Tropospheric ozone plays a major role in Earth's atmospheric chemistry processes and also acts as an air pollutant and greenhouse gas. Due to its short lifetime, and dependence on sunlight and precursor emissions from natural and anthropogenic sources, tropospheric ozone's abundance is highly variable in space and time on seasonal, interannual and decadal time-scales. Recent, and sometimes rapid, changes in observed ozone mixing ratios and ozone precursor emissions inspired us to produce this up-to-date overview of tropospheric ozone's global distribution and trends. Much of the text is a synthesis of in situ and remotely sensed ozone observations reported in the peer-reviewed literature, but we also include some new and extended analyses using well-known and referenced datasets to draw connections between ozone trends and distributions in different regions of the world. In addition, we provide a brief evaluation of the accuracy of rural or remote surface ozone trends calculated by three state-of-the-science chemistry-climate models, the tools used by scientists to fill the gaps in our knowledge of global tropospheric ozone distribution and trends.
Submicron aerosol particles (PM<sub>1</sub>) were measured in-situ using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer during the summer 2009 Field Intensive Study at Queens College in New York, NY. Organic aerosol (OA) and sulfate are the two dominant species, accounting for 54% and 24%, respectively, of the total PM<sub>1</sub> mass. The average mass-based size distribution of OA presents a small mode peaking at ~150 nm (<i>D</i><sub>va</sub>) and an accumulation mode (~550 nm) that is internally mixed with sulfate, nitrate, and ammonium. The diurnal cycles of both sulfate and OA peak between 01:00–02:00 p.m. EST due to photochemical production. The average (±σ) oxygen-to-carbon (O/C), hydrogen-to-carbon (H/C), and nitrogen-to-carbon (N/C) ratios of OA in NYC are 0.36 (±0.09), 1.49 (±0.08), and 0.012 (±0.005), respectively, corresponding to an average organic mass-to-carbon (OM/OC) ratio of 1.62 (±0.11). Positive matrix factorization (PMF) of the high resolution mass spectra identified two primary OA (POA) sources, traffic and cooking, and three secondary OA (SOA) components including a highly oxidized, regional low-volatility oxygenated OA (LV-OOA; O/C = 0.63), a less oxidized, semi-volatile SV-OOA (O/C = 0.38) and a unique nitrogen-enriched OA (NOA; N/C = 0.053) characterized with prominent C<sub>x</sub>H<sub>2x + 2</sub>N<sup>+</sup> peaks likely from amino compounds. Our results indicate that cooking and traffic are two distinct and mass-equivalent POA sources in NYC, together contributing ~30% of the total OA mass during this study. The OA composition is dominated by secondary species, especially during high PM events. SV-OOA and LV-OOA on average account for 34% and 30%, respectively, of the total OA mass. The chemical evolution of SOA in NYC appears to progress with a continuous oxidation from SV-OOA to LV-OOA, which is further supported by a gradual increase of O/C ratio and a simultaneous decrease of H/C ratio in total OOA. Detailed analysis of NOA (5.8% of OA) presents evidence that organic nitrogen species such as amines might have played an important role in the atmospheric processing of OA in NYC, likely involving both acid-base chemistry and photochemistry. In addition, analysis of air mass trajectories and satellite imagery of aerosol optical depth (AOD) indicates that the high potential source regions of secondary sulfate and aged OA are mainly located in regions to the west and southwest of the city
.[1] Nitric acid (HNO 3 ) is the dominant end product of NO x (= NO + NO 2 ) oxidation in the troposphere, and its dry deposition is considered to be a major removal pathway for the atmospheric reactive nitrogen. Here we present both field and laboratory results to demonstrate that HNO 3 deposited on ground and vegetation surfaces may undergo effective photolysis to form HONO and NO x , 1 -2 orders of magnitude faster than in the gas phase and aqueous phase. With this enhanced rate, HNO 3 photolysis on surfaces may significantly impact the chemistry of the overlying atmospheric boundary layer in remote low-NO x regions via the emission of HONO as a radical precursor and the recycling of HNO 3 deposited on ground surfaces back to NO x .
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.
[I] Top-down constraints on global sulfur dioxide (S02) emissions are inferred through inverse modeling using S02 column observations from two satellite instruments (SCIAMACHY and OMI). We first evaluated the S02 column observations with surface S02 measurements by applying local scaling factors from a global chemical transport model (GEOS-Chem) to S02 columns retrieved from the satellite instruments. The resulting annual mean surface S02 mixing ratios for 2006 exhibit a significant spatial correlation (r 0.86, slope 0.91 for SCIAMACHY and r 0.80, slope 0.79 for OMI) with coincident in situ measurements from monitoring networks throughout the United States and Canada. We evaluate the GEOS-Chem simulation of the S02 lifetime with that inferred from in situ measurements to verity the applicability of GEOS-Chem for inversion of S02 columns to emissions. The seasonal mean S02 lifetime calculated with the GEOSChem model over the eastern United States is 13 h in summer and 48 h in winter, compared to lifetimes inferred from in situ measurements of 19 ± 7 h in summer and 58 ± 20 h in winter. We apply S02 columns from SCIAMACHY and OMI to derive a top-down anthropogenic S02 emission inventory over land by using the local GEOS-Chem relationship between S02 columns and emissions. There is little seasonal variation in the top-down emissions «15%) over most major industrial regions providing some confidence in the method. Our global estimate for annual land surface anthropogenic S02 emissions (52.4 Tg S yr-I from SCIAMACHY and 49.9 Tg S yr-I from OMI) closely agrees with the bottom-up emissions (54.6 Tg S yr-1 ) in the GEOS-Chem model and exhibits consistency in global distributions with the bottom-up emissions (r 0.78 for SCIAMACHY, and r 0.77 for OMI). However, there are significant regional differences.Citation: Lee, C., R. V. Martin, A. van Donkelaar, H. Lee, R. R. Dickerson, J. C. Hains, N. Krotkov, A. Richter, K. Vinnikov, and J. 1. Schwab (20ll), S02 emissions and lifetimes: Estimates from inverse modeling using in situ and global, space-based (SCIAMACHY and OMI) observations, J. Geophys. Res., 116, D06304, doi:1O.1029
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] Ambient measurements of HONO and HNO 3 , using a highly sensitive coil scrubbing/ HPLC/visible detection technique, were made at a rural site in southwestern New York State from 26 June to 14 July 1998, along with concurrent measurements of NO x , NO y , O 3 , and various meteorological parameters. The mean (and median) half-hour concentrations of HONO and HNO 3 during this period were 63 (and 56) pptv and 418 (and 339) pptv, respectively. On average, there were two HONO concentration peaks, the first around 0200-0300 LT and the second around 0700-0800 LT, and a minimum at about 2000 LT. The sum of NO x , HONO, and HNO 3 (AENO yi ) was highly correlated with the measured NO y concentration (r 2 = 0.64). The average HONO/NO x ratio was 0.07, while the average AENO yi /NO y ratio was 0.66. During the early morning hours, the photolysis of HONO appeared to be a dominant source of HO x radicals in boundary layer near the ground surface. The average daily radical production from HONO photolysis was 2.3 ppbv, accounting for 24% of the total production from photolyses of HONO, O 3 , and HCHO at the measurement height of 4 m above the ground. Diurnal patterns of HONO and relative humidity suggest that the ground and vegetation surfaces were sinks for HONO in the boundary layer when dew droplets were formed at night and that the subsequent release of the trapped nitrous acid/nitrite from the surfaces acted as a strong HONO source in the morning as the dew droplets evaporated. Our data also suggest that, in order to maintain the observed daytime HONO concentration of $60 pptv, there should be a strong daytime source of 220 pptv hr À1 , which was much greater than the nighttime source of 13 pptv hr À1and the estimated production of $ 40 pptv hr À1 from the gas-phase NO-OH reaction. Photolysis of HNO 3 , which deposits and accumulates on the ground and vegetation surfaces, may contribute significantly to the ''missing'' daytime HONO sources.
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