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Hydroperoxymethyl thioformate (HPMTF), is a newly identified major oxidation product of dimethyl sulfide (DMS). It is speculated that it could act as a major reservoir of marine sulfur, but its fate in the atmosphere is currently unknown. In this study we have investigated the formation of HPMTF through the oxidation of DMS by OH, NO3 and Cl radicals and its losses through photolysis, OH oxidation, dry deposition and wet deposition using the global 3-dimensional chemical transport model, STOCHEM-CRI. The model results suggest that the distribution of loss processes for HPMTF are photolysis (46%) and wet deposition (31%) with additional contributions from dry deposition (13%) and OH-oxidation (10%). The global burden and the tropospheric lifetime of HPMTF are found to be 0.052 Tg and 0.5 days, respectively. The model integrations agree well with both aircraft derived measurements of HPMTF over the ocean surface between 80oN and 85oS latitudes and ground-based measurements made in the central Amazon. These ground-based data reveal a clear diurnal cycle with a maximum during midday, consistent with other recently reported data and possibly due to the dominance of the photochemical production rather than the photolytic loss. Accounting for HPMTF chemistry results in a significant decrease in boundary layer levels of SO2 and H2SO4 but increases in sulfate aerosol in the upper troposphere. We suggest that these changes could have important consequences on pre-industrial to present-day radiative forcing from sulfate aerosol.

Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban volatile organic compound (VOC) emissions that contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich Plant Atmosphere Chamber as part of a series of experiments examining benzene oxidation by OH under high- and low-NOx conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses, ranging from intermediate volatile organic compounds (IVOCs) to extremely low volatile organic compounds (ELVOCs), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOCs and semi-volatile organic compounds (SVOCs) but very few ELVOCs and low volatile organic compounds (LVOCs). A total of 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low- and high-NOx experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21 and 26 of the products listed in the Master Chemical Mechanism (MCM) were detected. The time series of highly oxidised (O≥6) and ring-retaining oxidation products (C6 and double-bond equivalent = 4) equilibrate quickly, characterised by a square form profile, compared to MCM and ring-breaking products which increase throughout oxidation, exhibiting sawtooth profiles. Under low-NOx conditions, all CHO formulae attributed to radical termination reactions of first-generation benzene products, and first-generation auto-oxidation products are observed. Several N-containing species that are either first-generation benzene products or first-generation auto-oxidation products are also observed under high-NOx conditions. Hierarchical cluster analysis finds four clusters, of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio, indicating it is sensitive to NO concentration and thus likely to contain NO addition products and alkoxy-derived termination products. This cluster has the highest average carbon oxidation state (OSC‾) and the lowest average carbon number. Where nitrogen is present in a cluster member of cluster 2, the oxygen number is even, as expected for alkoxy-derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO2 addition and peroxy-derived termination products. This cluster contains fewer fragmented species, as the average carbon number is higher and OSC‾ lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests that clustering of time series which have features pertaining to distinct chemical regimes, for example, NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.

Measurements of OH, HO2, complex RO2 (alkene- and aromatic-related RO2) and total RO2 radicals taken during the integrated Study of AIR Pollution PROcesses in Beijing (AIRPRO) campaign in central Beijing in the summer of 2017, alongside observations of OH reactivity, are presented. The concentrations of radicals were elevated, with OH reaching up to 2.8×107moleculecm−3, HO2 peaking at 1×109moleculecm−3 and the total RO2 concentration reaching 5.5×109moleculecm−3. OH reactivity (k(OH)) peaked at 89 s−1 during the night, with a minimum during the afternoon of ≈22s−1 on average. An experimental budget analysis, in which the rates of production and destruction of the radicals are compared, highlighted that although the sources and sinks of OH were balanced under high NO concentrations, the OH sinks exceeded the known sources (by 15 ppbv h−1) under the very low NO conditions (

Measurements of OH, HO2, RO2-complex (alkene and aromatic-related RO2) and total RO2 radicals taken during the AIRPRO campaign in central Beijing in the summer of 2017, alongside observations of OH reactivity are presented. The concentrations of radicals were elevated with OH reaching up to 2.8 × 107 molecule cm−3, HO2 peaked at 1 × 109 molecule cm−3 and the total RO2 concentration reached 5.5 × 109 molecule cm−3. OH reactivity (k(OH)) peaked at 89 s−1 during the night, with a minimum during the afternoons of ~ 22 s−1 on average. An experimental budget analysis, in which the rates of production and destruction of the radicals are compared, highlighted that although the sources and sinks of OH were balanced under high NO concentrations, the OH sinks exceeded the known sources (by 15 ppbv hr−1) under the very low NO conditions (2 production rate exceeded the rate of destruction by ~ 50 ppbv hr−1, whilst the rate of destruction of total-RO2 exceeded the production by the same rate indicating that the net propagation rate of RO2 to HO2 may be substantially slower than assumed. If just 10 % of the RO2 radicals propagate to HO2 upon reaction with NO, the HO2 and RO2 budgets could be closed at high NO, but at low NO this lower RO2 to HO2 propagation rate revealed a missing RO2 sink that was similar in magnitude to the missing OH source. A detailed box model that incorporated the latest MCM chemical mechanism (MCM3.3.1) reproduced the observed OH concentrations well, but over-predicted the observed HO2 under low concentrations of NO (2 (both the complex-RO2 fraction and other RO2 types which we classify as simple-RO2) most significantly at the highest NO concentrations. The model also under-predicted the observed k(OH) consistently by ~ 10 s−1 across all NOx levels highlighting that the good agreement for OH was fortuitous due to a cancellation of missing OH source and sink terms in its budget. Including heterogeneous loss of HO2 to aerosol surfaces did reduce the modelled HO2 concentrations in-line with the observations, but only at NO mixing ratios 2 concentration on several mornings when the Cl atom concentration was calculated to exceed 1 × 104 atoms cm−3 and could reconcile the modelled and measured RO2 concentrations at these times. However, on other mornings, when the Cl atom concentration was lower, large under-predictions in total RO2 remained. Furthermore, the inclusion of Cl atom chemistry did not enhance the modelled RO2 beyond the first few hours after sunrise and so was unable to resolve the modelled under-prediction in RO2 observed at other times of the day. Model scenarios, in which missing VOC reactivity was included as an additional reaction that converted OH to RO2, highlighted that the modelled OH, HO2 and RO2 concentrations were sensitive to the choice of RO2 product. The level of modelled to measured agreement for HO2 and RO2 (both complex and simple) could be improved if the missing OH reactivity formed a larger RO2 species that was able to undergo reaction with NO, followed by isomerisation reactions reforming other RO2 species, before eventually generating HO2. In this work an α-pinene-derived RO2 species was used as an example. In this simulation, consistent with the experimental budget analysis, the model underestimated the observed OH indicating a missing OH source. The model uncertainty, with regards to the types of RO2 species present and the radicals they form upon reaction with NO (HO2 directly or another RO2 species), leads to over an order of magnitude less O3 production calculated from the predicted peroxy radicals than calculated from the observed peroxy radicals at the highest NO concentrations. This demonstrates the rate at which the larger RO2 species propagate to HO2 or to another RO2 or indeed to OH needs to be understood to accurately simulate the rate of ozone production in environments such as Beijing where large multifunctional VOCs are likely present.

Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban VOC emissions that contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time of flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich plant chamber as part of a series of experiments examining benzene oxidation by OH under high and low NOx conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses ranging from intermediate volatile organic compounds (IVOC) to extremely low volatile organic compounds (ELVOC), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOC and semi volatile organic compounds (SVOC) but very few ELVOC and low volatile organic compounds (LVOC). 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low and high NOx experiments respectively. Ring breaking products make up the dominant fraction of detected signal (89–91 %). 21 and 26 of the products listed in the master chemical mechanism (MCM) were detected and account for 6.4–7.3 % of total signal. The time series of highly oxidised (O≥6) and ring retaining oxidation products (C6 and double bond equivalent = 4) equilibrate quickly characterised by a square form profile, compared to MCM and ring breaking products which increase throughout oxidation exhibiting saw tooth profiles. Under low NOx conditions, all CHO formulae attributed to radical termination reactions of 1st generation benzene products and 1st generation autoxidation products are observed, and one exclusively 2nd generation autoxidation product is also measured (C6H8O8). Several N containing species that are either 1st generation benzene products or 1st generation autoxidation products are also observed under high NOx conditions. Hierarchical cluster analysis finds four cluster of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio indicating it is sensitive to NO concentration thus likely to contain NO addition products and alkoxy derived termination products. This cluster has the highest average carbon oxidation state (OSc) and the lowest average carbon number and where nitrogen is present in cluster member, the oxygen number is even, as expected for alkoxy derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO2 addition and peroxy derived termination products. This cluster contains less fragmented species, as the average carbon number is higher and OSc lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests clustering of time series which have features pertaining to distinct chemical regimes e.g. NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) has a significant impact on both nocturnal particulate nitrate formation and photochemistry on the following day through the photolysis of nitryl chloride (ClNO2), yet these processes in highly polluted urban areas remain poorly understood. Here we present measurements of gas-phase N2O5 and ClNO2 by high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) during summer in urban Beijing, China as part of the Air Pollution and Human Health (APHH) campaign. N2O5 and ClNO2 show large day-to-day variations with average (±1σ ) mixing ratios of 79.2±157.1 and 174.3±262.0 pptv, respectively. High reactivity of N2O5, with ., (N2O5)'1 ranging from 0.20 × 10'2 to 1.46 × 10'2 s'1, suggests active nocturnal chemistry and a large nocturnal nitrate formation potential via N2O5 heterogeneous uptake. The lifetime of N2O5, ., (N2O5), decreases rapidly with the increase in aerosol surface area, yet it varies differently as a function of relative humidity with the highest value peaking at 1/4 40 %. The N2O5 uptake coefficients estimated from the product formation rates of ClNO2 and particulate nitrate are in the range of 0.017-0.19, corresponding to direct N2O5 loss rates of 0.00044-0.0034 s'1. Further analysis indicates that the fast N2O5 loss in the nocturnal boundary layer in urban Beijing is mainly attributed to its indirect loss via NO3, for example through the reactions with volatile organic compounds and NO, while the contribution of the heterogeneous uptake of N2O5 is comparably small (7-33 %). High ClNO2 yields ranging from 0.10 to 0.35 were also observed, which might have important implications for air quality by affecting nitrate and ozone formation.

Anthropogenic biomass burning is poorly represented in models due to a lack of observational data but represents a significant source of short-lived toxic gases. Guy Fawkes Night (bonfire night) is a regular UK-wide event where open fires are lit and fireworks are set off on 5 November. Previous gas phase studies of bonfire night focus on persistent organic pollutants primarily using off-line techniques. Here the first simultaneous online gas phase measurements of several classes of compounds including isocyanates, amides, nitrates, and nitro-organics are made during bonfire night (2014) in Manchester, UK, using a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) using iodide reagent ions. A shallow boundary layer and low wind speeds favor pollutant buildup with typical HCN, HNCO, and CH3NCO concentrations of tens of parts per thousand increasing by a factor of 13 to potentially harmful levels >1 ppb. Normalized excess mixing ratios relative to CO for a range of isocyanates and amides are reported for the first time. Using a HNCO:CO ratio of 0.1%, we distinguish emissions from flaming and smoldering combustion and report more accurate normalized excess mixing ratios for the distinct burning phases. While bonfire night is a highly polluting event, NO2 concentrations measured at this location are higher at other times, highlighting the importance of traffic as an NO2 emission source at this location. A risk communication methodology is used to equate enhancements in hourly averaged black carbon and NO2 concentrations caused by bonfire night as an equivalent of 26.1 passively smoked cigarettes.