changes over interannual to decadal periods. However, models are consistently biased high in the northern hemisphere and biased low in the southern hemisphere, throughout the depth of the troposphere, and are unable to replicate particular metrics that define the longer term trends in tropospheric ozone as derived from some background sites. When the models compare unfavorably against observations, we discuss the potential causes of model biases and propose directions for future developments, including improved evaluations that may be able to better diagnose the root cause of the model-observation disparity. Overall, model results should be approached critically, including determining whether the model performance is acceptable for the problem being addressed, whether biases can be tolerated or corrected, whether the model is appropriately constituted, and whether there is a way to satisfactorily quantify the uncertainty.

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Modeling the observed tropospheric BrO background: Importance of multiphase chemistry and implications for ozone, OH, and mercury

Cited by 148 publications

References 80 publications

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

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