In support of the first Tropospheric Ozone Assessment Report (TOAR) a relational database of global surface ozone observations has been developed and populated with hourly measurement data and enhanced metadata. A comprehensive suite of ozone data products including standard statistics, health and vegetation impact metrics, and trend information, are made available through a common data portal and a web interface. These data form the basis of the TOAR analyses focusing on human health, vegetation, and climate relevant ozone issues, which are part of this special feature. Cooperation among many data centers and individual researchers worldwide made it possible to build the world’s largest collection of in-situ hourly surface ozone data covering the period from 1970 to 2015. By combining the data from almost 10,000 measurement sites around the world with global metadata information, new analyses of surface ozone have become possible, such as the first globally consistent characterisations of measurement sites as either urban or rural/remote. Exploitation of these global metadata allows for new insights into the global distribution, and seasonal and long-term changes of tropospheric ozone and they enable TOAR to perform the first, globally consistent analysis of present-day ozone concentrations and recent ozone changes with relevance to health, agriculture, and climate. Considerable effort was made to harmonize and synthesize data formats and metadata information from various networks and individual data submissions. Extensive quality control was applied to identify questionable and erroneous data, including changes in apparent instrument offsets or calibrations. Such data were excluded from TOAR data products. Limitations of a posteriori data quality assurance are discussed. As a result of the work presented here, global coverage of surface ozone data for scientific analysis has been significantly extended. Yet, large gaps remain in the surface observation network both in terms of regions without monitoring, and in terms of regions that have monitoring programs but no public access to the data archive. Therefore future improvements to the database will require not only improved data harmonization, but also expanded data sharing and increased monitoring in data-sparse regions.

Since the nineties, atmospheric measurement systems have been deployed at Reunion Island, mainly for monitoring the atmospheric composition in the framework of NDSC/NDACC (Network for the Detection of Stratospheric Change/Network for the Detection of Atmospheric Composition Change). The location of Reunion Island presents a great interest because there are very few multi-instrumented stations in the tropics and particularly in the southern hemisphere. In 2012, a new observatory was commissioned in Maïdo at 2200 m above sea level: it hosts various instruments for atmospheric measurements, including lidar systems, spectro-radiometers and in situ gas and aerosol measurements. This new high-altitude Maïdo station provides an opportunity: 1. to improve the performance of the optical instruments above the marine boundary layer, and to open new perspectives on upper troposphere and lower stratosphere studies; 2. to develop in situ measurements of the atmospheric composition for climate change surveys, in a reference site in the tropical/subtropical region of the southern hemisphere; 3. to offer trans-national access to host experiments or measurement campaigns for focused process studies.

Long-term monitoring of the chemical composition of clouds (73 cloud events representing 199 individual samples) sampled at the puy de Dôme (pdD) station (France) was performed between 2001 and 2011. Physicochemical parameters, as well as the concentrations of the major organic and inorganic constituents, were measured and analyzed by multicomponent statistical analysis. Along with the corresponding back-trajectory plots, this allowed for distinguishing four different categories of air masses reaching the summit of the pdD: polluted, continental, marine and highly marine. The statistical analysis led to the determination of criteria (concentrations of inorganic compounds, pH) that differentiate each category of air masses. Highly marine clouds exhibited high concentrations of Na+ and Cl−; the marine category presented lower concentration of ions but more elevated pH. Finally, the two remaining clusters were classified as "continental" and "polluted"; these clusters had the second-highest and highest levels of NH4+, NO3−, and SO24−, respectively. This unique data set of cloud chemical composition is then discussed as a function of this classification. Total organic carbon (TOC) is significantly higher in polluted air masses than in the other categories, which suggests additional anthropogenic sources. Concentrations of carboxylic acids and carbonyls represent around 10% of the organic matter in all categories of air masses and are studied for their relative importance. Iron concentrations are significantly higher for polluted air masses and iron is mainly present in its oxidation state (+II) in all categories of air masses. Finally, H2O2 concentrations are much more varied in marine and highly marine clouds than in polluted clouds, which are characterized by the lowest average concentration of H2O2. This data set provides concentration ranges of main inorganic and organic compounds for modeling purposes on multiphase cloud chemistry.

Chemical Ionisation Mass Spectrometer measurements of hydroxyl radical (OH) and the sum of hydroperoxy and organic peroxy (HO₂+RO₂) radicals were conducted during the MEGAPOLI summer field campaign at the SIRTA observatory near Paris, France, in July 2009. OH and (HO₂+RO₂) showed a typical diurnal variation with averaged daytime maxima values around 5×10⁶ and 1.2×10⁸ molecule cm⁻³, respectively. Simultaneously, a large number of ancillary measurements, such as NO_x, O₃, HONO, HCHO and other VOCs were also conducted. These data provide an opportunity to assess our understanding of the radical chemistry in a suburban environment by comparing the radical observations to calculations. First, OH mixing ratios were estimated by a simple Photo Stationary State (PSS) calculation. PSS calculations overestimate the OH mixing ratio by 50%, especially at NO_x mixing ratios lower than 10 ppb, suggesting that some loss processes were missing in the calculation at low NO_x. Then, a photochemical box model simulation based on the Master Chemical Mechanism (MCM) and constrained by ancillary measurements was run to calculate radical concentrations. Three different modelling procedures were tested, varying the way the unconstrained secondary species were estimated, to cope with the unavoidable lack of their measurements. They led to significant differences in simulated radical concentrations. OH and (HO₂+RO₂) concentrations estimated by two selected model version were compared with measurements. These versions of the model were chosen because they lead, respectively, to the higher and lower simulated radical concentrations and are thus the two extremes versions. The box model showed better results than PSS calculations, with a slight overestimation of 12% and 5%, for OH and (HO₂+RO₂) respectively, in average for the reference model, and an overestimation of approximately 20% for OH and an underestimation for (HO₂+RO₂) for the other selected model version. Thus, we can conclude from our study that OH and (HO₂+RO₂) radical levels agree on average with observations within the uncertainty range. Finally, an analysis of the radical budget, on a daily basis (06:00–18:00 UTC), indicates that HONO photolysis (~35%), O₃ photolysis (~23%), and aldehydes and ketones photolysis (~16% for formaldehyde and 18% for others) are the main radical initiation pathways. According to the MCM modelling, the reactions of RO₂ with NO₂ (~19%), leading mainly to PAN formation, is a significant termination pathway in addition to the main net loss via reaction of OH with NO₂ (~50%).

Nitrous acid measurements were carried out during the MEGAPOLI summer and winter field campaigns at SIRTA observatory in Paris surroundings. Highly variable HONO levels were observed during the campaigns, ranging from 10 ppt to 500 ppt in summer and from 10 ppt to 1.7 ppb in winter. Significant HONO mixing ratios have also been measured during daytime hours, comprised between some tenth of ppt and 200 ppt for the summer campaign and between few ppt and 1 ppb for the winter campaign. Ancillary measurements, such as NO_x, O₃, photolysis frequencies, meteorological parameters (pressure, temperature, relative humidity, wind speed and wind direction), black carbon concentration, total aerosol surface area, boundary layer height and soil moisture, were conducted during both campaigns. In addition, for the summer period, OH radical measurements were made with a CIMS (Chemical Ionisation Mass Spectrometer). This large dataset has been used to investigate the HONO budget in a suburban environment. To do so, calculations of HONO concentrations using PhotoStationary State (PSS) approach have been performed, for daytime hours. The comparison of these calculations with measured HONO concentrations revealed an underestimation of the calculations making evident a missing source term for both campaigns. This unknown HONO source exhibits a bell-shaped like average diurnal profile with a maximum around noon of approximately 0.7 ppb h⁻¹ and 0.25 ppb h⁻¹, during summer and winter respectively. This source is the main HONO source during daytime hours for both campaigns. In both cases, this source shows a slight positive correlation with J(NO₂) and the product between J(NO₂) and soil moisture. This original approach had, thus, indicated that this missing source is photolytic and might be heterogeneous occurring at ground surface and involving water content available on the ground.

Measurements of the mole fraction of the CO₂ and its isotopes were performed in Paris during the MEGAPOLI winter campaign (January–February 2010). Radiocarbon (¹⁴CO₂) measurements were used to identify the relative contributions of 77% CO₂ from fossil fuel consumption (CO₂ff from liquid and gas combustion) and 23% from biospheric CO₂ (CO₂ from the use of biofuels and from human and plant respiration: CO₂bio). These percentages correspond to average mole fractions of 26.4 ppm and 8.2 ppm for CO₂ff and CO₂bio, respectively. The ¹³CO₂ analysis indicated that gas and liquid fuel contributed 70% and 30%, respectively, of the CO₂ emission from fossil fuel use. Continuous measurements of CO and NO_x and the ratios CO/CO₂ff and NO_x/CO₂ff derived from radiocarbon measurements during four days make it possible to estimate the fossil fuel CO₂ contribution over the entire campaign. The ratios CO/CO₂ff and NO_x/CO₂ff are functions of air mass origin and exhibited daily ranges of 7.9 to 14.5 ppb ppm⁻¹ and 1.1 to 4.3 ppb ppm⁻¹, respectively. These ratios are consistent with different emission inventories given the uncertainties of the different approaches. By using both tracers to derive the fossil fuel CO₂, we observed similar diurnal cycles with two maxima during rush hour traffic.