The Tropospheric Ozone Assessment Report (TOAR) is an activity of the International Global Atmospheric Chemistry Project. This paper is a component of the report, focusing on the present-day distribution and trends of tropospheric ozone relevant to climate and global atmospheric chemistry model evaluation. Utilizing the TOAR surface ozone database, several figures present the global distribution and trends of daytime average ozone at 2702 non-urban monitoring sites, highlighting the regions and seasons of the world with the greatest ozone levels. Similarly, ozonesonde and commercial aircraft observations reveal ozone’s distribution throughout the depth of the free troposphere. Long-term surface observations are limited in their global spatial coverage, but data from remote locations indicate that ozone in the 21st century is greater than during the 1970s and 1980s. While some remote sites and many sites in the heavily polluted regions of East Asia show ozone increases since 2000, many others show decreases and there is no clear global pattern for surface ozone changes since 2000. Two new satellite products provide detailed views of ozone in the lower troposphere across East Asia and Europe, revealing the full spatial extent of the spring and summer ozone enhancements across eastern China that cannot be assessed from limited surface observations. Sufficient data are now available (ozonesondes, satellite, aircraft) across the tropics from South America eastwards to the western Pacific Ocean, to indicate a likely tropospheric column ozone increase since the 1990s. The 2014–2016 mean tropospheric ozone burden (TOB) between 60˚N–60˚S from five satellite products is 300 Tg ± 4%. While this agreement is excellent, the products differ in their quantification of TOB trends and further work is required to reconcile the differences. Satellites can now estimate ozone’s global long-wave radiative effect, but evaluation is difficult due to limited in situ observations where the radiative effect is greatest.

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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.

[1] The impact of the African dust on levels of atmospheric suspended particulate matter (SPM) and on wet deposition was evaluated in eastern Iberia for the period 1996-2002. An effort was made to compile both the SPM and wet episodes. To this end, the time series of levels of TSP and PM10 in Levantine air quality monitoring stations were evaluated and complemented with the computation of back trajectories, satellite images, and meteorological analysis. Wet deposition frequency was obtained from weekly collected precipitation data at a rural background station in which the African chemical signature was identified (mainly pH and Ca 2+ concentrations). A number of African dust episodes (112) were identified (16 episodes per year). In 93 out of the 112 (13 episodes per year) the African dust influence caused high SPM levels. In 49 out of 112 (7 episodes per year), wet deposition was detected, and the chemistry was influenced by dust. There is a clear seasonal trend with higher frequency of dust outbreaks in May-August, with second modes in March and October. Wet events followed a different pattern, with a marked maximum in May. Except for one event, December was devoid of African air mass intrusions. On the basis of seasonal meteorological patterns affecting the Iberian Peninsula, an interpretation of the meteorological scenarios causing African dust transport over Iberia was carried out. Four scenarios were identified with a clear seasonal trend. The impact of the different dust outbreak scenarios on the levels of PM10 recorded at a rural site (Monagrega, Teruel, Spain) in the period 1996-2002 was also evaluated.

Abstract. We provide an atmospheric aerosol characterization for North Africa, Northeastern Atlantic, Mediterranean and Middle East based on the analysis of quality-assured direct-sun observations of 39 stations of the AErosol RObotic NETwork (AERONET) which include at least an annual cycle within the 1994-2007 period. We extensively test and apply the recently introduced graphical method of Gobbi and co-authors to track and discriminate different aerosol types and quantify the contribution of mineral dust. The method relies on the combined analysis of theÅngström exponent (α) and its spectral curvature δα. Plotting data in these coordinates allows to infer aerosol fine mode radius (R f ) and fractional contribution (η) to total Aerosol Optical Depth (AOD) and separate AOD growth due to fine-mode aerosol humidification and/or coagulation from AOD growth due to the increase in coarse particles or cloud contamination. Our results confirm the robustness of this graphical method. Large mineral dust is found to be the most important constituent in Northern Africa and Middle East. Under specific meteorological conditions, its transport to Southern Europe is observed from spring to autumn and decreasing with latitude. We observe "pure Saharan dust" conditions to show AOD>0.7 (ranging up to 5), α<0.3 and δα<0 corresponding to η<40% and R f ∼0.13 µm. Small pollution particles are abundant in sites close to urban and industrial areas of Correspondence to: S. Basart (sara.basart@bsc.es) Continental and Eastern Europe and Middle East, as well as, important contributions of biomass burning are observed in the sub-Sahel region in winter. These small aerosols are associated to AOD<1, α>1.5 and δα∼−0.2 corresponding to η>70% and R f ∼0.13 µm. Here, dust mixed with fine pollution aerosols shifts the observations to the region α<0.75, in which the fine mode contribution is less than 40%.