Ozone profile observations in Houston, Texas (1994 - 2010) from aircraft, balloons, and satellites

Houston, Texas has long been an urban area plagued with high levels of surface ozone, particularly in spring and late summer. The combination of a large commuter population and one of the largest concentrations of petrochemical plants in the world results in abundant and nearly co-located sources of NOx and hydrocarbons. The location of Houston on the South Coast of the United States in a subtropical climate results in meteorological conditions that favor ozone production. Using MOZAIC (1994 - 2004), ozonesonde (2000, 2004 - 2010), and TES (2005 – 2010) data, we examine the evolution of ozone profiles over Houston during a period in which various strategies have been implemented to alleviate the ozone pollution problem. Using meteorological data from associated soundings and analyses, we identify and evaluate influences on the ozone profiles from natural and anthropogenic sources, as well as local and remote sources. We further investigate how these various influences have changed with time

On the representation of IAGOS/MOZAIC vertical profiles in chemical transport models:contribution of different error sources in the example of carbon monoxide

Utilising a fleet of commercial airliners, MOZAIC/IAGOS provides atmospheric composition data on a regular basis that are widely used for modelling applications. Due to the specific operational context of the platforms, such observations are collected close to international airports and hence in an environment characterised by high anthropogenic emissions. This provides opportunities for assessing emission inventories of major metropolitan areas around the world, but also challenges in representing the observations in typical chemical transport models. We assess here the contribution of different sources of error to overall modeldata mismatch using the example of MOZAIC/IAGOS carbon monoxide (CO) profiles collected over the European regional domain in a time window of 5 yr (20062011). The different sources of error addressed in the present study are: 1) mismatch in modelled and observed mixed layer height; 2) bias in emission fluxes and 3) spatial representation error (related to unresolved spatial variations in emissions). The modelling framework combines a regional Lagrangian transport model (STILT) with EDGARv4.3 emission inventory and lateral boundary conditions from the MACC reanalysis. The representation error was derived by coupling STILT with emission fluxes aggregated to different spatial resolutions. We also use the MACC reanalysis to assess uncertainty related to uncertainty sources 2) and 3). We treat the random and the bias components of the uncertainty separately and found that 1) and 3) have a comparable impact on the random component for both models, while 2) is far less important. On the other hand, the bias component shows comparable impacts from each source of uncertainty, despite both models being affected by a low bias of a factor of 22.5 in the emission fluxes. In addition, we suggested methods to correct for biases in emission fluxes and in mixing heights. Lastly, the evaluation of the spatial representation error against modeldata mismatch between MOZAIC/IAGOS observations and the MACC reanalysis revealed that the representation error accounts for roughly 1520% of the modeldata mismatch uncertainty

Modeling lightning-NOx chemistry at sub-grid scale in a global chemical transport model

For the first time, a plume-in-grid approach is implemented in a chemical transport model (CTM) to parameterize the effects of the non-linear reactions occurring within high concentrated NOx plumes from lightning NOx emissions (LNOx) in the upper troposphere. It is characterized by a set of parameters including the plume lifetime, the effective reaction rate constant related to NOx-O3 chemical interactions and the fractions of NOx conversion into HNO3 within the plume. Parameter estimates were made using the DSMACC chemical box model, simple plume dispersion simulations and the mesoscale 3-D Meso-NH model. In order to assess the impact of the LNOx plume approach on the NOx and O3 distributions at large scale, simulations for the year 2006 were performed using the GEOS-Chem global model with a horizontal resolution of 2° × 2.5°. The implementation of the LNOx parameterization implies NOx and O3 decrease at large scale over the region characterized by a strong lightning activity (up to 25 and 8 %, respectively, over Central Africa in July) and a relative increase downwind of LNOx emissions (up to 18 and 2 % for NOx and O3, respectively, in July) are derived. The calculated variability of NOx and O3 mixing ratios around the mean value according to the known uncertainties on the parameter estimates is maximum over continental tropical regions with ΔNOx [−33.1; +29.7] ppt and ΔO3 [−1.56; +2.16] ppb, in January, and ΔNOx [−14.3; +21] ppt and ΔO3 [−1.18; +1.93] ppb, in July, mainly depending on the determination of the diffusion properties of the atmosphere and the initial NO mixing ratio injected by lightning. This approach allows (i) to reproduce a more realistic lightning NOx chemistry leading to better NOx and O3 distributions at the large scale and (ii) focus on other improvements to reduce remaining uncertainties from processes related to NOx chemistry in CTM

Interpretation of TOMS observations of tropical tropospheric ozone with a global model and in-situ observations

We interpret the distribution of tropical tropospheric ozone columns (TTOCs) from the Total Ozone Mapping Spectrometer (TOMS) by using a global three-dimensional model of tropospheric chemistry (GEOS-CHEM) and additional information from in situ observations. The GEOS-CHEM TTOCs capture 44% of the variance of monthly mean TOMS TTOCs from the convective cloud differential method (CCD) with no global bias. Major discrepancies are found over northern Africa and south Asia where the TOMS TTOCs do not capture the seasonal enhancements from biomass burning found in the model and in aircraft observations. A characteristic feature of these northern tropical enhancements, in contrast to southern tropical enhancements, is that they are driven by the lower troposphere where the sensitivity of TOMS is poor due to Rayleigh scattering. We develop an efficiency correction to the TOMS retrieval algorithm that accounts for the variability of ozone in the lower troposphere. This efficiency correction increases TTOCs over biomass burning regions by 3–5 Dobson units (DU) and decreases them by 2–5 DU over oceanic regions, improving the agreement between CCD TTOCs and in situ observations. Applying the correction to CCD TTOCs reduces by ∼5 DU the magnitude of the “tropical Atlantic paradox” [Thompson et al., 2000], i.e. the presence of a TTOC enhancement over the southern tropical Atlantic during the northern African biomass burning season in December–February. We reproduce the remainder of the paradox in the model and explain it by the combination of upper tropospheric ozone production from lightning NOx, persistent subsidence over the southern tropical Atlantic as part of the Walker circulation, and cross-equatorial transport of upper tropospheric ozone from northern midlatitudes in the African “westerly duct.” These processes in the model can also account for the observed 13–17 DU persistent wave-1 pattern in TTOCs with a maximum over the tropical Atlantic and a minimum over the tropical Pacific during all seasons. The photochemical effects of mineral dust have only a minor role on the modeled distribution of TTOCs, including over northern Africa, due to multiple competing effects. The photochemical effects of mineral dust globally decrease annual mean OH concentrations by 9%. A global lightning NOx source of 6 Tg N yr−1 in the model produces a simulation that is most consistent with TOMS and in situ observations

A tropospheric ozone maximum over the Middle East

The GEOS-CHEM global 3-D model of tropospheric chemistry predicts a summertime O3 maximum over the Middle East, with mean mixing ratios in the middle and upper troposphere in excess of 80 ppbv. This model feature is consistent with the few observations from commercial aircraft in the region. Its origin in the model reflects a complex interplay of dynamical and chemical factors, and of anthropogenic and natural influences. The anticyclonic circulation in the middle and upper troposphere over the Middle East funnels northern midlatitude pollution transported in the westerly subtropical jet as well as lightning outflow from the Indian monsoon and pollution from eastern Asia transported in an easterly tropical jet. Large-scale subsidence over the region takes place with continued net production of O3 and little mid-level outflow. Transport from the stratosphere does not contribute significantly to the O3 maximum. Sensitivity simulations with anthropogenic or lightning emissions shut off indicate decreases of 20–30% and 10–15% respectively in the tropospheric O3 column over the Middle East. More observations in this region are needed to confirm the presence of the O3 maximum

The Global Atmosphere Watch reactive gases measurement network

Long-term observations of reactive gases in the troposphere are important for understanding trace gas cycles and the oxidation capacity of the atmosphere, assessing impacts of emission changes, verifying numerical model simulations, and quantifying the interactions between short-lived compounds and climate change. The World Meteorological Organization’s (WMO) Global Atmosphere Watch (GAW) program coordinates a global network of surface stations some of which have measured reactive gases for more than 40 years. Gas species included under this umbrella are ozone, carbon monoxide, nitrogen oxides, and volatile organic compounds (VOCs). There are many challenges involved in setting-up and maintaining such a network over many decades and to ensure that data are of high quality, regularly updated and made easily accessible to users. This overview describes the GAW surface station network of reactive gases, its unique quality management framework, and discusses the data that are available from the central archive. Highlights of data use from the published literature are reviewed, and a brief outlook into the future of GAW is given. This manuscript constitutes the overview of a special feature on GAW reactive gases observations with individual papers reporting on research and data analysis of particular substances being covered by the program. - See more at: http://elementascience.org/article/info:doi/10.12952/journal.elementa.000067#sthash.cHvHu0T6.dpu

Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties

From the earliest observations of ozone in the lower atmosphere in the 19th century, both measurement methods and the portion of the globe observed have evolved and changed. These methods have different uncertainties and biases, and the data records differ with respect to coverage (space and time), information content, and representativeness. In this study, various ozone measurement methods and ozone datasets are reviewed and selected for inclusion in the historical record of background ozone levels, based on relationship of the measurement technique to the modern UV absorption standard, absence of interfering pollutants, representativeness of the well-mixed boundary layer and expert judgement of their credibility. There are significant uncertainties with the 19th and early 20th-century measurements related to interference of other gases. Spectroscopic methods applied before 1960 have likely underestimated ozone by as much as 11% at the surface and by about 24% in the free troposphere, due to the use of differing ozone absorption coefficients. There is no unambiguous evidence in the measurement record back to 1896 that typical mid-latitude background surface ozone values were below about 20 nmol mol–1, but there is robust evidence for increases in the temperate and polar regions of the northern hemisphere of 30–70%, with large uncertainty, between the period of historic observations, 1896–1975, and the modern period (1990–2014). Independent historical observations from balloons and aircraft indicate similar changes in the free troposphere. Changes in the southern hemisphere are much less. Regional representativeness of the available observations remains a potential source of large errors, which are difficult to quantify

Distribution, variability and sources of tropospheric ozone over south China in spring: intensive ozonesonde measurements at five locations and modeling analysis

We examine the characteristics of the spatial distribution and variability of tropospheric ozone (O3) by analysis of 93 ozonesonde profiles obtained at five stations over south China (18–30 N) during a field campaign in April–May 2004. We use a global 3-D chemical transport model (GEOS-Chem) to interpret these characteristics and to quantify the sources of tropospheric O3 over south China during this period. The observed tropospheric O3 mixing ratios showed strong spatiotemporal variability due to a complex interplay of various dynamical and chemical processes. A prominent feature in the upper and middle troposphere (UT/MT) was the frequent occurrence of high O3 mixing ratios shown as tongues extending down from the lower stratosphere or as isolated layers at all stations. The model largely captured the observed pattern of day-to-day variability in tropospheric O3 mixing ratios at all stations, but often underestimated those tongues or isolated layers of O3 enhancements observed in the UT/MT, especially at low-latitude stations. We found that tropospheric O3 along the southeast China coast was mainly produced within Asia. Lightning NOx emissions (over South Asia and equatorial Africa) and/or stratospheric influences were responsible for major events of high O3 observed in the UT/MT at all stations. Underestimated contributions of these sources likely led to the model’s underestimate in the low-latitude UT/MT O3. This study emphasizes the need for improved understanding of lightning NOx emissions and stratospheric influences over the Eurasian and African continents and for better representation of these processes in current global models

Tropospheric Ozone Assessment Report: Present-day distribution and trends of tropospheric ozone relevant to climate and global atmospheric chemistry model evaluation

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