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 Schultz et al: Tropospheric Ozone Assessment Report Art. 58, page 2 of 26 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.

Abstract. Ozone pollution in eastern China has become one of the top environmental issues. Quantifying the temporal trend of surface ozone helps to assess the impacts of the anthropogenic precursor reductions and the likely effects of emission control strategies implemented. In this paper, ozone data collected at the Shangdianzi (SDZ) regional atmospheric background station from 2003 to 2015 are presented and analyzed to obtain the variation in the trend of surface ozone in the most polluted region of China, north of eastern China or the North China Plain. A modified KolmogorovZurbenko (KZ) filter method was performed on the maximum daily average 8 h (MDA8) concentrations of ozone to separate the contributions of different factors from the variation of surface ozone and remove the influence of meteorological fluctuations on surface ozone. Results reveal that the short-term, seasonal and long-term components of ozone account for 36.4, 57.6 and 2.2 % of the total variance, respectively. The long-term trend indicates that the MDA8 has undergone a significant increase in the period of 2003-2015, with an average rate of 1.13 ± 0.01 ppb year −1 (R 2 = 0.92). It is found that meteorological factors did not significantly influence the long-term variation of ozone and the increase may be completely attributed to changes in emissions. Furthermore, there is no significant correlation between the longterm O 3 and NO 2 trends. This study suggests that emission changes in VOCs might have played a more important role in the observed increase of surface ozone at SDZ.

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Secondary organic aerosol (SOA) constitutes a large fraction of organic aerosol worldwide, however, the formation mechanisms in polluted environments remain poorly understood. Here we observed fast daytime growth of oxygenated organic aerosol (OOA) (with formation rates up to 10 μg m −3 h −1 ) during low relative humidity (RH, daytime average 38 ± 19%), high RH (53 ± 19%), and fog periods (77 ± 13%, fog occurring during nighttime with RH reaching 100%). Evidence showed that photochemical aqueous-phase SOA (aqSOA) formation dominantly contributed to daytime OOA formation during the periods with nighttime fog, while both photochemical aqSOA and gas-phase SOA (gasSOA) formation were important during other periods with the former contributing more under high RH and the latter under low RH conditions, respectively. Compared to daytime photochemical aqSOA production, dark aqSOA formation was only observed during the fog period and contributed negligibly to the increase in OOA concentrations due to fog scavenging processes. The rapid daytime aging, as indicated by the rapid decrease in m,p-xylene/ethylbenzene ratios, promoted the daytime formation of precursors for aqSOA formation, e.g., carbonyls such as methylglyoxal. Photooxidants related to aqSOA formation such as OH radical and H 2 O 2 also bear fast daytime growth features even under low solar radiative conditions. The simultaneous increases in ultraviolet radiation, photooxidant, and aqSOA precursor levels worked together to promote the daytime photochemical aqSOA formation. We also found that biomass burning emissions can promote photochemical aqSOA formation by adding to the levels of aqueous-phase photooxidants and aqSOA precursors. Therefore, future mitigation of air pollution in a polluted environment would benefit from stricter control on biomass burning especially under high RH conditions.

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detrimental to human health and vegetation (REVIHAAP, 2013; CLRTAP, 2016). Atmospheric ozone can only be formed in photochemical reactions. In the stratosphere, ozone is produced and sustained by the Chapman mechanism and catalytically destroyed by active species, such as OH, NO, and halogenated species (Solomon, 1999). A simulation study by Lelieveld and Dentener (2000) suggests that more than 85% of tropospheric ozone originates from photochemical oxidation of volatile organic compounds (VOCs), CO, and CH 4 , catalyzed by NO x , with

Abstract. Wet scavenging is one of the most efficient processes for removing aerosols from the atmosphere. This process is not well constrained in chemical transport models (CTMs) due to a paucity of localized parameterization regarding the below-cloud wet scavenging coefficient (BWSC). Here we conducted field measurements of the BWSC during the Atmospheric Pollution and Human Health Beijing (APHH-Beijing) campaign of 2016. Notably, the observed BWSC values based on the updated aerosol mass balance agree well with another estimation technique, and they fall in a range of 10−5 s−1. The measurement in this winter campaign, combined with that in summer of 2014, supported an exponential power distribution of BWSCs with rainfall intensity. The observed parameters were also compared with both the theoretical calculations and modeling results. We found that the theoretical estimations can effectively characterize the observed BWSCs of aerosols with sizes smaller than 0.2 µm and larger than 2.5 µm. However, the theoretical estimations were an order of magnitude lower than observed BWSCs within 0.2–2.5 µm, a domain size range of urban aerosols. Such an underestimation of BWSC through a theoretical method has been confirmed not only in APHH-Beijing campaign but also in all the rainfall events in summer of 2014. Since the model calculations usually originated from the theoretical estimations with simplified scheme, the significantly lower BWSC could well explain the underprediction of wet depositions in polluted regions as reported by the Model Inter-Comparison Study for Asia (MICS-Asia) and the global assessment of the Task Force on Hemispheric Transport of Atmospheric Pollutants (TF-HTAP). The findings highlighted that the wet deposition module in the CTMs requires improvement based on field measurement estimation to construct a more reasonable simulation scheme for BWSC, especially in polluted regions.

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