Main Atmospheric Oxidant Research Articles

Gas-Phase Kinetic Investigation of the OH-Initiated Oxidation of a Series of Methyl-Butenols under Simulated Atmospheric Conditions.

Five biogenic unsaturated alcohols have been investigated under simulated atmospheric conditions regarding their gas-phase OH reactivity. The gas-phase rate coefficients of OH radicals with 2-methyl-3-buten-2-ol (k1), 3-methyl-2-buten-1-ol (k2), 3-methyl-3-buten-1-ol (k3), 2-methyl-3-buten-1-ol (k4), and 3-methyl-3-buten-2-ol (k5) at 298 ± 2 K and 1000 ± 10 mbar total pressure of synthetic air were determined under low- and high-NOx conditions using the relative kinetic technique. The present work provides for the first time the rate coefficients of gas-phase reactions of hydroxyl radicals with 2-methyl-3-buten-1-ol and 3-methyl-3-buten-2-ol. The following rate constants were measured (in 10-11 cm3 molecule-1 s-1): k1 = 6.32 ± 0.49, k2 = 14.55 ± 0.93, k3 = 10.04 ± 0.78, k4 = 5.31 ± 0.37, and k5 = 11.71 ± 1.29. No significant differences in the measured rate coefficients were obtained when either 365 nm photolysis of CH3ONO in the presence of NO or 254 nm photolysis of H2O2 was used as a source of OH radicals. Reactivity toward other classes of related compounds such as alkenes and saturated alcohols is discussed. A comparison of the structure-activity relationship (SAR) estimates derived from the available accepted methodologies with experimental data available for unsaturated alcohols is provided. Atmospheric lifetimes for the investigated series of alkenols with respect to the main atmospheric oxidants are given and discussed.

Volatility of aerosol particles from NO3 oxidation of various biogenic organic precursors

Abstract. Secondary organic aerosol (SOA) is formed through the oxidation of volatile organic compounds (VOCs), which can be of both natural and anthropogenic origin. While the hydroxyl radical (OH) and ozone (O3) are the main atmospheric oxidants during the day, the nitrate radical (NO3) becomes more important during the nighttime. Yet, atmospheric nitrate chemistry has received less attention compared to OH and O3. The Nitrate Aerosol and Volatility Experiment (NArVE) aimed to study the NO3-induced SOA formation and evolution from three biogenic VOCs (BVOCs), namely isoprene, α-pinene, and β-caryophyllene. The volatility of aerosol particles was studied using isothermal evaporation chambers, temperature-dependent evaporation in a volatility tandem differential mobility analyzer (VTDMA), and thermal desorption in a filter inlet for gases and aerosols coupled to a chemical ionization mass spectrometer (FIGAERO-CIMS). Data from these three setups present a cohesive picture of the volatility of the SOA formed in the dark from the three biogenic precursors. Under our experimental conditions, the SOA formed from NO3 + α-pinene was generally more volatile than SOA from α-pinene ozonolysis, while the NO3 oxidation of isoprene produced similar although slightly less volatile SOA than α-pinene under our experimental conditions. β-Caryophyllene reactions with NO3 resulted in the least volatile species. Four different parameterizations for estimating the saturation vapor pressure of the oxidation products were tested for reproducing the observed evaporation in a kinetic modeling framework. Our results show that the SOA from nitrate oxidation of α-pinene or isoprene is dominated by low-volatility organic compounds (LVOCs) and semi-volatile organic compounds (SVOCs), while the corresponding SOA from β-caryophyllene consists primarily of extremely low-volatility organic compounds (ELVOCs) and LVOCs. The parameterizations yielded variable results in terms of reproducing the observed evaporation, and generally the comparisons pointed to a need for re-evaluating the treatment of the nitrate group in such parameterizations. Strategies for improving the predictive power of the volatility parameterizations, particularly in relation to the contribution from the nitrate group, are discussed.

Atmospheric reactivity of biogenic volatile organic compounds in a maritime pine forest during the LANDEX episode 1 field campaign

Trace gas measurements were performed during the LANDEX (the LANDes EXperiment) Episode 1 field campaign in the summer 2017, in one of the largest European maritime pine forests (> 95% Pinus pinaster) located in southwestern France. Efforts have been focused on obtaining a good speciation of 20 major biogenic volatile organic compounds (BVOCs, including pinenes, carenes, terpinenes, linalool, camphene, etc.). This was made possible by the development of a new and specific chromatographic method. In order to assess the role of BVOCs in the local gas phase chemistry budget, their reactivity with the main atmospheric oxidants (hydroxyl radicals (OH), ozone (O3) and nitrate radicals (NO3)) and the corresponding consumption rates were determined. When considering the OH reactivity with BVOCs, isoprene and linalool accounted for 10–47% of the OH depletion during daytime, and monoterpenes for 50–65%, whereas monoterpenes were the main contributors during the night (70–85%). Sesquiterpenes and monoterpenes were the main contributors to the ozone reactivity, especially β-caryophyllene (30–70%), with a maximum contribution during nighttime. Nighttime nitrate reactivity was predominantly due to monoterpenes (i.e. 90–95%). Five specific groups have been proposed to classify the 19 BVOCs measured in the forest, according to their reactivity with atmospheric oxidants and their concentrations. The total amount of BVOCs consumed under and above the forest canopy was evaluated for 7 BVOCs (i.e. isoprene, α-pinene, β-pinene, myrcene, limonene + cis-ocimene and Δ3-carene). The reactivity of atmospheric oxidants and BVOCs at a local level are discussed in order to highlight the compounds (BVOCs, other VOCs), the atmospheric oxidants and the main associated reactive processes observed under the canopy of a maritime pine forest.

Atmospheric fate of a series of saturated alcohols: kinetic and mechanistic study

Abstract. The atmospheric fate of a series of saturated alcohols (SAs) was evaluated through kinetic and reaction product studies with the main atmospheric oxidants. These SAs are alcohols that could be used as fuel additives. Rate coefficients (in cm3 molecule−1 s−1) measured at ∼298 K and atmospheric pressure (720±20 Torr) were as follows: k1 ((E)-4-methylcyclohexanol + Cl) = (3.70±0.16) ×10-10, k2 ((E)-4-methylcyclohexanol + OH) = (1.87±0.14) ×10-11, k3 ((E)-4-methylcyclohexanol + NO3) = (2.69±0.37) ×10-15, k4 (3,3-dimethyl-1-butanol + Cl) = (2.69±0.16) ×10-10, k5 (3,3-dimethyl-1-butanol + OH) = (5.33±0.16) ×10-12, k6 (3,3-dimethyl-2-butanol + Cl) = (1.21±0.07) ×10-10, and k7 (3,3-dimethyl-2-butanol + OH) = (10.50±0.25) ×10-12. The main products detected in the reaction of SAs with Cl atoms (in the absence/presence of NOx), OH radicals, and NO3 radicals were (E)-4-methylcyclohexanone for the reactions of (E)-4-methylcyclohexanol, 3,3-dimethylbutanal for the reactions of 3,3-dimethyl-1-butanol, and 3,3-dimethyl-2-butanone for the reactions of 3,3-dimethyl-2-butanol. Other products such as formaldehyde, 2,2-dimethylpropanal, and acetone have also been identified in the reactions of Cl atoms and OH radicals with 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol. In addition, the molar yields of the reaction products were estimated. The products detected indicate a hydrogen atom abstraction mechanism at different sites on the carbon chain of alcohol in the case of Cl reactions and a predominant site in the case of OH and NO3 reactions, confirming the predictions of structure–activity relationship (SAR) methods. Tropospheric lifetimes (τ) of these SAs have been calculated using the experimental rate coefficients. Lifetimes are in the range of 0.6–2 d for OH reactions, 7–13 d for NO3 radical reactions, and 1–3 months for Cl atoms. In coastal areas, the lifetime due to the reaction with Cl decreases to hours. The calculated global tropospheric lifetimes, and the polyfunctional compounds detected as reaction products in this work, imply that SAs could contribute to the formation of ozone and nitrated compounds at local, regional, and even global scales. Therefore, the use of saturated alcohols as additives in diesel blends should be considered with caution.

Constraints and biases in a tropospheric two-box model of OH

Abstract. The hydroxyl radical (OH) is the main atmospheric oxidant and the primary sink of the greenhouse gas CH4. In an attempt to constrain atmospheric levels of OH, two recent studies combined a tropospheric two-box model with hemispheric-mean observations of methyl chloroform (MCF) and CH4. These studies reached different conclusions concerning the most likely explanation of the renewed CH4 growth rate, which reflects the uncertain and underdetermined nature of the problem. Here, we investigated how the use of a tropospheric two-box model can affect the derived constraints on OH due to simplifying assumptions inherent to a two-box model. To this end, we derived species- and time-dependent quantities from a full 3-D transport model to drive two-box model simulations. Furthermore, we quantified differences between the 3-D simulated tropospheric burden and the burden seen by the surface measurement network of the National Oceanic and Atmospheric Administration (NOAA). Compared to commonly used parameters in two-box models, we found significant deviations in the magnitude and time-dependence of the interhemispheric exchange rate, exposure to OH, and stratospheric loss rate. For MCF these deviations can be large due to changes in the balance of its sources and sinks over time. We also found that changes in the yearly averaged tropospheric burden of CH4 and MCF can be obtained within 0.96 ppb yr−1 and 0.14 % yr−1 by the NOAA surface network, but that substantial systematic biases exist in the interhemispheric mixing ratio gradients that are input to two-box model inversions. To investigate the impact of the identified biases on constraints on OH, we accounted for these biases in a two-box model inversion of MCF and CH4. We found that the sensitivity of interannual OH anomalies to the biases is modest (1 %–2 %), relative to the uncertainties on derived OH (3 %–4 %). However, in an inversion where we implemented all four bias corrections simultaneously, we found a shift to a positive trend in OH concentrations over the 1994–2015 period, compared to the standard inversion. Moreover, the absolute magnitude of derived global mean OH, and by extent, that of global CH4 emissions, was affected much more strongly by the bias corrections than their anomalies (∼10 %). Through our analysis, we identified and quantified limitations in the two-box model approach as well as an opportunity for full 3-D simulations to address these limitations. However, we also found that this derivation is an extensive and species-dependent exercise and that the biases were not always entirely resolvable. In future attempts to improve constraints on the atmospheric oxidative capacity through the use of simple models, a crucial first step is to consider and account for biases similar to those we have identified for the two-box model.

Rate coefficients for the reactions of OH radical and ozone with a series of unsaturated esters

Unsaturated esters are emitted into the atmosphere from biogenic and anthropogenic sources. Their atmospheric degradation through reactions with the main atmospheric oxidants (OH, O3, NO3, Cl) contributes to the formation of important secondary pollutants. Kinetic and mechanistic parameters for these reactions are highly required in order to estimate the atmospheric lifetimes and hence the impacts of these important classes of VOCs. In this study, the gas-phase rate coefficients of the reaction of OH radical and O3 with a series of unsaturated esters are reported. The esters investigated include methyl methacrylate (MMA), ethyl methacrylate (EMA), n-propyl methacrylate (nPMA), n-butyl methacrylate (nBMA), iso-propyl methacrylate (iPMA), iso-butyl methacrylate (iBMA) and fully deuterated methyl methacrylate (MMA-D8). The measurement combined the pulsed laser photolysis-laser induced fluorescence for the OH studies in the temperature range 257–376K and a 7300 L atmospheric simulation chamber for the O3 reactions at 291 ± 2K. This work provides the first temperature-dependence studies of OH reaction with EMA, nPMA, iPMA and iBMA and of O3 reaction with nPMA, iPMA and iBMA at room temperature. The obtained data are discussed in terms of the structure reactivity relationships and their atmospheric consequences by estimating the atmospheric lifetimes for the studied esters with respect to O3, OH radical and other oxidant in the troposphere.

Nitrogen oxides in the global upper troposphere: interpreting cloud-sliced NO<sub>2</sub> observations from the OMI satellite instrument

Abstract. Nitrogen oxides (NOx≡NO+NO2) in the upper troposphere (UT) have a large impact on global tropospheric ozone and OH (the main atmospheric oxidant). New cloud-sliced observations of UT NO2 at 450–280 hPa (∼6–9 km) from the Ozone Monitoring Instrument (OMI) produced by NASA and the Royal Netherlands Meteorological Institute (KNMI) provide global coverage to test our understanding of the factors controlling UT NOx. We find that these products offer useful information when averaged over coarse scales (20∘×32∘, seasonal), and that the NASA product is more consistent with aircraft observations of UT NO2. Correlation with Lightning Imaging Sensor (LIS) and Optical Transient Detector (OTD) satellite observations of lightning flash frequencies suggests that lightning is the dominant source of NOx to the upper troposphere except for extratropical latitudes in winter. The NO2 background in the absence of lightning is 10–20 pptv. We infer a global mean NOx yield of 280±80 moles per lightning flash, with no significant difference between the tropics and midlatitudes, and a global lightning NOx source of 5.9±1.7 Tg N a−1. There is indication that the NOx yield per flash increases with lightning flash footprint and with flash energy.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism.

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O3, OH, Cl, and NO3) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm3 molecule-1 s-1) are (1.8 ± 0.3) × 10-18, (3.1 ± 0.7) × 10-11, (2.5 ± 0.5) × 10-10, and (1.1 ± 0.4) × 10-14, for reactions with O3, OH, Cl, and NO3, respectively. While results for reactions with O3, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO3 is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τO3 = 9 days, τOH = 5 h, τCl = 5 days, τNO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.

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

Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg0 ∕ HgII 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 ∕ HgII 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 HgII 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 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII–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 HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of HgII deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for HgII reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO2 and NO2 for second-stage HgBr oxidation.

Estimation of gas-phase rate coefficients for the reactions of a series of α,β-unsaturated esters with OH, NO3, O3 and Cl

Rate coefficients for reactions of a series of α,β-unsaturated esters with OH and NO3 radicals, O3 and Cl atoms have been estimated using different methods: i.e., physico-chemical correlations and Structure Activity Relationships (SARs). In this study new correlations to estimate rate coefficients have been proposed (only for α,β-unsaturated esters): logkNO3 = (13.9 ± 2.8) + (2.75 ± 0.26) × log (kOH); log (kOH) = (4.93 ± 1.14) + (1.60 ± 0.12) × log (kCl); ln(kOH/cm3 molecule−1 s−1) = (1.71 ± 0.21) × EHOMO/eV − (6.25 ± 2.24); ln(kNO3/cm3 molecule−1 s−1) =(5.94 ± 0.35) × EHOMO/eV + (27.87 ± 3.65); ln(kO3/cm3 molecule−1 s−1) = (2.20 ± 0.63) × EHOMO/eV − (16.75 ± 6.65) and ln(kCl/cm3 molecule−1 s−1) = (0.21 ± 0.23) × (EHOMO)/eV − (19.8 ± 2.4). The group reactivity factors necessary to estimate the rate coefficients by SAR methods have also been calculated for a series of α,β-unsaturated esters for the first time: f(–C(O)O(CH2)3CH3), f(–C(O)O(CH2)5CH3), f(–C(O)OCH2CH(CH2CH3)CH2CH2CH2CH3) for reactions with the NO3 radical, OH radical and O3. Furthermore, f(–C(O)OCH3), f(–C(O)OCH2CH3), f(–C(O)O(CH2)3CH3), f(–C(O)OCH2CH2CH2CH2CH2CH3), f(–C(O)OCH2CH(CH2CH3)CH2CH2CH2CH3) for reactions with chlorine atoms have been estimated.The results show that for NO3 and OH radicals these methods are generally satisfactory. In the case of Cl atoms, all methods used in this study reproduce the rate coefficient within a factor of 1.5. In general the best method for estimating the rate coefficient of α,β-unsaturated esters with the main atmospheric oxidants is the correlation of ln k versus EHOMO.

Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols

Abstract. The hydroperoxyl radical (HO2) is a major precursor of OH and tropospheric ozone. OH is the main atmospheric oxidant, while tropospheric ozone is an important surface pollutant and greenhouse gas. Standard gas-phase models for atmospheric chemistry tend to overestimate observed HO2 concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO2 uptake by aerosol involves conversion to H2O2, but this is of limited efficacy as an HO2 sink because H2O2 can photolyze to regenerate OH and from there HO2. Joint atmospheric observations of HO2 and H2O2 suggest that HO2 uptake by aerosols may in fact not produce H2O2. Here we propose a catalytic mechanism involving coupling of the transition metal ions Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO2 to H2O in aqueous aerosols. The implied HO2 uptake and conversion to H2O significantly affects global model predictions of tropospheric OH, ozone, carbon monoxide (CO) and other species, improving comparisons to observations in the GEOS-Chem model. It represents a previously unrecognized positive radiative forcing of aerosols through the effects on the chemical budgets of major greenhouse gases including methane and hydrofluorocarbons (HFCs).

Transference of Atmospheric Hydroxyl Radical to the Ocean Surface Induces High Phytoplankton Cell Death

Hydroxyl radical (OH), the main atmospheric oxidant at the global scale, is believed to play an important role in the dynamics of dissolved organic matter in the sea. Herein, we provide evidence, on the basis of seven experiments performed in contrasting ecosystems (subtropical NE Atlantic and Antarctic waters), of high fluxes of atmospheric OH into the surface oceanic layer, particularly during afternoon events. The experiments demonstrated a tight negative relationship between phytoplankton abundance and the concentration of OH in surface seawater, with acute cell death during afternoon atmospheric OH influx events. The effect of OH radical was higher for picophytoplankton organisms, with Prochlorococcus showing the highest decay rate and the shortest half-life among the phytoplankton populations habiting the ocean surface layers. Our results provide evidence for a high toxicity of atmospheric-derived OH radical to phytoplankton of the surface layer of the ocean.

The atmospheric chemistry of trace gases and particulate matter emitted by different land uses in Borneo

We report measurements of atmospheric composition over a tropical rainforest and over a nearby oil palm plantation in Sabah, Borneo. The primary vegetation in each of the two landscapes emits very different amounts and kinds of volatile organic compounds (VOCs), resulting in distinctive VOC fingerprints in the atmospheric boundary layer for both landscapes. VOCs over the Borneo rainforest are dominated by isoprene and its oxidation products, with a significant additional contribution from monoterpenes. Rather than consuming the main atmospheric oxidant, OH, these high concentrations of VOCs appear to maintain OH, as has been observed previously over Amazonia. The boundary-layer characteristics and mixing ratios of VOCs observed over the Borneo rainforest are different to those measured previously over Amazonia. Compared with the Bornean rainforest, air over the oil palm plantation contains much more isoprene, monoterpenes are relatively less important, and the flower scent, estragole, is prominent. Concentrations of nitrogen oxides are greater above the agro-industrial oil palm landscape than over the rainforest, and this leads to changes in some secondary pollutant mixing ratios (but not, currently, differences in ozone). Secondary organic aerosol over both landscapes shows a significant contribution from isoprene. Primary biological aerosol dominates the super-micrometre aerosol over the rainforest and is likely to be sensitive to land-use change, since the fungal source of the bioaerosol is closely linked to above-ground biodiversity.

Atmospheric degradation of 3-methylfuran: kinetic and products study

Abstract. A study of the kinetics and products obtained from the reactions of 3-methylfuran with the main atmospheric oxidants has been performed. The rate coefficients for the gas-phase reaction of 3-methylfuran with OH and NO3 radicals have been determined at room temperature and atmospheric pressure (air and N2 as bath gases), using a relative method with different experimental techniques. The rate coefficients obtained for these reactions were (in units cm3 molecule−1 s−1) kOH = (1.13 ± 0.22) × 10−10 and kNO3 = (1.26 ± 0.18) × 10−11. Products from the reaction of 3-methylfuran with OH, NO3 and Cl atoms in the absence and in the presence of NO have also been determined. The main reaction products obtained were chlorinated methylfuranones and hydroxy-methylfuranones in the reaction of 3-methylfuran with Cl atoms, 2-methylbutenedial, 3-methyl-2,5-furanodione and hydroxy-methylfuranones in the reaction of 3-methylfuran with OH and NO3 radicals and also nitrated compounds in the reaction with NO3 radicals. The results indicate that, in all cases, the main reaction path is the addition to the double bond of the aromatic ring followed by ring opening in the case of OH and NO3 radicals. The formation of 3-furaldehyde and hydroxy-methylfuranones (in the reactions of 3-methylfuran with Cl atoms and NO3 radicals) confirmed the H-atom abstraction from the methyl group and from the aromatic ring, respectively. This study represents the first product determination for Cl atoms and NO3 radicals in reactions with 3-methylfuran. The reaction mechanisms and atmospheric implications of the reactions under consideration are also discussed.

Analysis of a potential “solar radiation dose–dimethylsulfide–cloud condensation nuclei” link from globally mapped seasonal correlations

The CLAW postulate states that an increase in solar irradiance or in the heat flux to the ocean can trigger a biogeochemical response to counteract the associated increase in temperature and available sunlight. This natural (negative) feedback mechanism would be based on a multistep response: first, an increase in seawater dimethylsulfide concentrations (DMSw) and therefore its fluxes to the atmosphere (DMSflux); second, an increase in the atmospheric cloud condensation nuclei (CCN) burden as a consequence of DMS oxidation to form biogenic CCN (CCNbio); and third, an increase in cloud albedo due to higher CCN numbers. Monthly global climatological fields of the solar radiation dose in the upper mixed layer (SRD), surface oceanic DMSw, model outputs of hydroxyl radical concentrations (OH), and satellite‐derived CCN numbers (CCNs) are analyzed in order to evaluate the proposed “solar radiation dose‐DMS‐CCN” link from a global point of view. OH is included as the main atmospheric oxidant of the estimated DMSflux to produce CCNbio. Global maps of seasonal correlations between the variables show that the solar radiation dose is highly (positively) correlated with seawater dimethylsulfide over most of the global ocean and that atmospheric DMS oxidation is highly (positively) correlated with CCNs over large regions. These couplings are stronger at high latitudes, whereas the regions with negative or no correlation are located at low latitudes around the equator. However, CCNbio estimates for 15 regions of the global ocean show that DMS oxidation can be an important contributor to the CCNs burden only over pollution‐free regions, while it would have a minor contribution over regions with high loads of continental aerosols. Globally, the mean annual contribution of CCNbio to total CCNs is estimated to be ≈30%. Our results support that an oceanic biogenic mechanism that modulates cloud formation and albedo can indeed occur, although its impact seems rather weak over regions under a strong influence of continental aerosols. Nevertheless, our approach does not fully rule out that the observed correlations are due to an independent seasonal variation of the studied variables; seasonal couplings are necessary but not sufficient conditions to prove the CLAW hypothesis.

What controls CCN seasonality in the Southern Ocean? A statistical analysis based on satellite‐derived chlorophyll and CCN and model‐estimated OH radical and rainfall

A 3‐year time series set (from January 2002 to December 2004) of monthly means of satellite‐derived chlorophyll (CHL) and cloud condensation nuclei (CCN), as well as model outputs of hydroxyl radical (OH), rainfall amount (RAIN), and wind speed (WIND) for the Southern Ocean (SO, 40°S–60°S) is analyzed in order to explain CCN seasonality. Chlorophyll is used as a proxy for oceanic dimethylsulfide (DMS) emissions since both climatological aqueous DMS and atmospheric methanesulfonate (MSA) concentrations are tightly coupled with chlorophyll seasonality over the Southern Ocean. OH is included as the main atmospheric oxidant of DMS to produce CCN, and rainfall amount as the main loss factor for CCN through aerosol scavenging. Wind speed is used as a proxy for sea salt (SS) particles production. The CCN concentration seasonality is characterized by a clear pattern of higher values during austral summer and lower values during austral winter. Linear and multiple regression analyses reveal high significant correlations between CCN and the product of chlorophyll and OH (in phase) and rainfall amount (in antiphase). Also, CCN concentrations are anticorrelated with wind speed, which shows very little variability and a slight wintertime increase, in agreement with the sea salt seasonality reported in the literature. Finally, the fraction of the total aerosol optical depth contributed by small particles (ETA) exhibits a seasonality with a 3.5‐fold increase from austral winter to austral summer. The biogenic contribution to CCN is estimated to vary between 35% (winter) and 80% (summer). Sea salt particles, although contributing an important fraction of the CCN burden, do not play a role in controlling CCN seasonality over the SO. These findings support the central role of biogenic DMS emissions in controlling not only the number but also the variability of CCN over the remote ocean.

A lumped parameter model for the atmosphere‐to‐snow transfer function for hydrogen peroxide

Of the main atmospheric oxidants, only hydrogen peroxide (H2O2) is preserved in polar ice cores. To make use of the peroxide record, however, requires a quantitative understanding of the “transfer function” or relation between atmospheric concentrations of H2O2 and those preserved in the ice core. Snow‐pit H2O2 profiles adjacent to three automatic snow‐depth gages from Summit, Greenland were used to estimate parameters and evaluate the performance of a lumped parameter model to relate concentrations in the atmosphere with those in surface snow and shallow firn. Three of the model parameters define an equilibrium partitioning coefficient between snow and atmosphere as a nonlinear function of depositional temperature. Model parameters yielded a function that closely matched previous laboratory estimates [Conklin et al., 1993]. A fourth parameter reflects the disequilibrium that may be preserved during periods of rapid accumulation. The final model parameter describes the exchange of H2O2 between near‐surface snow and the atmosphere, allowing already buried snow to either take up or release H2O2 as conditions in and above the snowpack change. We simulated snow pit profiles by combining this transfer function model with a finite‐difference model of gas‐phase diffusion in the snowpack. Two applications for this transfer function are (1) to estimate the local seasonal or annual atmospheric H2O2 concentration in the past from snow‐pit and ice‐core records and (2) to invert snow‐pit and ice‐core H2O2 profiles to obtain estimates of the seasonal or annual accumulation time series. In the first case, an independent estimate of snow accumulation is needed, and in the second application, an independent estimate of the annual H2O2 atmospheric cycle is needed.