The oxidative capacity of the troposphere: Coupling of field measurements of OH and a global chemistry transport model

Atmospheric oxidation of furanones by •OH and •Cl radicals: In situ FTIR rate coefficient determinations, SAR and theoretical studies

Gas-phase kinetics of the overall reactions of •OH and •Cl radicals with dihydrofuran-3(2H)-one (oxolan-3-one) and dihydro-2-methyl-3(2H)-furanone (2MTHF-3-one) were studied at 298 K and atmospheric pressure. The rate coefficients were determined using the relative method in a 480 L multipass glass reactor coupled to an FT-IR detection system. The rate coefficients found for oxolan 3-one and 2MTHF-one with •OH radicals (k1 and k2) and with •Cl atoms (k3 and k4) at 298 K and atmospheric pressure (in cm3 molecule-1 s-1) were: k1 = (1.86 ± 0.29) × 10-11, k2 = (2.64 ± 0.47) × 10-11, k3= (1.15 ± 0.28) × 10 -10, and k4 = (1.33 ± 0.32) × 10-10, respectively. Reactivity trends were developed by comparison with other similar structures and Fukui indices employed to determine the reactivity of different sites on the ring. The singularity of the reaction with •OH was assessed by computational studies which showed the formation of several stable hydrogen bonded complexes, explaining the difference with the reaction with the •Cl atom. SAR estimations of the rate coefficients were calculated and compared to the experimental values.

Volatile Organic Compounds Released During the Fast Pyrolysis of Peanut Shells and Environmental Implications

The biomass conversion technologies, especially different types of pyrolysis, have been intensively studied to improve biomass energy transformation suggesting a low impact on the environment. In particular, fast pyrolysis of biomass is considered to be a thermal process in which the starting material is converted to bio-oil, char and gas products. In this work, volatile organic compounds (VOCs) of the gaseous fraction of peanut shells fast pyrolysis were collected and identified at atmospheric pressure. Aromatic compounds, hydrocarbons, furans and other oxygenated compounds were identified using solid phase microextraction (SPME) and gas chromatography coupled to mass spectrometry (CG-MS) as a detection system. The composition of volatiles was analyzed and compared with the constituents of liquid fraction for comparative purposes. Atmospheric implications of the main compounds identified in the gases fraction were assessed by determining tropospheric lifetimes of the VOCs identified and its impact on environment at the local, regional or global scale.

PM2.5 and O3 relationships affected by the atmospheric oxidizing capacity in the Yangtze River Delta, China

The atmospheric oxidizing capacity (AOC), reflecting the self-cleansing capacity of the atmosphere, plays an important role in the chemical evolution of secondary fine particulate matter (PM_2.5) and ozone (O₃). In this work, the AOC and its relationships with PM_2.5 and O₃ were investigated with a chemical transport model (CTM) in the Yangtze River Delta (YRD) region during the four seasons of 2017. The region-wide average AOC is ~4.5×10^-4 min^-1 in summer and ~ 6.4×10^-5 min^-1 in winter. Hydroxyl (OH) radicals oxidation contributes 55-69% to the total AOC, followed by nitrate (NO₃) radicals and O₃ (accounting for 19-34% and < 10%, respectively). The AOC attains a strong positive correlation with the O₃ level in all seasons. However, it is weakly or insignificantly correlated with PM_2.5 except in summer. Additionally, AOC×(SO₂ + NO₂ + volatile organic compound (VOC)) is well correlated with the PM_2.5 level, and high levels of precursors counteract lower AOC values in cold seasons. Collectively, the results indicate that the abundance of precursors could drive secondary aerosol formation in winter, and aqueous or heterogeneous reactions (not considered in the AOC estimates) are likely of importance at high aerosol loadings in the YRD. The relationship between the daily PM_2.5 and O₃ levels is affected by the AOC magnitude. PM_2.5 and O₃ are strongly correlated when the AOC is relatively high, but PM_2.5 is independent of O₃ under low-AOC (<6.6×10^-5 min^-1, typically in winter) conditions. This work reveals the dependence of PM_2.5-O₃ relationships on the AOC, suggesting that joint PM_2.5 and O₃ reduction could be realized at moderate to high AOC levels, especially in spring and autumn when the cooccurrence of high O₃ and PM_2.5 events is frequently observed.

Enhanced atmospheric oxidation capacity and associated ozone increases during COVID-19 lockdown in the Yangtze River Delta

Aggressive air pollution control in China since 2013 has achieved sharp decreases in fine particulate matter (PM_2.5), along with increased ozone (O₃) concentrations. Due to the pandemic of coronavirus disease 2019 (COVID-19), China imposed nationwide restriction, leading to large reductions in economic activities and associated emissions. In particular, large decreases were found in nitrogen oxides (NO_x) emissions (>50%) from transportation. However, O₃ increased in the Yangtze River Delta (YRD), which cannot be fully explained by changes in NO_x and volatile organic compound (VOCs) emissions. In this study, the Community Multi-scale Air Quality model was used to investigate O₃ increase in the YRD. Our results show a significant increase of atmospheric oxidation capacity (AOC) indicated by enhanced oxidants levels (up to +25%) especially in southern Jiangsu, Shanghai and northern Zhejiang, inducing the elevated O₃ during lockdown. Moreover, net P(HO_x) of 0.4 to 1.6 ppb h^-1 during lockdown (Case 2) was larger than the case without lockdown (Case 1), mainly resulting in the enhanced AOC and higher O₃ production rate (+12%). This comprehensive analysis improves our understanding on AOC and associated O₃ formation, which helps to design effective strategies to control O₃.

Significant decreases in the volatile organic compound concentration, atmospheric oxidation capacity and photochemical reactivity during the National Day holiday over a suburban site in the North China Plain

To what extent anthropogenic emissions could influence volatile organic compound (VOCs) concentrations and related atmospheric reactivity is still poorly understood. China's 70th National Day holidays, during which anthropogenic emissions were significantly reduced to ensure good air quality on Anniversary Day, provides a unique opportunity to investigate these processes. Atmospheric oxidation capacity (AOC), OH reactivity, secondary transformation, O₃ formation and VOCs-PM_2.5 sensitivity are evaluated based on parameterization methods and simultaneous measurements of VOCs, O₃, NO_x, CO, SO₂, PM_2.5, JO¹D, JNO₂, JNO₃ carried out at a suburban site between Beijing and Tianjin before, during, and after the National Day holiday 2019. During the National Day holidays, the AOC, OH reactivity, O₃ formation potential (OFP) and secondary organic aerosol formation potential (SOAP) were 1.6 × 10⁷ molecules cm^-3 s^-1, 41.8 s^-1, 299.2 μg cm^-3 and 1471.8 μg cm^-3, respectively, which were 42%, 29%, 47% and 42% lower than pre-National Day values and -12%, 42%, 36% and 42% lower than post-National Day values, respectively. Reactions involving OH radicals dominated the AOC during the day, but OH radicals and O₃ reactions at night. Alkanes (the degree of unsaturation = 0, (D, Equation (1)) accounted for the largest contributions to the total VOCs concentration, oxygenated VOCs (OVOCs; D ≤ 1) to OH reactivity and OFP, and aromatics (D = 4) to the SOAP. O₃ production was identified as VOCs-limited by VOCs (ppbC)/NOx (ppbv) ratios during the sampling campaign, with greater VOCs limitation during post- National Day and more-aged air masses during the National Day. The VOCs-sensitivity coefficient (VOCs-S) suggested that VOCs were more sensitive to PM_2.5 in low-pollution domains and during the National Day holiday. This study emphasizes the importance of not only the abundance, reactivity, and secondary transformation of VOCs but also the effects of VOCs on PM_2.5 for the development of effective control strategies to minimize O₃ and PM_2.5 pollution.

Mechanism and kinetics of the atmospheric reaction of 1,3,5-trimethylbenzene bicyclic peroxy radical with OH

The bicyclic peroxy radical (BPR) is the key intermediate during atmospheric oxidation of aromatics. In this paper, the reaction mechanisms and kinetics of the atmospheric reaction of the 1,3,5-trimethylbenzene (1,3,5-TMB) BPR with the OH radical were studied by density functional theory (DFT) and conventional transition-state theory (CTST) calculations. The product channels of formation of the 1,3,5-TMB trioxide (ROOOH), OH-adducts and Criegee intermediate (CI) have been identified, and the geometries and energies of all the stationary points were calculated at the M08-HX/6-311 + g(2df,2p) level of theory. In addition, the rate constants for the individual reaction pathway at 298 K were calculated. The results showed that OH addition reactions including the formation of ROOOH and OH-adducts are the main pathways, whereas Criegee intermediate formation is of minor importance.

The major pathways in the reaction of the 1,3,5-trimethylbenzene bicyclic peroxy radical with OH.

Kinetic and mechanistic study on gas phase reactions of ozone with a series ofcis-3-hexenyl esters

Reaction kinetics of O₃with fourcis-3-hexenyl esters were studied using experimental methods in a flow tube reactor as well as using theoretical methods.

OH production from the photolysis of isoprene-derived peroxy radicals: cross-sections, quantum yields and atmospheric implications

The OH radical production from the near-ultraviolet photolysis of peroxy radicals derived from isoprene has been investigated.

Kinetics and mechanisms of gas phase reactions of hexenols with ozone

The reactivity of hexenols, especially for the (E)-2-hexen-1-ol, with O₃ shows a strong dependence on their chemical structure.

The environmental impact of unsaturated fluoroesters: atmospheric chemistry towards OH radicals and Cl atoms, radiative behavior and cumulative ozone creation

Atmospheric lifetimes, reaction mechanisms, infrared spectra, radiative forcing efficiencies, global warming potentials and ozone creation potentials of the allyl and vinyl trifluoroacetate were determined to assess their environmental impact.

Experimental and Theoretical Study of Reactions of OH Radicals with Hexenols: An Evaluation of the Relative Importance of the H-Abstraction Reaction Channel

C6 hexenols are one of the most significant groups of volatile organic compounds with biogenic emissions. The lack of corresponding kinetic parameters and product information on their oxidation reactions will result in incomplete atmospheric chemical mechanisms and models. In this paper, experimental and theoretical studies are reported for the reactions of OH radicals with a series of C6 hexenols, (Z)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (Z)-4-hexen-1-ol, (E)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (E)-4-hexen-1-ol, at 298 K and 1.01 × 10(5) Pa. The corresponding rate constants were 8.53 ± 1.36, 10.1 ± 1.6, 7.86 ± 1.30, 8.08 ± 1.33, 9.10 ± 1.50, and 7.14 ± 1.20 (in units of 10(-11) cm(3) molecule(-1) s(-1)), respectively, measured by gas chromatography with a flame ionization detector (GC-FID), using a relative technique. Theoretical calculations concerning the OH-addition and H-abstraction reaction channels were also performed for these reactions to further understand the reaction mechanism and the relative importance of the H-abstraction reaction. By contrast to previously reported results, the H-abstraction channel is a non-negligible reaction channel for reactions of OH radicals with these hexenols. The rate constants of the H-abstraction channel are comparable with those for the OH-addition channel and contribute >20% for most of the studied alcohols, even >50% for (E)-3-hexen-1-ol. Thus, H-abstraction channels may have an important role in the reactions of these alcohols with OH radicals and must be considered in certain atmospheric chemical mechanisms and models.

Current challenges in air sampling of semivolatile organic contaminants: sampling artifacts and their influence on data comparability

With current science and policy needs, more attention is being given to expanding and improving air sampling of semivolatile organic contaminants (SVOCs). However, a wide range of techniques and configurations are currently used (active and passive samplers, different deployment times, different sorbents, etc.) and as the SVOC community looks to assess air measurements on a global scale, questions of comparability arise. We review current air sampling techniques, with a focus on sampling artifacts that can lead to uncertainties or biases in reported concentrations, in particular breakthrough, degradation, meteorological influences, and assumptions regarding passive sampling. From this assessment, we estimate the bias introduced for SVOC concentrations from all factors. Due to the effects of breakthrough, degradation, particle fractions and sampler uptake periods, some current passive and active sampler configurations may underestimate certain SVOCs by 30-95%. We then recommend future study design, appropriateness of sampler types for different study goals, and finally, how the SVOC community should move forward in both research and monitoring to best achieve comparability and consistency in air measurements.

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

Long-term observations of the reactive chemical composition of the tropical marine boundary layer (MBL) are rare, despite its crucial role for the chemical stability of the atmosphere. Recent observations of reactive bromine species in the tropical MBL showed unexpectedly high levels that could potentially have an impact on the ozone budget. Uncertainties in the ozone budget are amplified by our poor understanding of the fate of NOx (= NO + NO2), particularly the importance of nighttime chemical NOx sinks. Here, we present year-round observations of the multiisotopic composition of atmospheric nitrate in the tropical MBL at the Cape Verde Atmospheric Observatory. We show that the observed oxygen isotope ratios of nitrate are compatible with nitrate formation chemistry, which includes the BrNO3 sink at a level of ca. 20 ± 10% of nitrate formation pathways. The results also suggest that the N2O5 pathway is a negligible NOx sink in this environment. Observations further indicate a possible link between the NO2/NOx ratio and the nitrogen isotopic content of nitrate in this low NOx environment, possibly reflecting the seasonal change in the photochemical equilibrium among NOx species. This study demonstrates the relevance of using the stable isotopes of oxygen and nitrogen of atmospheric nitrate in association with concentration measurements to identify and constrain chemical processes occurring in the MBL.

Megacities and large urban agglomerations in the coastal zone: interactions between atmosphere, land, and marine ecosystems

Megacities are not only important drivers for socio-economic development but also sources of environmental challenges. Many megacities and large urban agglomerations are located in the coastal zone where land, atmosphere, and ocean meet, posing multiple environmental challenges which we consider here. The atmospheric flow around megacities is complicated by urban heat island effects and topographic flows and sea breezes and influences air pollution and human health. The outflow of polluted air over the ocean perturbs biogeochemical processes. Contaminant inputs can damage downstream coastal zone ecosystem function and resources including fisheries, induce harmful algal blooms and feedback to the atmosphere via marine emissions. The scale of influence of megacities in the coastal zone is hundreds to thousands of kilometers in the atmosphere and tens to hundreds of kilometers in the ocean. We list research needs to further our understanding of coastal megacities with the ultimate aim to improve their environmental management.

Multiannual observations of acetone, methanol, and acetaldehyde in remote tropical atlantic air: implications for atmospheric OVOC budgets and oxidative capacity

Oxygenated volatile organic compounds (OVOCs) in the atmosphere are precursors to peroxy acetyl nitrate (PAN), affect the tropospheric ozone budget, and in the remote marine environment represent a significant sink of the hydroxyl radical (OH). The sparse observational database for these compounds, particularly in the tropics, contributes to a high uncertainty in their emissions and atmospheric significance. Here, we show measurements of acetone, methanol, and acetaldehyde in the tropical remote marine boundary layer made between October 2006 and September 2011 at the Cape Verde Atmospheric Observatory (CVAO) (16.85° N, 24.87° W). Mean mixing ratios of acetone, methanol, and acetaldehyde were 546 ± 295 pptv, 742 ± 419 pptv, and 428 ± 190 pptv, respectively, averaged from approximately hourly values over this five-year period. The CAM-Chem global chemical transport model reproduced annual average acetone concentrations well (21% overestimation) but underestimated levels by a factor of 2 in autumn and overestimated concentrations in winter. Annual average concentrations of acetaldehyde were underestimated by a factor of 10, rising to a factor of 40 in summer, and methanol was underestimated on average by a factor of 2, peaking to over a factor of 4 in spring. The model predicted summer minima in acetaldehyde and acetone, which were not apparent in the observations. CAM-Chem was adapted to include a two-way sea-air flux parametrization based on seawater measurements made in the Atlantic Ocean, and the resultant fluxes suggest that the tropical Atlantic region is a net sink for acetone but a net source for methanol and acetaldehyde. Inclusion of the ocean fluxes resulted in good model simulations of monthly averaged methanol levels although still with a 3-fold underestimation in acetaldehyde. Wintertime acetone levels were better simulated, but the observed autumn levels were more severely underestimated than in the standard model. We suggest that the latter may be caused by underestimated terrestrial biogenic African primary and/or secondary OVOC sources by the model. The model underestimation of acetaldehyde concentrations all year round implies a consistent significant missing source, potentially from secondary chemistry of higher alkanes produced biogenically from plants or from the ocean. We estimate that low model bias in OVOC abundances in the remote tropical marine atmosphere may result in up to 8% underestimation of the global methane lifetime due to missing model OH reactivity. Underestimation of acetaldehyde concentrations is responsible for the bulk (∼70%) of this missing reactivity.

Multiphase halogen chemistry in the tropical Atlantic Ocean

We used a one-dimensional model to simulate the chemical evolution of air masses in the tropical Atlantic Ocean, with a focus on halogen chemistry. The model results were compared to the observations of inorganic halogen species made in this region. The model could largely reproduce the measurements of most chlorine species, especially under unpolluted conditions, but overestimated sea salt chloride, BrCl, and bromine species. Agreement with the measurements could be improved by taking into account the reactivity with aldehydes and the effects of dimethyl sulfide (DMS) and Saharan dust on aerosol pH; a hypothetical HOX → X(-) aqueous-phase reaction could also improve the agreement with measured Cl(2) and HOCl, especially under semipolluted conditions. The results also showed that halogens speciation and concentrations are very sensitive to cloud processing. The model was used to calculate the impact of the observed levels of halogens: Cl atoms accounted for 5.4-11.6% of total methane sinks and halogens (mostly bromine and iodine) accounted for 35-40% of total ozone destruction.

Heterogeneous uptake of NO2 on soils under variable temperature and relative humidity conditions

Heterogeneous reactions of nitrogen dioxide (NO2) on soils collected from Dalian (S1) and Changsha (S2) were investigated over the relative humidity (RH) range of 5%-80% and temperature range of 278-328 K using a horizontal coated-wall flow tube. The initial uptake coefficients of NO2 on S2 exhibited a decreasing trend from (10 +/- 1.3) x 10(-8) to (3.1 +/- 0.5) x 10(-8) with the relative humidity increasing from 5% to 80%. In the temperature effect studies, the initial uptake coefficients of S1 and S2 decreased from (10 +/- 1.2) x 10(-8) to (3.8 +/- 0.5) x 10(-8) and from (16 +/- 2.2) x 10(-8) to (3.8 +/- 0.4) x 10(-8) when temperature increased from 278 to 288 K for S1 and from 278 to 308 K for S2, respectively. As the temperature continued to increase, the initial uptake coefficients of S1 and S2 returned to (7.9 +/- 1.1) x 10(-8) and (20 +/- 3.1) x 10(-8) at 313 and 328 K, respectively. This study shows that relative humidity could influence the uptake kinetics of NO2 on soil and temperature would impact the heterogeneous chemistry of NO2.

Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean

Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is approximately 50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.

Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging

Most primary organic-particulate emissions are semivolatile; thus, they partially evaporate with atmospheric dilution, creating substantial amounts of low-volatility gas-phase material. Laboratory experiments show that photo-oxidation of diesel emissions rapidly generates organic aerosol, greatly exceeding the contribution from known secondary organic-aerosol precursors. We attribute this unexplained secondary organic-aerosol production to the oxidation of low-volatility gas-phase species. Accounting for partitioning and photochemical processing of primary emissions creates a more regionally distributed aerosol and brings model predictions into better agreement with observations. Controlling organic particulate-matter concentrations will require substantial changes in the approaches that are currently used to measure and regulate emissions.

Changes in tropospheric composition and air quality due to stratospheric ozone depletion and climate change

It is well-understood that reductions in air quality play a significant role in both environmental and human health. Interactions between ozone depletion and global climate change will significantly alter atmospheric chemistry which, in turn, will cause changes in concentrations of natural and human-made gases and aerosols. Models predict that tropospheric ozone near the surface will increase globally by up to 10 to 30 ppbv (33 to 100% increase) during the period 2000 to 2100. With the increase in the amount of the stratospheric ozone, increased transport from the stratosphere to the troposphere will result in different responses in polluted and unpolluted areas. In contrast, global changes in tropospheric hydroxyl radical (OH) are not predicted to be large, except where influenced by the presence of oxidizable organic matter, such as from large-scale forest fires. Recent measurements in a relatively clean location over 5 years showed that OH concentrations can be predicted by the intensity of solar ultraviolet radiation. If this relationship is confirmed by further observations, this approach could be used to simplify assessments of air quality. Analysis of surface-level ozone observations in Antarctica suggests that there has been a significant change in the chemistry of the boundary layer of the atmosphere in this region as a result of stratospheric ozone depletion. The oxidation potential of the Antarctic boundary layer is estimated to be greater now than before the development of the ozone hole. Recent modeling studies have suggested that iodine and iodine-containing substances from natural sources, such as the ocean, may increase stratospheric ozone depletion significantly in polar regions during spring. Given the uncertainty of the fate of iodine in the stratosphere, the results may also be relevant for stratospheric ozone depletion and measurements of the influence of these substances on ozone depletion should be considered in the future. In agreement with known usage and atmospheric loss processes, tropospheric concentrations of HFC-134a, the main human-made source of trifluoroacetic acid (TFA), is increasing rapidly. As HFC-134a is a potent greenhouse gas, this increasing concentration has implications for climate change. However, the risks to humans and the environment from substances, such as TFA, produced by atmospheric degradation of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are considered minimal. Perfluoropolyethers, commonly used as industrial heat transfer fluids and proposed as chlorohydrofluorocarbon (CHFC) substitutes, show great stability to chemical degradation in the atmosphere. These substances have been suggested as substitutes for CHFCs but, as they are very persistent in the atmosphere, they may be important contributors to global warming. It is not known whether these substances will contribute significantly to global warming and its interaction with ozone depletion but they should be considered for further evaluation.