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Observations are reported of HClO3 and HClO4 in the atmosphere and their widespread occurrence over the pan-Arctic during spring, providing further insights into atmospheric chlorine cycling in the polar environment.

Nitrate radical (NO3) is a critical nocturnal atmospheric oxidant in the troposphere, which widely affects the fate of air pollutants and regulates air quality. Many previous works have reported the chemistry of NO3 in inland regions of China, while less study targets marine regions. Here, we present a field measurement of the NO3 reservoir, dinitrogen pentoxide (N2O5), and related species at a typical marine site (Da Wan Shan Island) located in the South China Sea in the winter of 2021. Two patterns of air masses were captured during the campaign, including the dominant airmass from inland China (IAM) with a percentage of ~84%, and the airmass from eastern coastal areas (CAM) with ~16%. During the IAM period, the NO3 production rate reached 1.6 ± 0.9 ppbv h-1 due to the transportation of the polluted urban plume with high NOx and O3. While the average nocturnal N2O5 and the calculated NO3 mixing ratio were 119.5 ± 128.6 pptv and 9.9 ± 12.5 pptv, respectively, and the steady state lifetime of NO3 was 0.5 ± 0.7 min on average, indicating intensive nighttime chemistry and rapid NO3 loss at this site. By examining the reaction of NO3 with volatile organic compounds (VOCs) and N2O5 heterogeneous hydrolysis, we revealed that these two reaction pathways were not responsible for the NO3 loss (<20%), since the NO3 reactivity (k(NO3)) towards VOCs was small (5.2×10-3 s-1) and the aerosol loading was low. Instead, NO was proposed to significantly contribute to nocturnal NO3 loss at this site, despite the nocturnal NO concentration always at sub-ppbv level and near the instrument detection limit. It might be from the local soil emission. We infer that the nocturnal chemical NO3 reactions would be largely enhanced once without NO emission in the open ocean after the air mass passes through this site, thus highlighting the strong influences of the urban outflow to the downward marine areas in terms of nighttime chemistry. During the CAM period, nocturnal ozone was higher, while NOx was much lower. The NO3 production was still very fast, with a rate of 1.2 ppbv h-1. With the absence of N2O5 measurement in this period, the NO3 reactivity towards VOCs and N2O5 uptake were calculated to assess NO3 loss processes. We showed that the average k(NO3) from VOCs (56.5%, 2.6 ± 0.9×10-3 s-1) was higher than N2O5 uptake (43.5%, 2.0 ± 1.5× 10-3 s-1) during the CAM period, indicating a longer NO3/N2O5 lifetime than that during IAM period. This study improves the understanding of the nocturnal NO3 budget and environmental impacts with the interaction of anthropogenic and natural activities in marine regions. [ABSTRACT FROM AUTHOR]

Several studies have investigated New Particle Formation (NPF) events from various sites ranging from pristine locations, including (boreal) forest sites to urban areas. However, there is still a dearth of studies investigating NPF processes and subsequent aerosol growth in coastal yet semi-urban sites, where the tropospheric layer is a concoction of biogenic and anthropogenic gases and particles. The investigation of factors leading to NPF becomes extremely complex due to the highly dynamic meteorological conditions at the coastline especially when combined with both continental and oceanic weather conditions. Herein, we engage a comprehensive study of particle number size distributions and aerosol-forming precursor vapors at the coastal semi-urban site in Helsinki, Finland. The measurement period, 25 June 2019-18 August 2019, was timed with the recurring cyanobacterial summer bloom in the Baltic Sea region and coastal regions of Finland. Our study recorded several regional/local NPF and aerosol burst events during this period. Although the overall anthropogenic influence on Sulfuric Acid (SA) concentrations was low during the measurement period, we observed that the regional or local NPF events, characterized by SA concentrations in the order of 107 molecules per cm-3 occurred mostly when the air mass travelled over the land areas. Interestingly, when the air mass travelled over the Baltic Sea, an area enriched with Algae and cyanobacterial blooms, high Iodic Acid (IA) concentration coincided with an aerosol burst or a spike event at the measurement site. Further, SA-rich bursts were seen when the air mass travelled over the Gulf of Bothnia, enriched with cyanobacterial blooms. The two most important factors affecting aerosol precursor vapor concentrations, and thus the aerosol formation, were (1) the type of phytoplankton species and intensity of bloom present in the coastal regions of Finland/ Baltic Sea and (2) the wind direction. During the events, most of the growth of sub-3 nm particles was probably due to SA, rather than IA or MSA, however much of the particle growth remained unexplained indicative of the strong role of organics in the growth of particles, especially in the 3-7 nm particle size range. Further studies are needed to explore the role of organics in NPF events and the potential influence of cyanobacterial blooms in coastal locations. [ABSTRACT FROM AUTHOR]

Nitryl chloride (ClNO2) is an important chlorine reservoir in the atmosphere that affects the oxidation of volatile organic compounds (VOCs) and the production of ROx radicals and ozone (O3). This study presents measurements of ClNO2 and related compounds at urban, rural, and mountain sites in the winter of 2017-2018 over the North China Plain (NCP). The nocturnal concentrations of ClNO2 were lower at the urban and rural sites but higher at the mountain site. The winter concentrations of ClNO2 were generally lower than the summer concentrations that were previously observed at these sites, which was due to the lower nitrate radical (NO3) production rate (P(NO3) and the smaller N2O5 uptake coefficients (?(N2O5)) in winter, despite the higher dinitrogen pentoxide (N2O5) to NO3 ratios in winter. Significant daytime peaks of ClNO2 were observed at all the sites during the winter campaigns, with ClNO2 mixing ratios of up to 1.3 ppbv. Vertical transport of ClNO2 from the residual layers and prolonged photochemical lifetime of ClNO2 in winter may explain the elevated daytime concentrations. The daytime-averaged chlorine radical (Cl) production rates (P(Cl)) from the daytime ClNO2 were 0.17, 0.11, and 0.12 ppbv h-1 at the rural, urban, and mountain sites, respectively, which were approximately 3-4 times higher than the campaign-averaged conditions. Box model calculations showed that the Cl atoms liberated during the daytime peaks of ClNO2 increased the ROx levels by up to 27-37 % and increased the daily O3 productions by up to 13-18 %. [ABSTRACT FROM AUTHOR]

Iodine species are important in the marine atmosphere for oxidation and new-particle formation. Understanding iodine chemistry and iodine new-particle formation requires high time resolution, high sensitivity, and simultaneous measurements of many iodine species. Here, we describe the application of bromide chemical ionization mass spectrometers (Br-CIMS) to this task. During iodine new-particle formation experiments in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phase iodine species and sulfuric acid using two Br-CIMS, one coupled to a Multi-scheme chemical IONization inlet (Br-MION-CIMS) and the other to a Filter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offline calibrations and inter-comparisons with other instruments attached to the CLOUD chamber, we have quantified the sensitivities of the Br-MION-CIMS to HOI, I2, and H2SO4 and obtain detection limits of 5.8 x 106, 6.3 x 105, and 2.0 x 105 molec cm-3, respectively, for a 2-min integration time. From binding energy calculations, we estimate the detection limit for HIO3 to be 1.2 x 105 molec cm-3, based on an assumption of maximum sensitivity. Detection limits in the Br-FIGAERO-CIMS are around one order of magnitude higher than those in the Br-MION-CIMS; for example, the detection limits for HOI and HIO3 are 3.3 x 107 and 5.1 x 106 molec cm-3, respectively. Our comparisons of the performance of the MION inlet and the FIGAERO inlet show that bromide chemical ionization mass spectrometers using either atmospheric pressure or reduced pressure interfaces are well-matched to measuring iodine species and sulfuric acid in marine environments. [ABSTRACT FROM AUTHOR]

Heterogeneous reactivity of N2O5 on aerosols is a critical parameter in assessing NOx fate, nitrate production, and particulate chloride activation. Accurate measurement of its uptake coefficient (γN2O5) and representation in air quality model are challenging, especially in the polluted environment. With an in-situ aerosol flow tube system, the γN2O5 was directly measured on ambient aerosols at two rural sites in northern and southern China. The results were analyzed together with the γN2O5 derived from previous field studies in China to obtain a holistic picture of N2O5 uptake and the influencing factors under various climatic and chemical conditions. The field derived/measured γN2O5 was generally promoted by the aerosol water content and suppressed by particle nitrate. Significant discrepancies were found between the measured γN2O5 and that estimated from laboratory-determined parameterizations. An observation-based empirical parameterization was derived in the present work, which better reproduced the mean value and variability of the observed γN2O5. Incorporating this new parameterization in a regional air quality model (WRF-CMAQ) has improved the simulation of nitrogen oxides and secondary nitrate in the polluted regions of China. [ABSTRACT FROM AUTHOR]

The uptake of methanesulfonic acid (MSA) on existing particles is a major route of the particulate MSA formation, however, MSA uptake on different particles is still lack of knowledge. Characteristics of MSA uptake on different aerosol particles were investigated in polynya regions of the Ross Sea, Antarctica. Particulate MSA mass concentrations, as well as aerosol populations and size distributions, were observed simultaneously for the first time to access the uptake of MSA on different particles. The results showed that MSA mass concentration did not always reflect MSA particle population in the marine atmosphere. MSA uptake on aerosol particles increased the particle size and changed aerosol chemical compositions, but did not increase the particle population. The uptake rates of MSA on existing particles were significantly influenced by aerosol chemical properties. The favor uptake of MSA occurred on the sea salt particles, as MSA-Na and MSA-Mg particles were abundant in the Na and Mg particles, accounting for 0.43 ± 0.21 and 0.41 ± 0.20 of the total Na and Mg particles, respectively. However, acidic and hydrophobic particles suppressed the MSA uptake, as MSA-EC and MSA-SO42- accounted only 0.24 ± 0.68 and 0.26 ± 0.47 of the total EC and SO42- particles, respectively. The results extended the knowledge of the formation and environmental behavior of MSA in the marine atmosphere. [ABSTRACT FROM AUTHOR]

Heterogeneous uptake of dinitrogen pentoxide (N2O5) and production of nitryl chloride (ClNO2) are important nocturnal atmospheric processes that have significant implications for the production of secondary pollutants. However, the understanding of N2O5 uptake processes and ClNO2 production remains limited, especially in China. This study presents a field investigation of the N2O5 heterogeneous uptake coefficient (γ(N2O5)) and ClNO2 production yield (ϕ) in a polluted area of northern China during the summer of 2014. The N2O5 uptake coefficient and ClNO2 yield were estimated in 10 selected cases using simultaneously measured concentrations of ClNO2 and particulate nitrate. The determined γ(N2O5) and ϕ values varied greatly, with an average of 0.022 for γ(N2O5) (in range of 0.006–0.034) and 0.34 for ϕ (range, 0.07–1.04). The variations in γ(N2O5) could not be fully explained by the previously derived parameterizations of N2O5 uptake that consider nitrate, chloride, and the organic coating. Heterogeneous uptake of N2O5 was found to have a strong positive dependence on the relative humidity and aerosol water content. This result suggests that the heterogeneous uptake of N2O5 in Wangdu is governed mainly by the amount of water in the aerosol, a phenomenon that differs from other field observations in the United States and Europe. Laboratory-derived parameterization also overestimated the ClNO2 yield. The observation-derived ϕ showed a decreasing trend with an increasing ratio of acetonitrile to carbon monoxide, an indicator of biomass burning emissions, which suggests a possible suppressive effect on the production yield of ClNO2 in the plumes influenced by biomass burning in this region. The findings of this study illustrate the need to improve our understanding and to parameterize the key factors for γ(N2O5) and ϕ to accurately assess the photochemical and haze pollution. [ABSTRACT FROM AUTHOR]

Dinitrogen pentoxide (N2O5) and nitryl chloride (ClNO2) are key species in nocturnal tropospheric chemistry and have significant effects on particulate nitrate formation and the following day's photochemistry through chlorine radical production and NOx recycling upon photolysis of ClNO2. To better understand the roles of N2O5 and ClNO2 in the high-aerosol-loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465ma.s.l.) in the North China Plain (NCP) during the summer of 2014. Elevated ClNO2 plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1 min), whilst N2O5 was typically present at very low levels (< 30pptv), indicating fast heterogeneous N2O5 hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO2-laden air was caused by the transport of NOx - rich plumes from the coal-fired industry and power plants in the NCP. The heterogeneous N2O5 uptake coefficient (γ) and ClNO2 yield (φ) were estimated from steady-state analysis and observed growth rate of ClNO2. The derived γ and φ exhibited high variability, with means of 0.061 ± 0.025 and 0.28 ± 0.24, respectively. These values are higher than those derived from previous laboratory and field studies in other regions and cannot be well characterized by model parameterizations. Fast heterogeneous N2O5 reactions dominated the nocturnal NOx loss in the residual layer over this region and contributed to substantial nitrate formation of up to 17 µg m-3. The estimated nocturnal nitrate formation rates ranged from 0.2 to 4.8 µg m-3 h-1 in various plumes, with a mean of 2.2 ± 1.4 µg m-3 h-1. The results demonstrate the significance of heterogeneous N2O5 reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NOx transformation, and thus their potential impacts on regional haze pollution in northern China. [ABSTRACT FROM AUTHOR]

Dinitrogen pentoxide (N2O5) and nitryl chloride (ClNO2) are key species in nocturnal tropospheric chemistry, and have significant effects on particulate nitrate formation and the following day's photochemistry. To better understand the roles of N2O5 and ClNO2 in the high aerosol loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465 m a.s.l.) in the North China Plain (NCP) during the summer of 2014. Elevated ClNO2 plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1-min), whilst N2O5 was typically present at very low levels (<30 pptv), indicating fast heterogeneous N2O5 hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO2-laden air was caused by the transport of NOx-rich plumes from the coal-fired power plants in the NCP. The heterogeneous N2O5 uptake coefficient (γ) and ClNO2 yield during the campaign exhibited high variability, with means of 0.061 ± 0.025 and 0.27 ± 0.24, respectively. These derived values are higher than those derived from previous laboratory and field studies in other regions, and cannot be well characterized by model parameterizations. Fast heterogeneous N2O5 reactions dominated the nocturnal NOx loss in the residual layer over this region, and contributed to substantial nitrate formation of up to 17 μg m-3. The determined nocturnal nitrate formation rates ranged from 0.2 to 4.8 μg m-3 hr-1 in various plumes, with a mean of 2.2 ± 1.4 μg m-3 h-1. The results demonstrate the significance of heterogeneous N2O5 reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NOx transformation, and thus potential impacts on regional haze pollution in northern China. [ABSTRACT FROM AUTHOR]

Nitryl chloride (ClNO2) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO2 and its exact role are not fully understood under different environmental conditions. During the summer of 2014, we deployed a chemical ionization mass spectrometer to measure ClNO2 and dinitrogen pentoxide (N2O5) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO2 (> 350 pptv) were observed at most of the nights with low levels of N2O5 (< 200 pptv). The highest ClNO2 mixing ratio of 2070 pptv (1 min average) was observed in a plume from a megacity (Tianjin), and was characterized with a faster N2O5 heterogeneous loss rate and ClNO2 production rate compared to average conditions. The abundant ClNO2 concentration kept increasing even after sunrise, and reached a peak 4 h later. Such highly sustained ClNO2 peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis shows that the sustained ClNO2 morning peaks are caused by significant ClNO2 production in the residual layer at night followed by downward mixing after breakup of the nocturnal inversion layer in the morning. We estimated that ~1.7-4.0 ppbv of ClNO2 would exist in the residual layer in order to maintain the observed morning ClNO2 peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO2 produced chlorine radical with a rate up to 1.12 ppbv h-1, accounting for 10-30% of primary ROx production in the morning hours. The perturbation in total radical production leads to an increase of integrated daytime net ozone production by 3% (4.3 ppbv) on average, and with a larger increase of 13% (11 ppbv) in megacity outflow that was characterized with higher ClNO2 and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon (NMHC) ratio. [ABSTRACT FROM AUTHOR]

The uptake of dinitrogen pentoxide (N2O5/ on aerosol surfaces and the subsequent production of nitryl chloride (ClNO2/ can have a significant impact on the oxidising capability and thus on secondary pollutants such as ozone. The range of such an impact, however, has not been well quantified in different geographical regions. In this study, we applied the Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model to investigate the impact of the N2O5 uptake processes in the Hong Kong-Pearl River Delta (HK-PRD) region, where the highest ever reported N2O5 and ClNO2 concentrations were observed in our recent field study. We first incorporated into the WRF-Chem an aerosol thermodynamics model (ISORROPIA II), recent parameterisations for N2O5 heterogeneous uptake and ClNO2 production and gas-phase chlorine chemistry. The revised model was then used to simulate the spatiotemporal distribution of N2O5 and ClNO2 over the HK-PRD region and the impact of N2O5 uptake and Cl activation on ozone and reactive nitrogen in the planetary boundary layer (PBL). The updated model can generally capture the temporal variation of N2O5 and ClNO2 observed at a mountaintop site in Hong Kong, but it overestimates N2O5 uptake and ClNO2 production. The model results suggest that under average conditions, elevated levels of ClNO2 (> 0.25 ppb within the PBL) are present in the south-western PRD, with the highest values (> 1.00 ppb) predicted near the ground surface (0-200m above ground level; a.g.l.). In contrast, during the night when very high levels of ClNO2 and N2O5 were measured in well-processed plumes from the PRD, ClNO2 is mostly concentrated within the residual layer (~300ma.g.l.). The addition of N2O5 heterogeneous uptake and Cl activation reduces the NO and NO2 levels by as much as 1.93 ppb (~7.4%) and 4.73 ppb (~16.2%), respectively, and it increases the total nitrate and ozone concentrations by up to 13.45 μgm-3 (~57.4%) and 7.23 ppb (~16.3%), respectively, in the PBL. Sensitivity tests show that the simulated chloride and ClNO2 concentrations are highly sensitive to chlorine emission. Our study suggests the need to measure the vertical profiles of N2O5 = ClNO2 under various meteorological conditions, to consider the chemistry of N2O5 = ClNO2 in the chemical transport model and to develop an updated chlorine emission inventory over China. [ABSTRACT FROM AUTHOR]

The uptake of dinitrogen pentoxide (N2O5) on aerosol surfaces and the subsequent production of nitryl chloride (ClNO2) can have significant impact on the oxidising capability and thus on secondary pollutants such as ozone. The range of such impact, however, has not well been quantified in different geographical regions. In this study, we applied Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model to investigate the impact of the N2O5 uptake processes in the Hong Kong-Pearl River Delta (HK-PRD) region, where the highest ever-reported N2O5 and ClNO2 concentrations were observed in our recent field study. We first incorporated into the WRF-Chem an aerosol thermodynamics model (ISORROPIA II), recent parameterisations for N2O5 heterogeneous uptake and ClNO2 production and gas-phase chlorine chemistry. The revised model was then used to simulate the spatiotemporal distribution of N2O5 and ClNO2 over the HK-PRD region and the impact of N2O5 uptake and Cl activation on ozone and reactive nitrogen in the planetary boundary layer (PBL). The updated model is capable of reproducing the temporal patterns of N2O5 and ClNO2 observed at a mountain-top site in Hong Kong, but overestimates N2O5 uptake and ClNO2 production. The model results suggest that under average meteorological conditions, elevated levels of ClNO2 (> 0.25 ppb within the PBL) are present in the south-western PRD, with the highest values (> 1.00 ppb) predicted near the ground surface (0-200 m above ground level (a.g.l.)). In contrast, during the night when very high levels of ClNO2 and N2O5 were measured in well-processed plumes from the PRD, ClNO2 is mostly concentrated within the residual layer (~ 300 m a.g.l.). The addition of N2O5 heterogeneous uptake and Cl activation reduces the NO and NO2 levels by as much as 1.93 ppb (~ 7.4%) and 4.73 ppb (~ 16.2%), respectively, increases the total nitrate and ozone concentrations by up to 13.45 µg m-3 (~ 57.4%) and 7.23 ppb (~ 16.3%), respectively, in the PBL. Sensitivity tests show that the simulated chloride and ClNO2 concentrations are highly sensitive to chlorine emission. Our study suggests the need to measure the vertical profiles of N2O5/ClNO2 under various meteorological conditions, to consider the chemistry of N2O5/ClNO2 in the chemical transport model, and to develop an updated chlorine emission inventory over China. [ABSTRACT FROM AUTHOR]