Atmospheric Oxidation Research Articles

Changes in the Dominant Contributions of Nitrate Formation and Sources During Haze Episodes: Insights From Dual Isotopic Evidence

AbstractInorganic nitrate (NO3−), a crucial component of fine particulate matter (PM2.5), has not shown a consistent decrease, despite an obvious decrease of nitrogen oxide (NOx) and PM2.5. The atmospheric oxidation process for nitrate formation has been deemed a key factor in pollution; however, the changes of sources and oxidation pathways during a particular haze episode require further investigation. Here, daily dual isotopes (δ15N and δ18O) were used to quantify the sources and oxidation pathways of nitrate formation in Qingdao, a port city in Northern China, from September 2017 to February 2018. This study also includes a detailed introduction to two haze episodes. δ15N and δ18O results show that both fractions of nocturnal oxidation for nitrate formation and NOx from coal combustion were lower in warmer season and higher in colder season. The fractions increased with increasing PM2.5 under low PM2.5 concentration while the fractions were not significantly changed under higher PM2.5 concentration, dominated by nocturnal oxidation (70.6% ± 9.7%) and coal combustion (66.1% ± 18.2%), respectively. The haze episode 1 was attributed to smoke transported over long distances, which provided a large amount of aerosol particles to absorb more locally formed gaseous HNO3 or N2O5. In this episode, meteorological and air quality factors, nitrate sources, and formation mechanism did not obviously change. Haze episode 2 was caused by unfavorable meteorological factors that enhanced local nitrate accumulation. As pollution worsened, the oxidation pathway shifted from OH oxidation to N2O5 hydrolysis, and the primary source changed from coal combustion to more vehicle exhaust.

Novel technique to produce porous thermochromic VO2 nanoparticle films using gas aggregation source

Vanadium dioxide (VO2) is a phase transition material that undergoes semiconductor-to-metal transition at the temperature of about 68 °C. This extraordinary feature triggered intensive research focused on the controlled synthesis of VO2. In this study, we introduce and investigate an original linker- and solvent-free strategy enabling the production of highly porous VO2 nanoparticle-based films. This technique combines a gas-phase synthesis of vanadium nanoparticles and their subsequent atmospheric pressure thermal oxidation. It is shown that the thermochromic behaviour of such produced nanomaterial is at the fixed oxidation temperature strongly dependent on the oxidation time. Concerning this, it was found that there exists an optimal oxidation time (60 s in our study) that assures the production of crystalline VO2 nanoparticles with the highest, reproducible and temporally stable semiconductor-to-metal transition with the resistive switching ratio close to 2 orders of magnitude and dramatic switching of optical properties in the near infra-red spectral region.

Potential Air Quality Side-Effects of Emitting H2O2 to Enhance Methane Oxidation as a Climate Solution.

Methane (CH4) is a greenhouse gas with a global warming potential 81.2 times higher than carbon dioxide (CO2). The intentional emission of oxidants into the atmosphere has been proposed as a geoengineering solution to accelerate the oxidation of CH4 to CO2, thereby reducing surface warming. However, there has been little consideration for competing atmospheric oxidation pathways that will reduce CH4 oxidation efficiencies and result in the formation of secondary pollutants such as ozone and particulate matter. Using a global chemical-transport model, we simulate a proposed technology to intentionally emit H2O2 into the atmosphere to elevate OH concentrations and enhance CH4 oxidation. We find that proposed emission rates of oxidants have minimal impacts on monthly average tropospheric ozone and particulate matter concentrations. However, competition for the oxidation of CH4 would necessitate widespread adoption of such technology to remove substantial concentrations of atmospheric CH4, which would in-turn cause considerable increases in regional winter-time particulate matter. Our work underscores the need to consider competing chemistry in evaluating the efficacy and side effects of proposals to enhance the atmospheric oxidation of CH4 as a climate solution.

Quantification and source apportionment of atmospheric oxidation capacity in urban atmosphere by considering reactive chlorine species and photochemical loss of VOCs.

Atmospheric oxidation capacity (AOC) reflects the potential of the atmosphere in converting primary pollutants into secondary aerosols and ozone (O3). In this study, the AOC at an urban supersite in Wuhan, a megacity in central China, was quantified by considering the reactions of volatile organic compounds (VOCs) and carbon monoxide (CO) with atmospheric oxidants (OH, NO3, O3, and Cl). Photochemical loss of total VOCs (13.7-23.7%) during transport was accounted for by monitoring the concentration ratio of o-xylene and ethylbenzene, a VOC pair with diverse reaction rate constants with OH. AOC would be underestimated by 9.0-25.2% in the 4 seasons if being estimated from the observed VOCs instead of photochemical-loss corrected VOCs. The atmospheric oxidants were measured or indirectly estimated using well-established parameterizations. In particular, hourly reactive chlorine species were measured using an iodide-based chemical ionization mass spectrometer (I-CIMS). Cl radical concentration was calculated by assuming a steady-state between the production from reactive chlorine species and the removal by O3 and VOCs. The result showed that AOC would be underestimated by 14.5% and 1.9% in winter and spring if Cl radicals were neglected. Based on the above quantification, the composition of AOC was further apportioned to atmospheric oxidants, VOC categories, as well as the sources of VOC and CO. In winter, CNG combustion exhaust, electronics industry, and secondary formation were critical for regulating the AOC. In spring, secondary formation was the most important AOC source, followed by electronics industry and biogenic sources.

Methanesulfonic acid (MSA) and SO3 formation from the addition channel of atmospheric dimethyl sulfide oxidation.

The formation of methanesulfonic acid (MSA) from the dimethyl sulfide addition channel primarily proceeds via the reaction of methylsulfonyloxy radicals (CH3SO3) with H-atom donors, other than HO2 radicals. In competition with it, thermal decomposition of CH3SO3 results in SO3 generation. The MSA/SO3 ratio is driven by the temperature dependence of CH3SO3 decomposition.

Atmospheric nitrogen oxides (NOx), hydrogen sulphide (H2S) and carbon monoxide (CO): Boon or Bane for plant metabolism and development?

Atmospheric nitrogen oxides (NOx), hydrogen sulphide (H2S) and carbon monoxide (CO): Boon or Bane for plant metabolism and development?

Chromatographic differentiation in SOA fingerprint identification: A study on naphthalene and α-pinene oxidation.

The evolution of precursors to form secondary organic aerosol (SOA) is still a challenge in atmospheric chemistry. Chamber experiments were conducted to simulate the ambient OH oxidation of naphthalene and α-pinene, which are typical markers of anthropogenic and biogenic emissions. Particulate matters were sampled by quartz filters and were analyzed by comprehensive two-dimensional gas chromatography (GC×GC) coupled with a thermal desorption system (TD) and a mass spectrometer (MS). A four-step procedure is proposed based on chromatographic differentiation in SOA fingerprint identification, including initial resolving, translation and alignment, pixel-based differentiation, and marker identification. Data processing begins by creating a 65-organic consensus template from peak identification results. Peaks are screened with matching factors >700. Retention time shifts of Naphthalene and α-pinene oxidation products are 0.5 min and 0.1 min respectively. A reverse matching factor >800 is used to define high-confidence compound recognition, and the ±50 retention index tolerance assists structure ID for SOA study. Two new markers are found for the first time, i.e., phthalimide in naphthalene-OH oxidation with the occurrence of NOx, and citronellol epoxide from α-pinene-OH oxidation. Our finding offers fresh insights into the complex chemical pathways of these precursors and improving our understanding of SOA formation mechanisms. Our workflow works well for fingerprint identification and could be utilized in finding SOA intermediates and source appointments.

Revealing the composition and optical properties of marine carbonaceous aerosols: A case of the eastern China marginal seas.

Marine aerosols are major components of atmospheric aerosols, playing substantial roles in influencing the regional and global environment and climate. Marine aerosols are not only produced by seawater directly, but also by indirect processes such as atmospheric oxidation of marine bioactive gases as well as terrestrial transport. Over the Eastern China Marginal Seas (ECMS), marine aerosols are strongly affected by marine emission and transport of terrestrial aerosols. However, only few studies have paid attention to the optical properties across three marginal seas. In this study, marine aerosol samples were collected from the entire ECMS in spring 2023 to explore the composition and properties of carbonaceous species. Due to the significant influence of terrestrial transport on Bohai Sea, the average concentration of total suspended particles (TSP) is as high as (359.65±150.45) μg m-3, while the average concentrations of organic carbon (OC) and element carbon (EC) can be up to (17.99±7.71) μg m-3 and (3.28±1.23) μg m-3, respectively. Besides, intense solar radiation may be a potential factor leading to an increase in the solubility of OC in aerosols over southern Yellow Sea. The light-absorbing capacity (MAE365) of water-soluble organic carbon (WSOC) is higher in northern sea region, being (0.58±0.11) m2 g-1 in Bohai Sea, (0.40±0.12) m2 g-1 in Yellow Sea and (0.29±0.11) m2 g-1 in East China Sea. The current results show that humic-like and protein-like substances are the main fluorescent components in water-soluble organic matter. Terrestrial sources enhance the warming effect of aerosols over ECMS by about 1.5-2 times more than marine sources. This study suggests that future research should focus on the impact of terrestrial sources on the northern region of ECMS and the impact of marine sources on the southern region of ECMS.

Kamchatka (southeastern Siberia) ice core records of dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls since 1690s: A signal for the tropospheric oxidizing capacity.

There has been much interest about how to identify an ice core signal for oxidizing capacity of the troposphere. This study broadly explains the air-snow transfer/deposition process using ice core records of dicarboxylic (DCAs), ω-oxocarboxylic as well as pyruvic acids and α-dicarbonyls, which are potentially formed by atmospheric oxidation of aromatic hydrocarbons from the continent, incloud-oxidation of isoprene and unsaturated fatty acids from the western North Pacific. An ice core (~152m long, 304years) was collected at an ice cap on the Gorshkov crater at the summit of Ushkovsky (56° 04'N, 160° 28'E, altitude: 3903m) in the Kamchatka Peninsula from southeastern Siberia. Molecular distributions of DCAs showed a predominance of oxalic (C2) or succinic (C4) acid, followed by malonic (C3) acid. ω-Oxoacids are characterized by the predominance of glyoxylic acid (ωC2) followed by 8-oxooctanoic (ωC8) or 9-oxononanoic (ωC9) and then 4-oxobutanoic acid (ωC4). These molecular distributions (C2≅C4>C3 and ωC2>ωC8=ωC9>ωC4) in this ice core are different than those from Greenland Site-J and Alaskan Aurora Peak ice cores. The concentration profiles of these DCAs showed significant correlation (R=0.78, p<0.01) with the Dalton solar minimum (~1790s-1830s). Diagnostic ratios of these DCAs have clear signal of the Pacific and North Atlantic decadal oscillation with increases in the recent periods, suggesting that the ice core profiles of organic compounds are involved with past biogenic emission from marine biota and incloud-isoprene oxidation. This study reveals the multidecadal variability of the lower tropospheric oxidizing capacity in the North Pacific rim. SYNOPSIS: Polar organic compounds preserved in snow and ice are concomitant with recent climate changes in the western North Pacific rim. SIGNIFICANCE STATEMENTS.

Enhancing the compatibility of low-value multilayer plastic waste in bitumen mixtures using atmospheric cold plasma and thermal oxidation

Multilayer plastic (MP) commonly used in food and beverage packaging is difficult to recycle due to its layered structure, resulting in its accumulation over time; the consequent environmental harm is further exacerbated by its short lifespan. This study investigates recycled low-value MP as a modifier for polymer-modified bitumen (PMB). However, the difference in polarity between MP and PMB mixtures is a challenge, resulting in their poor compatibility and reduced mechanical properties. To overcome this, low-value MP was treated with atmospheric cold plasma and thermal oxidation to enhance its compatibility with PMB. The results indicate that plasma and thermal treatments increase the hydrophilicity of low-value MP through the formation of low-molecular-weight oxidized molecules containing hydrophilic hydroxyl (–OH) and carbonyl (C = O) groups that act as an intermediary boundary layer between the low-value MP and asphaltene-rich bitumen. Further, the optimal oxidation conditions for MP are revealed as 60 s of plasma treatment followed by heating at 150 °C for 60 min. Mixtures of PMB and optimally oxidized MP have optimal compositions of 1 wt.%, with ductility and penetration values of 87.7 cm and 57.4 mm, respectively.

Daytime Production of Airborne Pollutants Including Brown Carbon by NO2 Interaction With Surface Microlayer of Lake Water in Southwestern China

AbstractSurface microlayer at freshwater (rivers, lakes, ponds, streams, and groundwater) and seawater is abundant with organic compounds compared to subsurface water. These organics adsorbed at the air‐water interface can interact with the atmospheric oxidants and influence the exchange of organic materials between the water and the atmosphere. Here, we assess the chemical interaction between gaseous NO2 and authentic surface microlayers collected at the lake water (Dianchi Lake) situated in China. The formation of the gas‐phase product compounds was evaluated in real time using a novel secondary electrospray ionization ultrahigh‐resolution quadrupole Orbitrap mass spectrometer (SESI‐UHR‐MS) upon exposure of surface microlayer to gaseous NO2 (20 or 50 ppb) in dark and under simulated sunlight irradiation at two different temperatures: 5°C and 25°C. The obtained results revealed that the sampling sites of the lake impacted by human activities (municipal sewage and agricultural activities) significantly impact the number and the composition of the formed gas‐phase product compounds. The formation of nitrogen (N)‐containing compounds was observed as well, which contain most likely nitro or amino functional groups, or alternatively, they could be aromatic compounds. The observed N‐containing compounds may contribute to the “brown carbon” which act as light‐absorbing compounds, thus influencing the radiative forcing of aerosols in the atmosphere.

Dynamic phase transitions dictate the size effect and activity of supported gold catalysts.

The landmark discovery of gold catalysts has aroused substantial interest in heterogeneous catalysis, yet the catalytic mechanism remains elusive. For carbon monoxide oxidation on gold nanoparticles (NPs) supported on ceria surfaces, it is widely believed that carbon monoxide adsorbs on the gold particles, while the reaction occurs at the gold/ceria interface. Here, we have investigated the dynamic changes of supported gold NPs with various sizes in a carbon monoxide oxidation atmosphere using deep potential molecular dynamics simulations. Our results reveal that the structure of tiny gold particles in carbon monoxide atmospheres becomes highly disordered and undergoes phase transition. Such a liquid-like structure provides massive reactive sites, enabling facile carbon monoxide oxidation on the solid-state gold NP rather than just at the gold/ceria interface. This result is further corroborated by catalytic experiments. This work sheds light on both the size effects and activity in noble metal catalysis and provides insights for the design of more effective nanocatalysts.

Organic compounds in valley fogwater in North and Mount Lebanon during COVID-19 period.

Caltech Active Strand Cloudwater collectors are used to collect valley fog samples from Mount and North-Lebanon during 2021 for the speciation of organic matter for the first time ever. Numerous compounds including pesticides, phenols, acids, and persistent organic pollutants such as polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), and polycyclic aromatic hydrocarbons (PAHs) have been identified in fogwater samples. They are extracted using the liquid-liquid extraction performed on the XTR chromabond columns. The highest contribution to the total organic fraction refers to phenols and acids (around 77%) due to their better water solubility than others inducing a good affinity to the aqueous phase of fogwater droplets. The lowest contribution refers to the hydrophobic families accounting together for <5% of the total fraction. Pearson analysis is employed in this study to check correlations between some variables. PAHs are found to be originated from combustion and vehicle exhaust and PCBs are found to be highly correlated with PAHs. Phenols and acids are highly associated with PAHs and PCBs as well as with sulfate and heavy metals (manganese and nickel). This shows that their sources can be either atmospheric oxidation or vehicle exhaust. Pesticides are found to be highly correlated with each other which suggests their simultaneous applications. Despite all, more research is still needed to have a bigger and more reliable data to adopt the beneficial effect of fogwater as an alternative for fresh water at least for the Lebanese agriculture.

Chemical Composition of Secondary Organic Aerosol Formed from the Oxidation of Semivolatile Isoprene Epoxydiol Isomerization Products.

3-Methylenebutane-1,2,4-triol and 3-methyltetrahydrofuran-2,4-diols, previously designated "C5-alkene triols", were recently confirmed as in-particle isomerization products of isoprene-derived β-IEPOX isomers that are formed upon acid-driven uptake and partition back into the gas phase. In chamber experiments, we have systematically explored their gas phase oxidation by hydroxyl radical (•OH) as a potential source of secondary organic aerosol (SOA). •OH-initiated oxidation of both compounds in the presence of ammonium bisulfate aerosol resulted in substantial aerosol volume growth. Compositions of low-volatility products in both the gas and particulate phases were established by high-resolution mass spectrometry measurements. Under conditions mimicking the Southeast USA (50% relative humidity, bulk seed aerosol pH 1.4), we estimate the SOA yield from •OH-initiated oxidation of 3-methylenebutane-1,2,4-triol to be 93.1%, equating to 1.95 ± 0.81 Tg C Yr-1, and from 3-methyltetrahydrofuran-2,4-diol oxidation to be 26.7%, equating to 1.76 ± 1.26 Tg C Yr-1. Previously unreported isoprene-derived oxidation products, 2,3-dihydroxy-2-(hydroxymethyl)propanal, 1,3,4-trihydroxybutan-2-one, and four organosulfates have been confirmed in ambient SOA, and aid in understanding isoprene oxidation pathways in HO2• dominated environments as NOx levels continue to decline in the US. This work underlines the need for inclusion of partitioning of in-particle formed semivolatile products and their atmospheric oxidation pathways in atmospheric models.

Copper-Catalyzed Cycloaddition/Coupling Cascades of Azomethine Imines with Alkynes for Divergent Synthesis of (Z)-Alkenyl and Alkynyl N,N'-Bicyclic Pyrazolidinones.

Copper-catalyzed cycloaddition/coupling cascades utilizing azomethine imines and alkynes have been developed for the divergent synthesis of (Z)-alkenyl and alkynyl N,N'-bicyclic pyrazolidinones by varying the reaction conditions. The choice of inert or oxidative atmosphere plays a crucial role in determining the transformation pathways. These reactions have broad substrate scopes and mild conditions, making them potentially useful.

The Observation of Urban Ambient 1,3‐Butadiene Based on Differential Optical Absorption Spectroscopy Technique and Its Potential Alteration on the Nighttime Chemistry

AbstractAs a highly reactive dialkenes similar to isoprene, 1,3‐butadiene (BD) is rapidly oxidized and fully involved in atmospheric oxidation processes. We have established a method for the online measurement of ambient BD using Differential Optical Absorption Spectroscopy (DOAS) technique. Lab testing demonstrated that DOAS technique can accurately measure BD concentrations in high temporal resolution of minute‐level, with accuracy and precision within ±5% and 1%, respectively. Following field measurement, the detection limit for BD reached 90 pptv, the r between DOAS and the online VOCs system results reached 0.885, and the nighttime hourly averaged concentration peaked at 0.75 ppbv. At this BD level, if the NO3 reaction pathway with BD was not considered, the maximum proportional variations in the NO3 concentration simulated by the box model exceeded 10%. In conclusion, DOAS is suitable for long‐term BD measurements, and continuous attention should be given to the potential alteration on atmospheric chemistry caused by this type of easily neglected compounds.

Atmospheric reaction of CH2=CHCH2OCF2CHF2 with OH radicals and Cl atoms, UV and IR absorption cross sections, and global warming potential.

In this work, the rate coefficients for OH radical, k1(T), and Cl atom, k2(T), reaction with allyl 1,1,2,2-tetrafluoroethyl ether, CH2=CHCH2OCF2CHF2, were studied as a function of temperature and pressure in a collaborative effort made between UCLM, Spain, and LAPKIN, Greece. OH rate coefficients were determined in UCLM, between 263 and 353K and 50-600Torr, using the absolute rate method of pulsed laser photolysis-laser-induced fluorescence technique, while Cl kinetics were studied in temperature (260-363K) and pressure (34-721Torr) ranges, using the relative rate method of the thermostated photochemical reactor equipped with Fourier transform infrared spectroscopy as the detection technique. In both OH and Cl reactions, a negative temperature dependence of the measured rate coefficients was observed, which is consistent with complex association reactions. The temperature dependence of OH rate coefficients was found to be well represented by the following expression: k1(T) = (2.30 ± 0.35) × 10-12 exp[(544 ± 46) K/T] cm3 molecule-1s-1. In the case of the Cl-initiated reaction, a slight curvature was observed in the Arrhenius plot for k2(T), and the kinetic data were fitted to a modified Arrhenius expression: k2(T) = (4.42 ± 0.32) × 10-16 T2 exp[(610 ± 22) K/T] cm3 molecule-1s-1. No pressure dependence was observed in either case. These results are consistent with a complex reaction mechanism that is not uncommon in radical association reactions to the unsaturated bond. As part of this work, UV (200-400nm) and infrared absorption spectra (500-3200cm-1) were also measured to further evaluate CH2=CHCH2OCF2CHF2 atmospheric impact. Atmospheric lifetimes with respect to OH radical and Cl atom reactions were estimated to be 19.8h and 38days, respectively, showing that OH radicals dominate atmospheric oxidation. CH2=CHCH2OCF2CHF2 is a very weak absorber in the solar actinic region, while its relatively low radiative efficiency in the atmospheric IR window, 0.0034 W m-2 ppbv-1, and the short lifetime led to a very low GWP value relative to CO2, 1.2 × 10-2 and 3.3 × 10-3, at time horizons of 20 and 100years, respectively.

Catalytic Reduction of the Compounds Generated When Heating Heet Tobacco in Presence of USY and Beta Zeolites and Silica Lovel 6000 and SBA-15 Silicate in Oxidative and Inert Atmospheres: Effect of Temperature and Catalyst Content

The thermal decomposition of a heat-not-burn (HNB) tobacco at four temperatures (250–400 °C) was studied via thermogravimetric analysis (TGA) and Multi-shot pyrolizer experiments (Py-GC/MS), and the effect of four potential additives, USY Beta and beta zeolites and Silica Lovel 6000 and SBA-15 silicates at three concentrations (5, 15 and 25% w/w) under an inert and oxidative atmosphere was analyzed. Different techniques were applied showing that the presence of the additives modifies the decomposition processes (TGA). Py-GC/MS showed that these tobaccos generate large amounts of Nicotine and Glycerine. Acid compounds are the most abundant compounds under an inert atmosphere, while Oxygenated compounds predominate under an oxidative atmosphere. In both atmospheres, Furans and Aromatics present in a significant abundance at high temperatures. The additives used reduce both the number and the concentration of most of the compounds generated, especially at high temperatures and concentrations. Moreover, SBA-15 shows good aptitudes to reduce the formation of some individual compounds included in the FDA’s HPHC list, such as Acetone and Acetaldehyde. Finally, smoking experiments corroborated that all additives produce marked reductions in TPM, i.e., the majority fraction obtained, and in practically all the compounds generated. Phenol, a toxicant compound that was detected in a significant amount, is also markedly reduced. SBA-15 is the material that presents a major reduction in the TPM and the principal compounds generated. These results may be of great interest for further reducing the toxicity of smoking this type of heat-not-burn tobacco product.

On the atmospheric budget of 1,2-dichloroethane and its impact on stratospheric chlorine and ozone (2002–2020)

Abstract. The chemical compound 1,2-dichloroethane (DCE), or ethylene dichloride, is an industrial very short-lived substance (VSLS) whose major use is as a feedstock in the production chain of polyvinyl chloride (PVC). Like other chlorinated VSLSs, transport of DCE (and/or its atmospheric oxidation products) to the stratosphere could contribute to ozone depletion there. However, despite annual production volumes greatly exceeding those of more prominent VSLSs (e.g. dichloromethane), global DCE observations are sparse; thus, the magnitude and distribution of DCE emissions and trends in its atmospheric abundance are poorly known. In this study, we performed an exploratory analysis of the global DCE budget between 2002 and 2020. Combining bottom-up data on annual production and assumptions around fugitive losses during production and feedstock use, we assessed the DCE source strength required to reproduce atmospheric DCE observations. We show that the TOMCAT/SLIMCAT 3-D chemical transport model (CTM) reproduces DCE measurements from various aircraft missions well, including HIPPO (2009–2011), ATom (2016–2018), and KORUS-AQ (2016), along with surface measurements from Southeast Asia, when assuming a regionally varying production emission factor in the range of 0.5 %–1.5 %. Our findings imply substantial fugitive losses of DCE and/or substantial emissive applications (e.g. solvent use) that are poorly reported. We estimate that DCE's global source increased by ∼ 45 % between 2002 (349 ± 61 Gg yr−1) and 2020 (505 ± 90 Gg yr−1), with its contribution to stratospheric chlorine increasing from 8.2 (± 1.5) to ∼ 12.9 (± 2.4) ppt Cl (where ppt denotes parts per trillion) over this period. DCE's relatively short overall tropospheric lifetime (∼ 83 d) limits, although does not preclude, its transport to the stratosphere, and we show that its impact on ozone is small at present. Annually averaged, DCE is estimated to have decreased ozone in the lower stratosphere by up to several parts per billion (&lt; 1 %) in 2020, although a larger effect in the springtime Southern Hemisphere polar lower stratosphere is apparent (decreases of up to ∼ 1.3 %). Given strong potential for growth in DCE production tied to demand for PVC, ongoing measurements would be of benefit to monitor potential future increases in its atmospheric abundance and its contribution to ozone depletion.

Short‐Lived Air Pollutants and Climate Forcers Through the Lens of the COVID‐19 Pandemic

AbstractDramatic reductions in anthropogenic emissions during the lockdowns of the COVID‐19 pandemic provide an unparalleled opportunity to assess responses of the Earth system to human activities. Here, we synthesize the latest progress in understanding changes in short‐lived atmospheric constituents, that is, aerosols, ozone (O3), nitrogen oxides (NOx), and methane (CH4), in response to COVID‐19 induced emission reductions and the associated climate impacts on regional and global scales. The large‐scale emission reduction in the transportation sector reduced near‐surface particulate and ozone concentrations, with certain regional enhancements modulated by atmospheric oxidizing capacity and abnormal meteorological conditions. The methane increase during the pandemic is a combined effect of fluctuations in methane emissions and chemical sinks. Global net radiative forcing of all short‐lived species was found to be small, but regionally, aerosol radiative impacts during the lockdowns were discernible near China and India. Aerosol microphysical effects on clouds and precipitation were reported from modeling assessments only, except for observed reductions in aircraft contrails. There exist moderate climatic impacts of the pandemic on regional surface temperature, atmospheric circulations, and ecosystems, mainly over populous and polluted areas. Novel methodologies emerge in the pandemic‐related research to achieve the synergy between observations from multiple platforms and model simulations and to overcome the enormous hurdles and sophistication in detection and attribution studies. The insight gained from COVID‐19 research concerning the complex interplay between emission, chemistry, and meteorology, as well as the unexpected climate forcing‐responses relationships, underscores future challenges for cleaning up the air and alleviating the adverse impacts of global warming.