N₂O₅ uptake onto saline mineral dust: a potential missing source of tropospheric ClNO₂ in inland China

Nitryl chloride (ClNO2), an important precursor of Cl atoms, significantly affects atmospheric oxidation capacity and O3 formation. However, sources of ClNO2 in inland China have not been fully elucidated. In this work, laboratory experiments were conducted to investigate heterogeneous reaction of N2O5 with eight saline mineral dust samples collected from different regions in China, and substantial formation of ClNO2 was observed. ClNO2 yields, φ(ClNO2), showed large variations (ranging from < 0.05 to ~0.77) for different saline mineral dust samples, largely depending on mass fractions of particulate chloride. In addition, for different saline mineral dust samples, φ(ClNO2) could increase, decrease or show insignificant change as RH increased from 18 % to 75 %. We further found that current parameterizations significantly overestimated φ(ClNO2) for heterogeneous uptake of N2O5 onto saline mineral dust. Assuming a uniform φ(ClNO2) value of 0.10 for N2O5 uptake onto mineral dust, we used a 3-D chemical transport model to assess the impact of this reaction on tropospheric ClNO2 in China, and found that weekly mean nighttime maximum ClNO2 mixing ratios could be increased by up to 85 pptv during a severe dust event in May 2017. Overall, our work showed that heterogeneous reaction of N2O5 with saline mineral dust could be an important source of tropospheric ClNO2 in inland China.

Urban inland wintertime N₂O₅ and ClNO₂ influenced by snow-covered ground, air turbulence, and precipitation

The atmospheric multiphase reaction of dinitrogen pentoxide (N2O5) with chloride-containing aerosol particles produces nitryl chloride (ClNO2), which has been observed across the globe. The photolysis of ClNO2 produces chlorine radicals and nitrogen dioxide (NO2), which alter pollutant fates and air quality. However, the effects of local meteorology on near-surface ClNO2 production are not yet well understood, as most observational and modeling studies focus on periods of clear conditions. During a field campaign in Kalamazoo, Michigan from January–February 2018, N2O5 and ClNO2 were measured using chemical ionization mass spectrometry, with simultaneous measurements of atmospheric particulate matter and meteorological parameters. We examine the impacts of atmospheric turbulence, precipitation (snow, rain) and fog, and ground cover (snow-covered and bare ground) on the abundances of ClNO2 and N2O5. N2O5 mole ratios were lowest during periods of lower turbulence and were not statistically significantly different between snow-covered and bare ground. In contrast, ClNO2 mole ratios were highest, on average, over snow-covered ground, due to saline snowpack ClNO2 production. Both N2O5 and ClNO2 mole ratios were lowest, on average, during rainfall and fog because of scavenging, with N2O5 scavenging by fog droplets likely contributing to observed increased particulate nitrate concentrations. These observations, specifically those during active precipitation and with snow-covered ground, highlight important processes, including N2O5 and ClNO2 wet scavenging, fog nitrate production, and snowpack ClNO2 production, that govern the variability in observed atmospheric chlorine and nitrogen chemistry and are missed when considering only clear conditions.

Effects of chlorine chemistry combined with Heterogeneous N2O5 reactions on PM2.5 and Ozone formation in China

&lt;p&gt;Heterogeneous reactivity of N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; on Cl-containing aerosols can produce nitric acid (HNO&lt;sub&gt;3&lt;/sub&gt;) and nitryl chloride (ClNO&lt;sub&gt;2&lt;/sub&gt;), which is a critical parameter in assessing O&lt;sub&gt;3&lt;/sub&gt; variation, nitrate production, and chloride activation. In this study, we used the GEOS-Chem to quantify the effects of chlorine chemistry on fine particulate matter (PM2.5) and O&lt;sub&gt;3 &lt;/sub&gt;formation across China, with comprehensive anthropogenic chlorine emissions (HCl + Cl&lt;sub&gt;2&lt;/sub&gt; + particulate Cl&lt;sup&gt;-&lt;/sup&gt;). We extended GEOS-Chem to include the heterogeneous reactions of N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; and assess the impact of different parameterizations of uptake coefficient of N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;(&amp;#947;(N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;)), and ClNO&lt;sub&gt;2&lt;/sub&gt; yield (&amp;#934;(ClNO&lt;sub&gt;2&lt;/sub&gt;)). Observation from three representative sites in the north, east and south China were selected to assess the model performance with regard to particulate chloride. With the addition of anthropogenic chlorine emissions, model bias in particulate chloride decreased from -79.10% to -39.64% (Dongying), -60.55% to -34.14% (Shenzhen), and -77.53% to -39.97% (Gucheng), respectively. The results show that N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;-ClNO&lt;sub&gt;2&lt;/sub&gt; chemistry can reduce the concentration of NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; and NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;, but increase the concentration of SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2- &lt;/sup&gt;slightly, consequently leading to a reduction in the concentration of PM2.5 in China(0.5 &amp;#956;g/m&lt;sup&gt;3&lt;/sup&gt; on average and 1.8 &amp;#956;g/m&lt;sup&gt;3 &lt;/sup&gt;on haze days). On the other hand, the monthly average O&lt;sub&gt;3&lt;/sub&gt; MDA8 concentration in China increased by up to 2 ppbv(8 ppbv on haze days), which is mainly due to the increase of OH concentration associated with the photolysis of ClNO&lt;sub&gt;2.&lt;/sub&gt;&lt;/p&gt;

Formation of highly oxygenated organic molecules from chlorine-atom-initiated oxidation of alpha-pinene

Highly oxygenated organic molecules (HOMs) from atmospheric oxidation of alpha-pinene can irreversibly condense to particles and contribute to secondary organic aerosol (SOA) formation. Recently, the formation of nitryl chloride (ClNO2) from heterogeneous reactions, followed by its subsequent photolysis, is suggested to be an important source of chlorine atoms in many parts of the atmosphere. However, the oxidation of monoterpenes such as alpha-pinene by chlorine atoms has received very little attention, and the ability of this reaction to form HOMs is completely unstudied. Here, chamber experiments were conducted with alpha-pinene and chlorine under low- and high-nitrogen-oxide (NOx, NOx=NO+NO2) conditions. A nitrate-based CI-APi-ToF (chemical ionization–atmospheric pressure interface–time of flight) mass spectrometer was used to measure HOM products. Clear distributions of monomers with 9–10 carbon atoms and dimers with 18–20 carbon atoms were observed under low-NOx conditions. With increased concentration of NOx within the chamber, the formation of dimers was suppressed due to the reactions of peroxy radicals with NO. We estimated the HOM yields from chlorine-initiated oxidation of alpha-pinene under low-NOx conditions to be around 1.8 %, though with a substantial uncertainty range (0.8 %–4 %) due to lack of suitable calibration methods. Corresponding yields at high NOx could not be determined because of concurrent ozonolysis reactions. Our study demonstrates that also the oxidation of alpha-pinene by chlorine atoms and yield low-volatility organic compounds.

Effect of nitryl chloride chemistry on oxidants concentrations during the KORUS-AQ campaign

&lt;p&gt;Nitryl chloride (ClNO&lt;sub&gt;2&lt;/sub&gt;) plays an important role as a night-time reservoir of NO&lt;sub&gt;X&lt;/sub&gt; and the source of Cl radical during the daytime, which consequently affects the ozone photochemistry. Its impacts on regional air quality in East Asia, however, are not fully understood so far. We here use extensive observations during the international KORea-US cooperative Air Quality field study in Korea (KORUS-AQ), which occurred in May-June 2016, with a 3-D chemistry transport model to examine the impacts of ClNO&lt;sub&gt;2&lt;/sub&gt; chemistry on radical species and total nitrate concentrations in East Asia. We first update the model by implementing chlorine chemistry and latest anthropogenic chlorine emissions of China and South Korea. We conduct model simulations for May-June, 2016 and validate the model by comparing against the observations from the KORUS-AQ campaign. We find that the ClNO&lt;sub&gt;2&lt;/sub&gt; chemistry in the model results in an increase of ozone by ~1.4 ppbv (~2.5%), Cl radical by ~ 4.6x10&lt;sup&gt;3&lt;/sup&gt; molec cm&lt;sup&gt;-3&lt;/sup&gt; (~3600%), OH ~8.2x10&lt;sup&gt;4&lt;/sup&gt; molec cm&lt;sup&gt;-3&lt;/sup&gt; (~5.3%), HO&lt;sub&gt;2&lt;/sub&gt; ~6.6 molec cm&lt;sup&gt;-3&lt;/sup&gt; (~3.0%), a decrease of TNO&lt;sub&gt;3&lt;/sub&gt; (HNO&lt;sub&gt;3&lt;/sub&gt; + nitrate aerosol) concentrations by ~2 &amp;#956;g m&lt;sup&gt;-3&lt;/sup&gt; on a daily mean basis during the campaign. Overall, the enhanced conversion of NO to NO&lt;sub&gt;2&lt;/sub&gt; driven by ClNO&lt;sub&gt;2&lt;/sub&gt; chemistry contributes to higher oxidant concentrations in the model. As a result, the updated model shows a better agreement with the observations in Korea during the KORUS-AQ campaign.&lt;/p&gt;

A novel spectroscopic approach for detection of chlorine reservoir species: HCl-TILDAS

&lt;p&gt;Atomic chlorine radicals are known to affect atmospheric oxidation and pollutant lifetimes, but are challenging to detect due to their low ambient concentrations.&amp;#160; A lack of field observations limits useful assessments of the impacts of tropospheric chlorine oxidation on important atmospheric processes, such as regional ozone production, reactive nitrogen loss, and global methane removal.&amp;#160; In the last decade, instrumental innovations have enabled detection and speciation of much more stable chlorine atom reservoir species, such as nitryl chloride, through techniques such as cavity ring down spectroscopy and mass spectrometry.&amp;#160; HCl is the most abundant and long-lived tropospheric chlorine reservoir species, yet few observations exist.&amp;#160; Here, we present a specific method for detection of HCl via Tunable Laser Infrared Direct Absorption Spectrometer (TILDAS), which has been further extended for the detection of nitryl chloride.&amp;#160; This analytical method has several advantages over current observational techniques (e.g. chemical ionisation mass spectrometry), and will provide a much needed constraint on the tropospheric chlorine atom budget.&lt;/p&gt;

Integration of airborne and ground observations of nitryl chloride in the Seoul metropolitan area and the implications on regional oxidation capacity during KORUS-AQ 2016

Nitryl chloride (ClNO2) is a radical reservoir species that releases chlorine radicals upon photolysis. An integrated analysis of the impact of ClNO2 on regional photochemistry in the Seoul metropolitan area (SMA) during the Korea–United States Air Quality Study (KORUS-AQ) 2016 field campaign is presented. Comprehensive multiplatform observations were conducted aboard the NASA DC-8 and at two ground sites (Olympic Park, OP; Taehwa Research Forest, TRF), representing an urbanized area and a forested suburban region, respectively. Positive correlations between daytime Cl2 and ClNO2 were observed at both sites, the slope of which was dependent on O3 levels. The possible mechanisms are explored through box model simulations constrained with observations. The overall diurnal variations in ClNO2 at both sites appeared similar but the nighttime variations were systematically different. For about half of the observation days at the OP site the level of ClNO2 increased at sunset but rapidly decreased at around midnight. On the other hand, high levels were observed throughout the night at the TRF site. Significant levels of ClNO2 were observed at both sites for 4–5 h after sunrise. Airborne observations, box model calculations, and back-trajectory analysis consistently show that these high levels of ClNO2 in the morning are likely from vertical or horizontal transport of air masses from the west. Box model results show that chlorine-radical-initiated chemistry can impact the regional photochemistry by elevating net chemical production rates of ozone by ∼25 % in the morning.

Formation of highly oxygenated organic molecules from chlorine atom initiated oxidation of alpha-pinene

Highly oxygenated organic molecules (HOMs) from atmospheric oxidation of alpha-pinene can irreversibly condense to particles and contribute to secondary organic aerosol (SOA) formation. Recently, the formation of nitryl chloride (ClNO2) from heterogeneous reactions, followed by its subsequent photolysis is suggested to be an important source of chlorine atoms in many parts of the atmosphere. However, the oxidation of monoterpenes such as alpha-pinene by chlorine atoms has received very little attention, and the ability of this reaction to form HOM is completely unstudied. Here, chamber experiments were conducted with alpha-pinene and chlorine under low and high nitrogen oxide (NOx) conditions. A NO3-based CI-APi-TOF was used to measure HOM products. Clear distributions of monomers with 9–10 carbon atoms and dimers with 18–20 carbon atoms were observed under low NOx conditions. With increased concentration of NOx within the chamber, the formation of dimers was suppressed due to the reactions of peroxy radicals with NO. We estimated the HOM yields from chlorine-initiated oxidation of alpha-pinene under low-NOx conditions to be around 1.8 %, though with a substantial uncertainty range (0.8–4 %) due to lack of suitable calibration methods. Corresponding yields at high NOx could not be determined because of concurrent ozonolysis reactions. Our study demonstrates that chlorine atoms also initiated oxidation of alpha-pinene and yields low volatility organic compounds.