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Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone (O3), the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide (CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new reduced Caltech isoprene mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbon and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one-third from each of the following: isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a mini Caltech isoprene mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms. 

Abstract. Bromine radicals influence global tropospheric chemistryby depleting ozone and by oxidizing elemental mercury and reduced sulfurspecies. Observations typically indicate a 50 % depletion of sea saltaerosol (SSA) bromide relative to seawater composition, implying that SSAdebromination could be the dominant global source of tropospheric bromine.However, it has been difficult to reconcile this large source with therelatively low bromine monoxide (BrO) mixing ratios observed in the marineboundary layer (MBL). Here we present a new mechanistic description of SSAdebromination in the GEOS-Chem global atmospheric chemistry model with adetailed representation of halogen (Cl, Br, and I) chemistry. We show thatobserved levels of SSA debromination can be reproduced in a mannerconsistent with observed BrO mixing ratios. Bromine radical sinks from theHOBr + S(IV) heterogeneous reactions and from ocean emission ofacetaldehyde are critical in moderating tropospheric BrO levels. Theresulting HBr is rapidly taken up by SSA and also deposited. Observations of SSA debromination at southern midlatitudes in summer suggest that modeluptake of HBr by SSA may be too fast. The model provides a successfulsimulation of free-tropospheric BrO in the tropics and midlatitudes in summer,where the bromine radical sink from the HOBr + S(IV) reactions iscompensated for by more efficient HOBr-driven recycling in clouds compared toprevious GEOS-Chem versions. Simulated BrO in the MBL is generally muchhigher in winter than in summer due to a combination of greater SSA emissionand slower conversion of bromine radicals to HBr. An outstanding issue inthe model is the overestimate of free-tropospheric BrO in extratropicalwinter–spring, possibly reflecting an overestimate of the HOBr∕HBr ratiounder these conditions where the dominant HOBr source is hydrolysis ofBrNO₃. 

We present a comprehensive simulation of tropospheric chlorine within the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmospheric chemistry. The simulation includes explicit accounting of chloride mobilization from sea salt aerosol by acid displacement of HCl and by other heterogeneous processes. Additional small sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are also included. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl, HOCl, ClNO3, ClNO2, and minor species, is produced by the HCl+OH reaction and by heterogeneous conversion of sea salt aerosol chloride to BrCl, ClNO2, Cl2, and ICl. The model successfully simulates the observed mixing ratios of HCl in marine air (highest at northern midlatitudes) and the associated HNO3 decrease from acid displacement. It captures the high ClNO2 mixing ratios observed in continental surface air at night and attributes the chlorine to HCl volatilized from sea salt aerosol and transported inland following uptake by fine aerosol. The model successfully simulates the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be largely explained by transport inland of the marine source. It does not reproduce the boundary layer Cl2 mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the daytime, low at night); the model is too high at night, which could be due to uncertainty in the rate of the ClNO2+Cl− reaction, but we have no explanation for the high observed Cl2 in daytime. The global mean tropospheric concentration of Cl atoms in the model is 620 cm−3 and contributes 1.0 % of the global oxidation of methane, 20 % of ethane, 14 % of propane, and 4 % of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85 %, mainly through the HOBr+Cl− reaction, and decreases global burdens of tropospheric ozone by 7 % and OH by 3 % through the associated bromine radical chemistry. ClNO2 chemistry drives increases in ozone of up to 8 ppb over polluted continents in winter. 

Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg⁰. Oxidation to water-soluble Hg^II plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg⁰∕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 atmosphere–ocean Hg⁰∕Hg^II cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg⁰ oxidant and that second-stage HgBr oxidation is mainly by the NO₂ and HO₂ radicals. The resulting chemical lifetime of tropospheric Hg⁰ 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 ≡ Hg⁰+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 Hg⁰ 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 NO₂ and HO₂ 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 HO₂ and NO₂ for second-stage HgBr oxidation. 

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.