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Heterogeneous chemical cycles of pyrogenic nitrogen and halides influence tropospheric ozone and affect the stratosphere during extreme Pyrocumulonimbus (PyroCB) events. We report field‐derived N2O5 uptake coefficients, γ(N2O5), and ClNO2 yields, φ(ClNO2), from two aircraft campaigns observing fresh smoke in the lower and mid troposphere and processed/aged smoke in the upper troposphere and lower stratosphere (UTLS). Derived φ(ClNO2) varied across the full 0–1 range but was typically <0.5 and smallest in a PyroCB (<0.05). Derived γ(N2O5) was low in agricultural smoke (0.2–3.6 × 10−3), extremely low in mid‐tropospheric wildfire smoke (0.1 × 10−3), but larger in PyroCB processed smoke (0.7–5.0 × 10−3). Aged biomass burning aerosol in the UTLS had a higher γ(N2O5) of 17 × 10−3 that increased with sulfate and liquid water, but that was 1–2 orders of magnitude lower than values for aqueous sulfuric aerosol used in stratospheric models. Plain Language Summary: The injection of reactive material into Earth's atmosphere from fires affects atmospheric composition at regional and hemispheric scales. Reported stratospheric ozone depletion during extreme events, such as the 2020 Australian wildfires, illustrates one example of fire impacts and the role of heterogeneous (gas‐particle) processes. We report field quantification of rates and product yields from airborne observations of smoke. Extremely slow heterogeneous reaction rates on young smoke increase with transport and aging, but upper atmospheric values are still a factor of 10 slower than parameterizations used in stratospheric models. Heterogeneous production of ClNO2, a major lower atmospheric chlorine activation pathway, may be active on biomass burning aerosol in the upper atmosphere. Key Points: ClNO2 formation is active on biomass burning (BB) particles but decreases with transport to the upper troposphere and lower stratosphere (UTLS)N2O5 uptake coefficients are low on young BB smoke and increase with transport through a PyroCB and UTLS agingN2O5 uptake coefficients on aged BB particles in the UTLS are significantly lower than those used in model parameterizations [ABSTRACT FROM AUTHOR]

Nighttime reactions of nitrogen oxides influence ozone, volatile organic compounds, and aerosol and are thus important to the understanding of regional air quality. Despite large emissions and rapid recent growth of nitrogen oxide concentrations, there are few studies of nighttime chemistry in China. Here we present measurements of nighttime nitrogen oxides, NO3 and N2O5, from a coastal mountaintop site in Hong Kong adjacent to the megacities of the Pearl River Delta region. This is the first study of nighttime chemistry from a site within the residual layer in China. Key findings include the following. First, highly concentrated urban NOx outflow from the Pearl River Delta region was sampled infrequently at night, with N2O5 mixing ratios up to 8 ppbv (1 min average) or 12 ppbv (1 s average) in nighttime aged air masses. Second, the average N2O5 uptake coefficient was determined from a best fit to the available steady state lifetime data as γ(N2O5) = 0.014 ± 0.007. Although this determination is uncertain due to the difficulty of separating N2O5 losses from those of NO3, this value is in the range of previous residual layer determinations of N2O5 uptake coefficients in polluted air in North America. Third, there was a significant contribution of biogenic hydrocarbons to NO3 loss inferred from canister samples taken during daytime. Finally, daytime N2O5 mixing ratios were in accord with their predicted photochemical steady state. Heterogeneous uptake of N2O5 in fog is determined to be an important production mechanism for soluble nitrate, even during daytime.

This study investigates selected secondary atmospheric responses to the widely reported emission change attributed to COVID‐19 lockdowns in the highly polluted Indo‐Gangetic Plain (IGP) using ground‐based measurements of trace gases and particulate matter. We used a chemical box‐model to show that production of nighttime oxidant, NO3, was affected mainly by emission decrease (average nighttime production rates 1.2, 0.8 and 1.5 ppbv hr−1 before, during and relaxation of lockdown restrictions, respectively), while NO3 sinks were sensitive to both emission reduction and seasonal variations. We have also shown that the maximum potential mixing ratio of nitryl chloride, a photolytic chlorine radical source which has not been previously considered in the IGP, is as high as 5.5 ppbv at this inland site, resulting from strong nitrate radical production and a potentially large particulate chloride mass. This analysis suggests that air quality measurement campaigns and modeling explicitly consider heterogeneous nitrogen oxide and halogen chemistry. Plain Language Summary: The Indo‐Gangetic Plain (IGP) is one of the most polluted regions on earth, with poor air quality affecting the majority of the Indian population. The atmospheric chemistry that transforms major regional emissions into harmful secondary pollutants is complex. Here, we quantify, for the first time, several important oxidative processes and show the potential for substantial oxidation of biogenic volatile organic compounds and the production of chlorine through unconventional chemistry in the IGP. We further show how these chemical cycles varied due to the emission reductions as a result of COVID‐19 lockdown, findings that will serve to define their sensitivity to future emission changes in the region. Key Points: Atmospheric response in the Indo‐Gangetic Plain varied according to seasonal changes and emissions reductions due to COVID‐19 lockdownNO3 production was mainly affected by emission changes, while NO3 sinks were sensitive to both emissions and seasonal changesNitryl chloride, a photolytic chlorine radical source not previously considered in the inland Indo‐Gangetic Plain, may be up to 5.5 ppbv [ABSTRACT FROM AUTHOR]

We use observations from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign to constrain the proposed mechanism of Cl2 production from ClNO2 reaction in acidic particles. To reproduce Cl2 concentrations observed during WINTER with a chemical box model that includes ClNO2 reactive uptake to form Cl2, the model required the ClNO2 reaction probability, γ (ClNO2), to range from 6 × 10−6 to 7 × 10−5, with a mean value of 2.3 × 10−5 (±1.8 × 10−5). These field‐determined γ (ClNO2) are more than an order of magnitude lower than those determined in previous laboratory experiments on acidic surfaces, even when calculated particle pH is ≤2. We hypothesize this is because thick salt films in the laboratory enhanced the reactive uptake ClNO2 compared to that which would occur in submicron aerosol particles. Using the reacto‐diffusive length‐scale framework, we show that the field and laboratory observations can be reconciled if the net aqueous‐phase reaction rate constant for ClNO2 (aq) + Cl‐(aq) in acidic particles is on the order of 104 s−1. We show that wet particle diameter and particulate chloride mass together explain 90% of the observed variance in the box model‐derived γ (ClNO2), implying that the availability of chloride and particle volume limit the efficiency of the reaction. Despite a much lower conversion of ClNO2 into Cl2, this mechanism can still be responsible for the nocturnal formation of 10–20 pptv of Cl2 in polluted regions, yielding an atmospherically relevant concentration of Cl atoms the following morning. Key Points: Observations of ClNO2 and Cl2 show that even under acidic conditions, the uptake of ClNO2 proceeds much slower than previously reportedThe availability of particulate chloride in acidic aerosols and particle size limit the efficiency of the conversion of ClNO2 to Cl2Despite a slower reaction, ClNO2 reactions on aerosol can be a significant source of Cl2 in polluted regions [ABSTRACT FROM AUTHOR]

Nitryl chloride (ClNO2) acts as a source of highly reactive chlorine atoms as well as an important NOx reservoir. Measurements of ClNO2 at an operational U.K. landfill site are reported here for the first time. A peak concentration of 4 ppb of ClNO2 was found with a peak mean nighttime maximum of 0.9 ppb. Using models based upon the photolysis of observed ClNO2 and atmospheric chlorine chemistry, chlorine atom concentrations reaching in excess of 1.20 × 105 molecules/cm3 in the early morning following sunrise are calculated. These concentrations are approximately 10 times higher than previously reported in the United Kingdom, suggesting a significant impact on the oxidizing capacity around such sites. Given the ubiquity of landfill sites regionally and globally, and the large abundances of Cl atoms from the photolysis of ClNO2, chlorine chemistry has a significant impact on ozone formation and volatile organic compounds oxidation as shown by WRF‐Chem modeling. Plain Language Summary: Landfill sites are a known source of traces gases into the atmosphere, but measurements often focus predominately on methane and carbon dioxide. A small subsection of trace gas measurements at landfill sites have shown, however, that these sites may be important halogen sources that could have subsequent impacts on air quality and climate. Spatially limited field measurements have previously been reported of a halogen species, ClNO2, showing that this species is consistently formed during nighttime hours, but no such measurements before now have been made at any landfill site. ClNO2 undergoes photolysis upon sunrise, releasing the extremely reactive Cl as well as NO2 into the atmosphere and therefore plays an important part in the total budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species, all of which are important to air quality. Here we present mass spectrometry measurements of ClNO2 taken at an undisclosed landfill, which show high concentrations in comparison to any other global study of this type. We use predictive modeling techniques to show the importance of this halogen species to air quality, using indicators such as ozone formation. Based on these results we recommend that landfill sources of Cl should be included in future air quality studies. Key Points: Up to 4 ppb of ClNO2 is measured at a U.K. landfill siteSubsequent chlorine chemistry can have a significant impact on ozone formation and VOCs oxidation as shown by WRF‐Chem modeling [ABSTRACT FROM AUTHOR]

Highlight • Nitryl chloride ClNO 2. • High-resolution Fourier transform spectroscopy. • First analysis of the ν 2 , 2ν 3 and 2ν 6 bands taking into account the relevant Coriolis and Fermi resonances. Abstract The 12.6 µm region of nitryl chloride (ClNO 2) has been recorded at high resolution (0.00102 cm−1) using a Fourier transform spectrometer and the SOLEIL synchrotron light source. ClNO 2 was found during our studies of chlorine nitrate and, after fractional distillations it was introduced into a multipass White cell made of inert materials, with an optical path length of 5.4 m. 320 scans were recorded for the range 600–980 cm−1 at a total pressure of 0.23 hPa and a temperature of 250 K. A thorough analysis of the ν 2 , 2ν 6 and 2ν 3 bands of 35ClNO 2 and of the ν 2 band of 37ClNO 2 falling in this region has been carried out using a Hamiltonian model which takes explicitly into account the numerous resonances affecting the ro-vibrational energy levels; especially the C-type Coriolis resonance between the ν 2 and the ν 3 + ν 6 modes. These two modes are only separated by less than 20 cm−1 and are thoroughly mixed. From the fittings, the following band centers were derived: ν o (ν 2) = 792.761264(60) cm−1, ν o (ν 3 + ν 6) = 775.8923(20) cm−1, ν o (2ν 3) = 734.183576(20) cm−1 and ν o (2ν 6) = 819.836727(90) cm−1 for 35ClNO 2 and ν o (ν 2) = 792.258284(30) cm−1 and ν o (ν 3 + ν 6) = 774.6826(30) cm−1 for 37ClNO 2 where the uncertainties are one standard deviation. Due to their sharp Q branches falling into an atmospheric "window", the detection of the ν 2 bands might be an advantageous route for future attempts to quantify atmospheric ClNO 2 , using infrared techniques. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Measurements of nitryl chloride (ClNO2) and its precursors (O3, NO2, particulate chloride) were made in 2014–2016 at three contrasting locations in the United Kingdom: Leicester, Penlee Point and Weybourne. ClNO2 was observed at all sites and in every season, with the highest concentrations between 00:00 and 04:00 GMT. The median nocturnal concentration of ClNO2 ranged between the detection limit (4.2 ppt) and 139 ppt. A clear seasonal cycle, with maxima in spring and winter, and significant differences between locations in the same season were observed. The main source of particulate chloride was sea salt aerosol (including at Leicester, ∼200 km from the coast). In general, ClNO2 levels were controlled by the concentrations of O3 and NO2, rather than by the uptake and reaction of N2O5 with particulate chloride. Under these conditions, the seasonality and geographical distribution of ClNO2 can be explained in terms of O3‐limited and NO2‐limited regimes affecting the formation of the N2O5 precursor. A global version of the GEOS‐Chem model at medium resolution (2° × 2.5°) was not able to fully capture the observed seasonality of ClNO2, mostly because the model overestimated the concentrations of the precursors, particularly of nocturnal O3. A higher‐resolution (0.25° × 0.3125°) version of GEOS‐Chem showed better agreement with the observations, although it still overestimated ClNO2 concentrations during summer. [ABSTRACT FROM AUTHOR]

• Nitryl chloride, ClNO 2 , is a trace component of environmental importance. • Higher resolution Fourier transform spectroscopy resulting in a well resolved spectrum. • New and extensive line positions analysis of the ν 3 bands of 35ClNO 2 and 37ClNO 2. • Line list of both bands has been generated for atmospheric retrieval. A new investigation of the ν 3 bands of 35ClNO 2 and 37ClNO 2 , located around 370 cm−1 has been performed using a high resolution (0.00102 cm−1) Fourier transform spectrum recorded at SOLEIL with highly improved experimental conditions as compared to previous work [Orphal J, Morillon-Chapey M, Klee S, Mellau GC, Winnewisser M. J Mol Spectrosc 1998;190:101–6]: (i) the use of synchrotron radiation which resulted in a better signal-to-noise ratio ; (ii) a resolution twice better ; (iii) a low temperature (221 K) with an optical path length of 8.16 m, allowing low pressure (0.025 hPa) in the sample leading to a well resolved spectrum. As a consequence, significantly better results than previously were obtained. Thanks to the new experimental conditions, the line assignments were pursued up to higher J and K a quantum number values, J = 83 and K a = 44. For both isotopomers, a total of 6331 transitions were reproduced with a root-mean-square deviation of 2 × 10−4 cm−1 using a Watson-type A -reduced Hamiltonian. Improved rotational and centrifugal distortion constants for the ν 3 fundamental bands of 35ClNO 2 and 37ClNO 2 have been determined. The band centers are 370.1510773(92) cm−1 for the ν 3 fundamental band of 35ClNO 2 and 364.5218094(96) cm−1 for the ν 3 fundamental band of 37ClNO 2. The synthetic line list obtained in this study could be interesting for future measurements of ClNO 2 in the atmosphere, e.g. using the new space mission FORUM which is one of the concepts chosen by ESA to be developed further and which opens up a new window (150–1400 cm−1) for understanding and quantifying the radiative processes, as well as air quality and pollution effects. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nitryl chloride (ClNO2) plays an important role in the budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species. ClNO2 is formed from the heterogeneous uptake and reaction of dinitrogen pentoxide (N2O5) on chloride‐containing aerosol, with a production yield, ϕ(ClNO2), defined as the moles of ClNO2 produced relative to N2O5 lost. The ϕ(ClNO2) has been increasingly incorporated into 3‐D chemical models where it is parameterized based on laboratory‐derived kinetics and currently accepted aqueous‐phase formation mechanism. This parameterization models ϕ(ClNO2) as a function of the aerosol chloride to water molar ratio. Box model simulations of night flights during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign derived 3,425 individual ϕ(ClNO2) values with a median of 0.138 and range of 0.003 to 1. Comparison of the box model median to those predicted by two other field‐based ϕ(ClNO2) derivation methods agreed within a factor of 1.3, within the uncertainties of each method. In contrast, the box model median was 75–84% lower than predictions from the laboratory‐based parameterization (i.e., [parameterization − box model]/parameterization). An evaluation of factors influencing this difference reveals a positive dependence of ϕ(ClNO2) on aerosol water, opposite to the currently parameterized trend. Additional factors may include aqueous‐phase competition reactions for the nitronium ion intermediate and/or direct ClNO2 loss mechanisms. Further laboratory studies of ClNO2 formation and the impacts of aerosol water, sulfate, organics, and ClNO2 aqueous‐phase reactions are required to elucidate and quantify these processes on ambient aerosol, critical for the development of a robust ϕ(ClNO2) parameterization. Key Points: Box modeling of wintertime measurements over the eastern United States provides the first determinations of ClNO2 yields (ϕ(ClNO2)) from aircraftThe median ϕ(ClNO2) value derived from the box model is overpredicted by 75‐84% by the current laboratory‐based ϕ(ClNO2) parameterizationModeled and parameterized ϕ(ClNO2) values show opposite trends with aerosol water, suggesting an incorrectly parameterized dependence [ABSTRACT FROM AUTHOR]