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.

High loadings of anthropogenic carbonaceous aerosols in Chinese air influence the air quality for over one billion people and impact the regional climate. A large fraction (17–80%) of this aerosol carbon is water-soluble, promoting cloud formation and thus climate cooling. Recent findings, however, suggest that water-soluble carbonaceous aerosols also absorb sunlight, bringing additional direct and indirect climate warming effects, yet the extent and nature of light absorption by this water-soluble "brown carbon" and its relation to sources is poorly understood. Here, we combine source estimates constrained by dual carbon isotopes with light-absorption measurements of water-soluble organic carbon (WSOC) for a March 2011 campaign at the Korea Climate Observatory at Gosan (KCOG), a receptor station in SE Yellow Sea for the outflow from northern China. The mass absorption cross section at 365 nm (MAC₃₆₅) of WSOC for air masses from N. China were in general higher (0.8–1.1 m² g⁻¹), than from other source regions (0.3–0.8 m² g⁻¹). However, this effect corresponds to only 2–10% of the radiative forcing caused by light absorption by elemental carbon. Radiocarbon constraints show that the WSOC in Chinese outflow had significantly higher fraction fossil sources (30–50%) compared to previous findings in S. Asia, N. America and Europe. Stable carbon (δ¹³C) measurements were consistent with aging during long-range air mass transport for this large fraction of carbonaceous aerosols.

PM_1.0, PM_2.5, and PM₁₀ were sampled at Gosan ABC Superstation on Jeju Island from August 2007 to September 2008. The carbonaceous aerosols were quantified with the thermal/optical reflectance (TOR) method, which produced five organic carbon (OC) fractions, OC1, OC2, OC3, OC4, and pyrolyzed organic carbon (OP), and three elemental carbon (EC) fractions, EC1, EC2, and EC3. The mean mass concentrations of PM_1.0, PM_2.5, and PM₁₀ were 13.7 μg m⁻³, 17.2 μg m⁻³, and 28.4 μg m⁻³, respectively. The averaged mass fractions of OC and EC were 23.0% and 10.4% for PM_1.0, 22.9% and 9.8% for PM_2.5, and 16.4% and 6.0% for PM₁₀. Among the OC and EC sub-components, OC2 and EC2+3 were enriched in the fine mode, but OC3 and OC4 in the coarse mode. The filter-based PM_1.0 EC agreed well with black carbon (BC) measured by an Aethalometer, and PM₁₀ EC was higher than BC, implying less light absorption by larger particles. EC was well correlated with sulfate, resulting in good relationships of sulfate with both aerosol scattering coefficient measured by Nephelometer and BC concentration. Our measurements of EC confirmed the definition of EC1 as char-EC emitted from smoldering combustion and EC2+3 as soot-EC generated from higher-temperature combustion such as motor vehicle exhaust and coal combustion (Han et al., 2010). In particular, EC1 was strongly correlated with potassium, a traditional biomass burning indicator, except during the summer, when the ratio of EC1 to EC2+3 was the lowest. We also found the ratios of major chemical species to be a useful tool to constrain the main sources of aerosols, by which the five air masses were well distinguished: Siberia, Beijing, Shanghai, Yellow Sea, and East Sea types. Except Siberian air, the continental background of the study region, Beijing plumes showed the highest EC1 (and OP) to sulfate ratio, which implies that this air mass had the highest net warming by aerosols of the four air masses. Shanghai-type air, which was heavily influenced by southern China, showed the highest sulfate enhancement. The highest EC2+3/EC1 ratio was found in aged East Sea air, demonstrating a significant influence of motor vehicle emissions from South Korea and Japan and less influence from industrial regions of China. The high ratio results from the longer residence time and less sensitivity to wet scavenging of EC2+3 compared to EC1, indicating that soot-EC could have greater consequence in regional-scale warming.

The NASA GTE TRACE A mission sampled air over the South Atlantic and western Indian Oceans. Thirteen flight legs were flown within the marine boundary layer (MBL). The MBL was typically the cleanest air sampled (e.g., CH₄ < 1680 ppb, CO < 70 ppb, C₂H₆ < 400 ppt, C₃H₈ < 40 ppt, NO_x < 15 ppt, and midday NO < 5 ppt) but was overlain by polluted air. The photochemistry of the MBL was influenced by oceanic emissions, surface deposition, and entrainment of pollutants from aloft. Chemical budgets were constructed for several species in the MBL in order to investigate these effects and are presented for ethane, ethylene, propane, propylene, n‐butane, formic acid (HFo), methylhydroperoxide (CH₃OOH), oxides of nitrogen (i.e., NO, NO₂, PAN, HNO₃), hydrogen peroxide (H₂O₂), and ozone (O₃). A photochemical point model was used to evaluate local chemical production and loss. An entrainment model was used to assess material exchange between the lower free troposphere (FT) and the MBL and a resistance deposition model was used to evaluate material exchange across the air‐sea interface. The results suggested the ocean to be the source of measured alkenes in the MBL and to be the most likely source of the shorter‐lived alkanes: propane and n‐butane. Ethane was the only hydrocarbon for which input from aloft may have exceeded its photochemical destruction. The estimated hydrocarbon sources from the ocean were in agreement with prior analyses. Transport from the lower FT together with surface loss could not account for measured concentrations of CH₂O, HFo, and HNO₃. The transport of peroxyacetylnitrate (PAN) from the FT to the MBL exceeded the rate of HNO₃ production and was more than sufficient to maintain observed NO_x levels without having to invoke an oceanic source for NO. The flux of NO_x, PAN, and HNO₃ was in balance with the surface deposition flux of HNO₃. However, the predicted rates of HNO₃ formation from the oxidation of NO₂ and HNO₃ entrainment from aloft were inadequate to maintain observed levels of HNO₃ unless HNO₃ was partitioned between the gas phase and a more slowly depositing aerosol phase. The estimated dry deposition flux of HNO₃ to the South Atlantic during TRACE A, 2–4 × 10⁹ molecules cm⁻² s⁻¹, was about 10 times the annual average estimate for this region. The destruction of O₃ within the MBL was found to be exceeded by transport into the MBL from aloft, 6 ± 2 × 10¹⁰ compared to 11 ± 10 × 10¹⁰ molecules cm⁻² s⁻¹. The principal O₃ destruction process was mediated by the formation and surface deposition of H₂O₂ and CH₃OOH, 4 ± 4 × 10¹⁰ and 1.1 ± 0.5 × 10¹⁰ molecules cm⁻² s⁻¹. The direct loss of O₃ to the sea surface was estimated to be 1.7 ± 0.2 × 10¹⁰ molecules cm⁻² s⁻¹. CH₃OOH was lost to the sea and transported into the FT from the MBL. Its first‐order loss rate was estimated to be 7 × 10⁻⁶ s⁻¹ for a mean MBL height of 700 m. H₂O₂ and CH₂O losses from the MBL were estimated at rates of 1.3 × 10⁻⁵ s⁻¹ for both species. The inclusion of surface deposition improved the agreement between predicted and measured concentrations of HNO₃, CH₃OOH, H₂O₂, and CH₂O. However, model CH₂O remained significantly greater than that measured in the MBL.

Rapid urbanization and economic development in East Asia in past decades has led to photochemical air pollution problems such as excess photochemical ozone and aerosol formation. Asian megacities such as Seoul, Tokyo, Shanghai, Guangzhou, and Beijing are surrounded by densely forested areas, and recent research has consistently demonstrated the importance of biogenic volatile organic compounds (VOCs) from vegetation in determining oxidation capacity in the suburban Asian megacity regions. Uncertainties in constraining tropospheric oxidation capacity, dominated by hydroxyl radical, undermine our ability to assess regional photochemical air pollution problems. We present an observational data set of CO, NO_x, SO₂, ozone, HONO, and VOCs (anthropogenic and biogenic) from Taehwa research forest (TRF) near the Seoul metropolitan area in early June 2012. The data show that TRF is influenced both by aged pollution and fresh biogenic volatile organic compound emissions. With the data set, we diagnose HO_x (OH, HO₂, and RO₂) distributions calculated using the University of Washington chemical box model (UWCM v2.1) with near-explicit VOC oxidation mechanisms from MCM v3.2 (Master Chemical Mechanism). Uncertainty from unconstrained HONO sources and radical recycling processes highlighted in recent studies is examined using multiple model simulations with different model constraints. The results suggest that (1) different model simulation scenarios cause systematic differences in HO_x distributions, especially OH levels (up to 2.5 times), and (2) radical destruction (HO₂ + HO₂ or HO₂ + RO₂) could be more efficient than radical recycling (RO₂ + NO), especially in the afternoon. Implications of the uncertainties in radical chemistry are discussed with respect to ozone–VOC–NO_x sensitivity and VOC oxidation product formation rates. Overall, the NO_x limited regime is assessed except for the morning hours (8 a.m. to 12 p.m. local standard time), but the degree of sensitivity can significantly vary depending on the model scenarios. The model results also suggest that RO₂ levels are positively correlated with oxygenated VOCs (OVOCs) production that is not routinely constrained by observations. These unconstrained OVOCs can cause higher-than-expected OH loss rates (missing OH reactivity) and secondary organic aerosol formation. The series of modeling experiments constrained by observations strongly urge observational constraint of the radical pool to enable precise understanding of regional photochemical pollution problems in the East Asian megacity region.