Nitryl chloride (ClNO₂) accumulation at night acts as a significant reservoir for active chlorine and impacts the following day's photochemistry when the chlorine atom is liberated at sunrise. Here, we report simultaneous measurements of N₂O₅ and a suite of inorganic halogens including ClNO₂ and reactions of chloride with volatile organic compounds (Cl–VOCs) in the gas and particle phases utilising the Filter Inlet for Gas and AEROsols time-of-flight chemical ionisation mass spectrometer (FIGAERO-ToF-CIMS) during an intensive measurement campaign 40 km northwest of Beijing in May and June 2016. A maximum mixing ratio of 2900 ppt of ClNO₂ was observed with a mean campaign nighttime mixing ratio of 487 ppt, appearing to have an anthropogenic source supported by correlation with SO₂, CO and benzene, which often persisted at high levels after sunrise until midday. This was attributed to such high mixing ratios persisting after numerous e-folding times of the photolytic lifetime enabling the chlorine atom production to reach 2.3 × 10⁵ molecules cm⁻³ from ClNO₂ alone, peaking at 09:30 LT and up to 8.4 × 10⁵ molecules cm⁻³ when including the supporting inorganic halogen measurements. Cl–VOCs were observed in the particle and gas phases for the first time at high time resolution and illustrate how the iodide ToF-CIMS can detect unique markers of chlorine atom chemistry in ambient air from both biogenic and anthropogenic sources. Their presence and abundance can be explained via time series of their measured and steady-state calculated precursors, enabling the assessment of competing OH and chlorine atom oxidation via measurements of products from both of these mechanisms and their relative contribution to secondary organic aerosol (SOA) formation.

A time-of-flight chemical ionization mass spectrometer (CIMS) utilizing the Filter Inlet for Gas and Aerosol (FIGAERO) was deployed at a regional site 40 km north-west of Beijing and successfully identified and measured 17 sulfur-containing organics (SCOs are organo/nitrooxy organosulfates and sulfonates) with biogenic and anthropogenic precursors. The SCOs were quantified using laboratory-synthesized standards of lactic acid sulfate and nitrophenol organosulfate (NP OS). The variation in field observations was confirmed by comparison to offline measurement techniques (orbitrap and high-performance liquid chromatography, HPLC) using daily averages. The mean total (of the 17 identified by CIMS) SCO particle mass concentration was 210 ± 110 ng m ⁻³ and had a maximum of 540 ng m ⁻³ , although it contributed to only 2 ± 1 % of the organic aerosol (OA). The CIMS identified a persistent gas-phase presence of SCOs in the ambient air, which was further supported by separate vapour-pressure measurements of NP OS by a Knudsen Effusion Mass Spectrometer (KEMS). An increase in relative humidity (RH) promoted partitioning of SCO to the particle phase, whereas higher temperatures favoured higher gas-phase concentrations. Biogenic emissions contributed to only 19 % of total SCOs measured in this study. Here, C ₁₀ H ₁₆ NSO ₇ , a monoterpene-derived SCO, represented the highest fraction (10 %) followed by an isoprene-derived SCO. The anthropogenic SCOs with polycyclic aromatic hydrocarbon (PAH) and aromatic precursors dominated the SCO mass loading (51 %) with C ₁₁ H ₁₁ SO ₇ , derived from methyl naphthalene oxidation, contributing to 40 ng m ⁻³ and 0.3 % of the OA mass. Anthropogenic-related SCOs correlated well with benzene, although their abundance depended highly on the photochemical age of the air mass, tracked using the ratio between pinonic acid and its oxidation product, acting as a qualitative photochemical clock. In addition to typical anthropogenic and biogenic precursors the biomass-burning precursor nitrophenol (NP) provided a significant level of NP OS. It must be noted that the contribution analysis here is only representative of the detected SCOs. There are likely to be many more SCOs present which the CIMS has not identified. Gas- and particle-phase measurements of glycolic acid suggest that partitioning towards the particle phase promotes glycolic acid sulfate production, contrary to the current formation mechanism suggested in the literature. Furthermore, the HSO ₄ ⋅H2SO4- cluster measured by the CIMS was utilized as a qualitative marker for acidity and indicates that the production of total SCOs is efficient in highly acidic aerosols with high SO42- and organic content. This dependency becomes more complex when observing individual SCOs due to variability of specific VOC precursors.

This study presents the first gaseous formic acid (HC(O)OH) concentration measurements collected over the Fenno-Scandinavian wetlands (67.9–68.0° N, 22.1–27.8° E) as part of the MAMM (Methane and other greenhouse gases in the Arctic-Measurements, process studies and Modelling) aircraft campaigns conducted in August and September 2013. A boundary layer box model approach has been used to calculate a regionally representative (~240 km²) surface flux for HC(O)OH of 0.0098 (±0.0057) mg_[HCOOH]·m⁻²·h⁻¹. A surface-type classification map was used to estimate proportional source contributions to the observed HC(O)OH flux over the measurement region. The removal of expected source contributions (using available literature parameterisations) from the calculated surface flux identified that 75% remained unaccounted for. This may suggest that HC(O)OH emission from wetland within the Fenno-Scandinavian region could contribute up to 29 times higher per unit area than previous theoretical HC(O)OH globally-averaged wetland estimates, highlighting a need for further constrained wetland studies of HC(O)OH emission to better understand its potentially significant impact on the Arctic HC(O)OH budget and consequent impacts on oxidative capacity.

A Time of Flight Chemical Ionisation Mass spectrometer (CIMS) utilizing the Fast Inlet for Gas and AEROsol (FIGAERO) was deployed at a regional site 40 km north west of Beijing and successfully identified and measured 17 sulfur containing organics (SCOs = organo/nitrooxyorgano sulfates and sulfonates) with biogenic and anthropogenic precursors. The SCOs were quantified using laboratory synthesized standards of lactic acid sulfate and nitrophenol organosulfate (NP OS). The mean total (of the 17 identified by CIMS) SCO particle mass concentration was 207 ± 106 ng m⁻³ and had a maxima of 542 ng m⁻³, although contributed to only 2 ± 2.8 % of the organic aerosol (OA). SCO contribution to submicron mass (PM₁) indicates a dominant secondary production of SCO due to the low contribution of SCOs to PM₁ during periods of high mass loading. The CIMS identified a persistent gas phase presence of SCOs in the ambient air, which was further supported by post campaign vapour pressure measurements of NP OS. An increase in RH appeared to promote partitioning to the particle phase whereas higher temperatures favored higher gas phase concentrations. On average 12 % of the total SCOs were observed in the gas phase with C₁₀H₁₆NSO₇ having just 5 % and IEPOX OS having 44 % on average in the gas phase. Biogenic emissions contributed to only 19 % of total SCOs detected. C₁₀H₁₆NSO₇, an alpha-pinene derived SCO, representing the highest fraction (10 %) followed by an isoprene-derived SCO. Anthropogenic SCOs with PAH and aromatic precursors dominated the SCO mass loading (51 % total SCOs) with C₁₁H₁₁SO₇, derived from methyl napthalene oxidation, contributing to 40 ng m⁻³ and 0.3 % of the OA mass. Biomass burning was also identified as a potential anthropogenic and biogenic source of SCOs, based on nitrophenol (NP) and acetonitrile time series via secondary production of NP OS. Gas and particle phase measurements of glycolic acid suggest that partitioning towards the particle phase promotes glycolic acid sulfate production, contrary to the current formation mechanism suggested in the literature. Highly oxidised multifunctional organic compounds (HOMS) and RO₂ radical diurnal profiles, as measured by the iodide ToF-CIMS, are similar to that of total SCOs, supporting results that indicate HOMS are able to play a role in SCO production. Anthropogenic related SCOs correlated well with benzene, although their abundance depended highly on the photochemical age of the air mass, tracked using the ratio between pinonic acid and its oxidation product, acting as a qualitative photochemical clock. The HSO₄.H₂SO₄⁻ cluster measured by the CIMS was utilized as a qualitative marker for acidity and provides further evidence that the production rate of total SCOs is efficient in highly acidic aerosols with high SO₄²⁻ and organic content. This dependency becomes more complex when observing individual SCOs due to variability of specific VOC precursors.

Nitryl chloride (ClNO₂) accumulation at night acts as a significant reservoir for active chlorine and impacts the following day's photochemistry when the chlorine atom is liberated at sunrise. Here, we report simultaneous measurements of N₂O₅ and a suite of inorganic halogens including ClNO₂ and reactions of chloride with volatile organic compounds (Cl–VOCs) in the gas and particle phases utilising the Filter Inlet for Gas and AEROsols time-of-flight chemical ionisation mass spectrometer (FIGAERO-ToF-CIMS) during an intensive measurement campaign 40 km northwest of Beijing in May and June 2016. A maximum mixing ratio of 2900 ppt of ClNO₂ was observed with a mean campaign nighttime mixing ratio of 487 ppt, appearing to have an anthropogenic source supported by correlation with SO₂, CO and benzene, which often persisted at high levels after sunrise until midday. This was attributed to such high mixing ratios persisting after numerous e-folding times of the photolytic lifetime enabling the chlorine atom production to reach 2.3 × 10⁵ molecules cm⁻³ from ClNO₂ alone, peaking at 09:30 LT and up to 8.4 × 10⁵ molecules cm⁻³ when including the supporting inorganic halogen measurements. Cl–VOCs were observed in the particle and gas phases for the first time at high time resolution and illustrate how the iodide ToF-CIMS can detect unique markers of chlorine atom chemistry in ambient air from both biogenic and anthropogenic sources. Their presence and abundance can be explained via time series of their measured and steady-state calculated precursors, enabling the assessment of competing OH and chlorine atom oxidation via measurements of products from both of these mechanisms and their relative contribution to secondary organic aerosol (SOA) formation.