Secondary organic aerosol (SOA) represents a major component of urban air pollution. This study presents observational evidence from summer 2017 in urban Beijing, supported by model simulations and a case study from summer 2023, demonstrating the crucial role of nighttime organic nitrates (ONs) production in subsequent daytime SOA formation. Our measurements revealed that total reactive nitrogen compound (NO z ) concentrations exceeded 40 ppb at night, resulting from nitrate radical (NO3)-initiated oxidation of volatile organic compounds (VOCs) in the surface layer and through aloft production followed by downward transport. While these NO z existed primarily in the gas phase during nighttime, they underwent atmospheric aging processes the following day, significantly contributing to SOA growth and potentially new particle formation. Model simulations identified reactive terpenoids as the dominant VOC precursors for nighttime ON formation. These findings underscore the need for an improved understanding of nocturnal ONs production mechanisms given their substantial impact on daytime SOA production.

Abstract. The hydroxyl radical (OH) is a critical determinant of global oxidative capacity and trace gas lifetimes, but its variation remains poorly constrained. This study investigates the sensitivity of modelled tropospheric OH concentration changes to physical and chemical processes using the FRSGC/UCI chemistry transport model. The simulated tropospheric O3 and NO2 agree well with satellite observations, but that annual variations in CO do not, likely due to uncertainties in biomass burning emissions in the southern hemisphere and overestimated CO trends over Asia. This discrepancy, along with the background increase in CO, may lead to an underestimation of OH increase or overestimation of OH decrease from 2000 to 2017. Changes in tropospheric OH column show substantial spatial heterogeneity, with increases in OH in high-emission regions and decreases in the tropics. Global mean OH trends are dependent on the assumed trend in emissions: with dynamic emissions there is little change in OH, while under annually invariant emissions, there is a substantial increase in OH due to meteorological conditions alone. Inclusion of water vapor UV absorption, heterogeneous reactions, and updates to the OH + NO2 reaction rate have a smaller impact on OH trend, but decrease OH levels by 3.6%, 5.8%, and 7.0%, respectively, increasing CH4 lifetimes by 4.2%, 5.2%, and 8.4%. Incorporating oceanic CH3CHO emissions reduces global mean OH by up to 1.5%, increasing the CH4 lifetime by up to 1.6%. These results provide a quantitative basis for understanding the drivers of tropospheric OH variability and their implications for global methane chemistry.

The methane pledge brought about during COP26 requires its signatories to reduce their 2020 methane emissions by 30% by 2030 (European Commission and United States of America, 2021). 65 % of all methane emissions are thought to be anthropogenic in nature (Saunois et al., 2025)One of the major anthropogenic sectors contributing to methane emissions is from the oil and gas industry. Fugitive emissions of methane are one of the major contributors to emissions from this industry, this may refer to unwanted emission during transportation of product (colloquially referred to as gas leaks), or flaring, where methane undergoes combustion to CO2. As of 2023 the oil and gas industry was responsible for 1.2 % of the UK’s methane emissions, with flaring emissions representing 69 % of this sector's emissions. (North Sea Transition Authority, 2025).Flaring may occur for one of three reasons; Routine flaring, where an oil producing facility is unable to use the produced gas; Safety flaring, where flaring ensures the safe operation of the facility; Non-routine flaring encompasses all other flaring. The North Sea is one of the most active areas in the world for oil and gas activities. Recent attempts between 2018 and 2022 have reduced flaring activities in the North Sea by 50%, with an aim for zero flaring to take place by 2030. This is an important step to reducing emissions in this region as one fifth of all emissions in the North Sea related to oil and gas production activities are attributed to flaring (North Sea Transition Authority, 2023). While flaring activity continues to be reduced, some facilities require the continued use of flares, in these cases attempts have been made to adapt the flare itself and improve its overall efficiency and ensure that more methane is converted to CO2. This work features data collected from one sampling flight of a platform with a highly efficient flare that was intentionally flaring while on task. We explore the feasibility of using previous detection methodologies, such that present in (Shaw et al., 2023), while adding additional stages to confirm the detection of a flare, including source identification using NOx : CO2 ratios as well as modelling the dispersion of multiple sources on the platform using ADMS to understand the likelihood of detecting the flare.

Secondary organic aerosol (SOA) represents a major component of urban air pollution. This study presents observational evidence from summer 2017 in urban Beijing, supported by model simulations and a case study from summer 2023, demonstrating the crucial role of nighttime organic nitrates (ONs) production in subsequent daytime SOA formation. Our measurements revealed that total reactive nitrogen compound (NOz) concentrations exceeded 40 ppb at night, resulting from nitrate radical (NO3)-initiated oxidation of volatile organic compounds (VOCs) in the surface layer and through aloft production followed by downward transport. While these NOz existed primarily in the gas phase during nighttime, they underwent atmospheric aging processes the following day, significantly contributing to SOA growth and potentially new particle formation. Model simulations identified reactive terpenoids as the dominant VOC precursors for nighttime ON formation. These findings underscore the need for an improved understanding of nocturnal ONs production mechanisms given their substantial impact on daytime SOA production.

Volatile organic compounds (VOCs) are important precursors to the formation of ozone (O3) and secondary organic aerosols (SOA) and can also have direct human health impacts. The emissions of VOCs remain poorly characterized due to the complexity and variability of their sources. The VOC levels in Beijing during the winter campaign (APHH) were investigated using a dispersion model (NAME), and a chemical box model (AtChem2) in order to understand how chemistry and transport affect the VOC concentrations in Beijing. Emissions of VOCs in Beijing and contributions from outside Beijing were modelled using the NAME dispersion model combined with the emission inventories and were used to initialize the AtChem2 box model. The modelled concentrations of VOCs from the NAME-AtChem2 combination were then compared to the output of a chemical transport model (GEOS-Chem). The results from the emission inventories and the NAME air mass pathways suggest that industrial sources to the south of Beijing and within Beijing during the winter campaign are very important in controlling the VOC levels in Beijing. A number of scenarios with different nitrogen oxides to ozone ratios (NOx/O3) and hydroxyl (OH) levels were simulated to determine the changes in VOC levels. In Beijing over 80 % of VOC are emitted locally during winter. Most scenarios are in good agreement with daily GEOS-Chem simulations, with the best agreements seen for the modelled concentrations of ethanol, benzene and propane with correlation coefficients of 0.67, 0.63 and 0.64 respectively. Furthermore, the production of formaldehyde in an air mass within 24 h of travel from Beijing was investigated, and it was estimated that 90 % of formaldehyde in Beijing is secondary, produced from oxidation of non-methane volatile organic compounds (NMVOCs). The benzene/CO and toluene/CO ratios during the campaign are very similar to the ratio derived from literature for 2014 in Beijing, however more data are needed to enable investigation of more species over longer timeframes to determine whether this ratio can be applied to predicting VOCs in Beijing. The results suggest that VOC concentrations in Beijing are driven predominantly by sources within Beijing and by local atmospheric chemistry during the winter. Moreover, the relationship of the NOx/VOC and O3 shows that the VOCs during the winter campaign are possibly emitted from similar sources as NOx.

Measurements in the remote unpolluted atmosphere have tremendous power to reveal processes that are happening on a global scale. In the marine atmosphere where nitrogen oxide (NOx) levels are very low, the photochemical loss rate of tropospheric ozone dominates over production, allowing loss processes to be sensitively explored. We showed that bromine and iodine emitted from open-ocean marine sources initiate important global-scale catalytic ozone-destroying cycles and found that the deposition of ozone and subsequent reactions at the sea surface are a substantial pathway for production of volatile iodine. Production of ozone in the remote atmosphere is predominantly regulated by the abundance of NOx, which also exerts substantial control over the hydroxyl radical (OH), the most important oxidant in the atmosphere. It is now emerging that NOx regeneration pathways, namely the photolysis of particulate nitrate, could provide the dominant source of NOx to the marine atmosphere. This has significant implications for our understanding of the chemistry of the remote troposphere. This presentation discusses advances made in understanding these important, predominantly natural, cycles and their impacts on the atmosphere.

The North Atlantic Climate System Integrated Study (ACSIS) was a large multidisciplinary research programme funded by the UK's Natural Environment Research Council (NERC). ACSIS ran from 2016 to 2022 and brought together around 80 scientists from seven leading UK-based environmental research institutes to deliver major advances in the understanding of North Atlantic climate variability and extremes. Here, we present an overview of the data generated by the ACSIS programme. The datasets described cover the North Atlantic Ocean, the atmosphere above it (including its composition), and Arctic sea ice. Atmospheric composition datasets include measurements from seven aircraft campaigns (45 flights in total, 0–10 km altitude range) in the northeastern Atlantic (∼ 15–55° N, ∼ 40° W–5° E) made at intervals of 6 months to 2 years between February 2017 and May 2022. The flights measured chemical species (including greenhouse gases; ozone precursors; and volatile organic compounds – VOCs) and aerosols (organic aerosol – OA; SO4; NH4; NO3; and non-sea salt chloride – nss-Cl) (https://doi.org/10.5285/6285564c34a246fc9ba5ce053d85e5e7, FAAM et al., 2024). Ground-based stations at the Cape Verde Atmospheric Observatory (CVAO), Penlee Point Atmospheric Observatory (PPAO), and Plymouth Marine Laboratory (PML) recorded ozone, ozone precursors, halocarbons, greenhouse gases (CO2 and methane), SO2, and photolysis rates (CVAO; http://catalogue.ceda.ac.uk/uuid/81693aad69409100b1b9a247b9ae75d5, National Centre for Atmospheric Science et al., 2010); O3 and CH4 (PPAO, https://catalogue.ceda.ac.uk/uuid/8f1ff8ea77534e08b03983685990a9b0 (Plymouth Marine Laboratory and Yang, 2017); and aerosols (PML, https://doi.org/10.5285/e74491c96ef24df29a9342a3d57b5939, Smyth, 2024), respectively. Complementary model simulations of atmospheric composition were performed with the UK Earth System Model (UKESM1) for the period from 1982 to 2020 using Coupled Model Intercomparison Project Phase 6 (CMIP6) historical forcing up to 2014 and Shared Socioeconomic Pathways (SSP) scenario SSP3-7.0 from 2015 to 2020. Model temperature and winds were relaxed towards ERA reanalysis. Monthly mean model data for ozone, NO, NO2, CO, methane, stratospheric ozone tracers, and 30 regionally emitted tracers are available for download (https://data.ceda.ac.uk/badc/acsis/UKESM1-hindcasts, Abraham, 2024). ACSIS also generated new ocean heat content diagnostics (https://doi.org/10/g6wm, https://doi.org/10/g8g2, Moat et al., 2021a–b) and gridded temperature and salinity based on objectively mapped Argo measurements (https://doi.org/10.5285/fe8e524d-7f04-41f3-e053-6c86abc04d51 King, 2023). An ensemble of atmosphere-forced global-ocean sea-ice simulations using the NEMO-CICE model was performed with horizontal resolutions of 1/4 and 1/12° covering the period from 1958 to 2020 using several different atmosphere-reanalysis-based surface forcing datasets, supplemented by additional global simulations and stand-alone sea-ice model simulations with advanced sea-ice physics using the CICE model (http://catalogue.ceda.ac.uk/uuid/770a885a8bc34d51ad71e87ef346d6a8, Megann et al., 2021e). Output is stored as monthly averages and includes 3D potential temperature, salinity, zonal velocity, meridional velocity, and vertical velocity; 2D sea-surface height, mixed-layer depth, surface heat, and freshwater fluxes; ice concentration and thickness; and a wide variety of other variables. In addition to the data presented here, we provide a very brief overview of several other datasets that were generated during ACSIS and have been described previously in the literature.

Road transport NO x emissions in many high-income countries have steadily reduced due to improved exhaust after-treatment technology. However, ambient concentrations of NO2, O3 and PM2.5 continue to exceed World Health Organization guidelines in many cities globally. The megacity of London has taken an international lead in mobility interventions through the use of low-emission zones. Using long-term air pollution flux measurements made from a communications tower, we show that the largest source of NO x emissions in central London has transitioned from road transport to space heating. Observations and supporting consumption/mobility data indicated that natural gas combustion in boilers was responsible for 72 +/- 17% of NO x emissions in the measurement footprint (average years 2021-2023). Road transport has dominated air quality thinking on NO2 for many decades. However, in urban environments that are reliant on natural gas, building heating may now be an effective sector to prioritize for further NO x emissions intervention. With system-wide changes in the heat and power sector expected in the coming decades to achieve decarbonisation pledges, we project that very low urban emissions of NO x are achievable. The trajectory will, however, depend on choices made around urban buildings and their associated infrastructure and whether low-carbon fuel combustion or electrification pathways are chosen. We estimate a damage cost penalty of up to 600 pound M in the U.K. should hydrogen combustion replace natural gas for heating rather than technologies such as heat pumps.

Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based datasets and provides insights into key photochemical and depositional loss processes that control the concentrations and spatiotemporal variability in ozone as well as the physicochemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational datasets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall dataset consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The dataset, consisting of more than 630 000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatiotemporal distribution and long-term trends over the 11 defined ocean/polar regions. The datasets from ships, buoys, and aircraft are complemented by ozonesonde data from 29 launch sites or field campaigns and by 21 non-polar and 17 polar ground-based station datasets. The datasets contain information on how long the observed air masses were isolated from land, as estimated by backward trajectories from the individual observation points. To extract observations representative of oceanic conditions, we recommend using a subset of the data with an isolation time of 72 h or longer, from the analysis with coincident radon observations. These filtered oceanic and polar data showed typically flat diurnal cycles at high latitudes, whereas daytime decreases in ozone (11 %-16 %) were observed at lower latitudes. The ship/buoy- and aircraft-based datasets presented here will supplement the land-based ones in the TOAR-II (Tropospheric Ozone Assessment Report Phase II) database to provide a fully global assessment of tropospheric ozone. The described dataset is available at 10.17596/0004044 (Kanaya et al., 2025).

The amount of methane released to the atmosphere from the Nord Stream subsea pipeline leaks remains uncertain, as reflected in a wide range of estimates1–18. A lack of information regarding the temporal variation in atmospheric emissions has made it challenging to reconcile pipeline volumetric (bottom-up) estimates1–8 with measurement-based (top-down) estimates8–18. Here we simulate pipeline rupture emission rates and integrate these with methane dissolution and sea-surface outgassing estimates9,10 to model the evolution of atmospheric emissions from the leaks. We verify our modelled atmospheric emissions by comparing them with top-down point-in-time emission-rate estimates and cumulative emission estimates derived from airborne11, satellite8,12–14 and tall tower data. We obtain consistency between our modelled atmospheric emissions and top-down estimates and find that 465 ± 20 thousand metric tons of methane were emitted to the atmosphere. Although, to our knowledge, this represents the largest recorded amount of methane released from a single transient event, it is equivalent to 0.1% of anthropogenic methane emissions for 2022. The impact of the leaks on the global atmospheric methane budget brings into focus the numerous other anthropogenic methane sources that require mitigation globally. Our analysis demonstrates that diverse, complementary measurement approaches are needed to quantify methane emissions in support of the Global Methane Pledge19. Modelling of the evolution of atmospheric methane emissions from the 2022 Nord Stream subsea pipeline leaks shows that the event emitted the largest recorded amount of methane from a single transient event.

Black carbon (BC) is a significant environmental health and climate forcing concern. Direct measurement of BC fluxes using eddy covariance can quantify emissions and identify sources. Previous studies have examined urban BC emissions in highly polluted countries such as China and India, but to date no equivalent research has been done in the UK and Europe. Here, we present black carbon flux data from a single particle soot photometer (SP2) deployed in an eddy covariance system at the BT (formerly British Telecommunications) Tower in central London. Mean BC mass (number) fluxes with a size range of 60 nm to 600 nm were 6.83 ng m-2 s-1 (443 cm-2 s-1) in summer and 13.3 ng m-2 s-1 (687 cm-2 s-1) in winter, indicating relatively low BC emission when compared to Delhi, which is likely due to the introduction of the ultra-low emission zone (ULEZ) and requirements for road diesel vehicles to meet Euro 6 standards or higher. However, flux footprint analysis identified strong point sources near construction sites during winter and summer observations, which implies that non-road mobile machinery (NRMM) emissions can dominate over traffic BC emissions. This implies that tightened NRMM regulations can help future air quality in London. Observations indicate that the UK's National Atmospheric Emissions Inventory (NAEI) overestimates BC emissions by a factor of 5, although large uncertainties are expected for the combustion sector in the manufacturing industry. The estimate of traffic emissions is more accurate.

From 2020, the atmospheric methane burden has grown at the fastest rate in the detailed observational record. This rise has been accompanied by an unprecedented plunge in d13C(CH4). The causes of recent accelerated growth are as yet uncertain but the geographic spread of growth and the rapid isotopic plunge suggest strong rises in isotopically light emissions from both Tropical and Boreal wetlands. These emissions may be due to rising precipitation and temperatures in parts of the tropics, and by rising temperatures in northern Canada, Siberia, and Europe. Over the longer period since 2007, methane’s actual growth is comparable to methane’s growth in the ‘worst case’ very high baseline emission scenario RCP8.5 (8.5 W/m2 forcing increase relative to pre-industrial). If the recent trend were to continue for more than another decade it could make the 2°C target as hard to achieve as the 1.5°C target is now. Natural feedbacks to climate warming in wetlands need to be included in future modelling and should be incorporated in climate modelling projects such as CMIP7. Methane’s recent accelerated growth also has wide implications for climate negotiations as it reduces the permissible total anthropogenic greenhouse gas emissions if the Paris Agreement is to be achieved. Strong growth in non-anthropogenic methane emissions, driven by feedback impacts on natural and quasi-natural sources, was not expected in modelling at the time of the Paris Agreement and shows the urgency of improving our understanding of the feedback impacts of climate change. The simplest way to limit methane’s growth is for all nations, including non-signatory countries, to cut anthropogenic emissions urgently and sharply, meeting or exceeding the targets of the Global Methane Pledge.

Heatwaves are phenomena that are occurring with increasing frequency, a trend which is expected to continue over the coming century as the effects of climate change continue to be felt, as has been well documented in the IPCC’s AR6 synthesis report. Heatwaves are of direct concern to human health due to exposure to extreme heat, but also for their secondary impacts including those on air pollution. A heatwave describes a period of hot weather where the air temperature exceeds a climatological average for that region, for example the UK’s Met Office defines these based on the average temperature in a region for the 15th July 1991 - 2020. The meteorology surrounding heatwaves is usually characterised by stagnant conditions, allowing air pollutants to remain closer to their sources and allowing secondary pollutants to form there. Tropospheric ozone is one of these air pollutants, created through the reactions of nitrogen oxides and volatile organic compounds and is harmful to human health.As a part of the World Meteorological Organisation’s Air Quality and Climate Bulletin (2023), we included a short report on the effects of the July 2022 heatwave on ozone concentrations measured across several hundred air quality monitoring stations which were located primarily in urban and rural background locations in Europe. Here we present an extended analysis examining heatwave events over the last decade and their contribution to the number of extreme ozone events experienced at these sites.

The shipping industry is a significant source of both SO2 and NOx, two air pollutants which have neg-active implications for climate and air quality. SO2 is heavily regulated such that January 2020 marked a global reduction of the maximum permitted sulfur fuel content (SFC) in shipping fuel from 3.5% to 0.5% by mass. There are also various limits on NOx in coastal and inland waterways with more widespread NOx emissions limits likely to be implemented in the future. The anticipated effect of the new regulations is an improvement of coastal air quality, but there is a potential drawback in terms of reducing the climate cooling effect of SO2 caused by a change in cloud properties. However, the difficulty in measuring emissions from ships means that the overall impact and level of compliance with new and future regulations is unclear. Therefore, monitoring strategies capable of providing regular and long-term measurements of air pollutant emissions from the shipping industry are essential for fundamental research and compliance monitoring. Here we present top-down methodologies for calculating the SFC, along with emission ratios of ∆NOx/∆CO2 from individual ships. First, we demonstrate the application of an airborne platform to perform targeted measurements of ship plumes in the English Channel and Atlantic shipping lanes, allowing the comparison of emissions inside and out of sulfur emission control areas (SECAs). Second, we implement a stationary, point-sampling approach to measure ships arriving and departing two European ports. Our results show there has been a significant reduction in the SFC of ships in the open ocean shipping lanes due to the new emission regulations, with most ships now well below the 0.5% limit imposed in 2020. Our measurements also show good agreement with a comprehensive ship specific emissions inventory (the Ship Traffic Emission Assessment Model – STEAM). This confirms the validity of models that use the new sulfur limit for radiative forcing calculations. In terms of NOx, the measured ∆NOx/∆CO2 ratios were generally greater than those from diesel vehicles operating within a typical European fleet, suggesting ships could be a significant source of NOx to cities, especially where ports are close to populated areas. It is envisaged that the presented methodologies could be implemented to facilitate widespread monitoring of ship emissions, which could provide the basis for policy formulation and validation.

Abstract. Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based data sets and provides insights into key photochemical and depositional loss processes that control the concentrations, spatio-temporal variability of ozone, and the physico-chemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational data sets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall data set consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The data set, consisting of more than 630,000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatio-temporal distribution and long-term trends over the defined 11 ocean/polar regions. The data sets from ships, buoys, and aircrafts are complemented with an ozonesonde data set from 29 launch sites or field campaigns, and by 21 non-polar and 17 polar ground-based stations data sets. The data were filtered by using backward trajectories calculated with the HYSPLIT model from the individual observation points to extract essentially oceanic observations, defined as air masses that have travelled over oceans for 72 hours or more, which were further tested with the coincident Radon observations. The oceanic and polar data thus selected showed typically flat diurnal patterns at high latitudes and daytime decreases (11–16 %) at low latitudes, indicating the adequacy of the data collection and processing procedures, as well as the potential for further studies of processes with statistical robustness and coverage. The ship/buoy- and aircraft-based data sets presented here will supplement the land-based ones in the TOAR-II database to provide a fully global assessment of tropospheric ozone.

Reducing fugitive emissions from natural gas networks is an important area to consider for future management of a country's methane emissions. The main problem with reducing these emissions comes from locating where leaks are commonly occurring, in order to determine which areas should be prioritised for replacing ageing or leaky pipes. This problem is especially relevant in the United Kingdom which has an extensive but ageing natural gas network. Two cities in the UK of varying sizes were selected in order to conduct vehicle-based measurement campaigns in summer and winter with a three-fold method to detect natural gas leaks; locating methane enhancements using 1Hz methane measurements; determining which areas had a recurring methane enhancement; confirming if these enhancements are natural gas emissions by use of ratios of other components contained within the natural gas mixture. This information can then be used to quantify the detected natural gas leaks and compare these to existing inventories.

The OH radical is the dominant daytime tropospheric oxidising agent, reacting with almost all Volatile Organic Compounds (VOCs). The majority of global methane is removed in the tropical troposphere by OH. The oxidation of VOCs by OH forms peroxy radicals, HO2 and RO2, with formaldehyde (HCHO) often formed as a product. In remote marine environments, ozone (O3) is destroyed during the day by reaction with OH or HO2, or photolysis.Ground-based measurements of OH, HO2, RO2, OH reactivity and HCHO, together with a comprehensive suite of supporting measurements, were made at the Cape Verde Atmospheric Observatory (CVAO), situated on the island of São Vicente located in the Tropical North Atlantic Ocean, during February 2023 as part of the NERC-funded PEROXY project. With no nearby emissions and prevailing winds from over the ocean, the clean marine air sampled was representative of the open ocean.In this work, time series and diurnal variations of the measured species are presented, and production and destruction rates of OH, HO2 and RO2 have been calculated. Reactions of the halogen oxides, IO and BrO, are shown to be important for understanding the chemistry of OH, HO2 and RO2. The CH3O2 reactions with halogen oxides are an important sink for RO2 and a possible source of HO2 that likely enhances O3 destruction in remote marine environments. Heterogeneous losses are also shown to be important for HO2. The OH budget analysis shows a possible minor missing source of OH while net HO2 production is observed, indicating either an overestimation of HO2 sources or an underestimation of HO2 sinks. Net RO2 production is observed, with the reactions of CH3O2 with halogen oxides and heterogeneous losses of CH3O2 to aerosols and the ocean surface needed to close the RO2 budget, but the magnitude of these loss processes is currently highly uncertain.