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Anne M. Thompson

Abstract

Efforts to measure the tropospheric hydroxyl radical concentration OH, the transient that plays a role in acid deposition formation and cleanses the atmosphere of many pollutants, began 20 years ago. In the past five years considerable progress has been made in local (in situ or point) measurement of OH (refer to other papers in this issue). New measurements put constraints on photochemical models that compute OH and other species. Reproducing observed OH with a model is more successful in some regimes than in others with calculated OH usually within 20% of measurements. This paper reviews OH photochemistry, comparisons of model-derived and measured OH in the continental boundary layer, and model calculation of OH in the marine boundary layer and free troposphere when high-frequency aircraft measurements of O3, and related trace gases are assimilated into a photochemical model. Factors affecting model calculation of OH—nonlinearities, imprecisions in kinetics, differences among model formulation-are also discussed.

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Gregory G. Garner
and
Anne M. Thompson

Abstract

Air quality forecasts produced by the National Air Quality Forecast Capability (NAQFC), human air quality forecasters, and persistence are evaluated for predictive skill and economic value when used to inform decisions regarding pollutant emission and exposure. Surface ozone forecasts and observations were collected from 40 monitors representing eight forecast regions throughout Washington, D.C.; Virginia; and Maryland over the 2005–09 ozone seasons (April–October). The skill of the forecasts are quantified using discrete statistics, such as correlation, mean bias, and root-mean-square error, and categorical statistics, such as exceedance hit rate, false alarm rate, and critical success index. The value of the forecasts are quantified using a decision model based on costs to protect the public against a poor air quality event and the losses incurred if no protective measures are taken. The results indicate that the most skillful forecast method is not necessarily the most valuable forecast method. Air shed managers need to consider multiple forecast methods when deciding on multiple protective measures, because a single measure of forecast skill can often hide the user’s sensitivity to forecast error for a specific decision.

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Nikolay V. Balashov
,
Anne M. Thompson
, and
George S. Young

Abstract

The recent change in the Environmental Protection Agency’s surface ozone regulation, lowering the surface ozone daily maximum 8-h average (MDA8) exceedance threshold from 75 to 70 ppbv, poses significant challenges to U.S. air quality (AQ) forecasters responsible for ozone MDA8 forecasts. The forecasters, supplied by only a few AQ model products, end up relying heavily on self-developed tools. To help U.S. AQ forecasters, this study explores a surface ozone MDA8 forecasting tool that is based solely on statistical methods and standard meteorological variables from the numerical weather prediction (NWP) models. The model combines the self-organizing map (SOM), which is a clustering technique, with a stepwise weighted quadratic regression using meteorological variables as predictors for ozone MDA8. The SOM method identifies different weather regimes, to distinguish between various modes of ozone variability, and groups them according to similarity. In this way, when a regression is developed for a specific regime, data from the other regimes are also used, with weights that are based on their similarity to this specific regime. This approach, regression in SOM (REGiS), yields a distinct model for each regime taking into account both the training cases for that regime and other similar training cases. To produce probabilistic MDA8 ozone forecasts, REGiS weighs and combines all of the developed regression models on the basis of the weather patterns predicted by an NWP model. REGiS is evaluated over the San Joaquin Valley in California and the northeastern plains of Colorado. The results suggest that the model performs best when trained and adjusted separately for an individual AQ station and its corresponding meteorological site.

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Kenneth E. Pickering
,
Anne M. Thompson
,
Donna P. McNamara
, and
Mark R. Schoeberl

Abstract

The authors have compared isentropic trajectories computed from meteorological fields from different analysis centers. The analysis was performed for the South Atlantic, where a springtime maximum in tropospheric ozone has sparked considerable interest in the transport meteorology. Using the model of Schoeberl et al., isentropic forward trajectories were computed from an array of points over southern Africa and backward trajectories from an array of points over the South Atlantic. The model was run for an 8-day period in October 1989 with input taken from the twice-daily global gridded data fields from the National Meteorological Center (NMC) and from the European Centre for Medium-Range Weather Forecasts (BCMWF). There were large differences between the trajectories based on the two fields in terms of travel distance, horizontal separation, and vertical separation. Best comparisons for individual trajectories were found in the low-latitude easterlies, and the poorest comparisons were found in the westerlies and in the vicinity of the center of the South Atlantic subtropical anticyclone. Significant differences in wind speeds between the two analyses also led to large trajectory differences.

Trajectories were also computed using once-daily NMC fields. The effect of this degradation of the data was small. Trajectories computed from balanced winds computed from the NMC geopotential height and temperature fields showed the largest differences when compared with the ECMWF trajectories. The balanced wind fields should not be used in trajectory construction in the tropical lower troposphere.

It is difficult to make a definitive recommendation concerning which set of fields should be used in future transport analysts in this region due to the very large trajectory differences found in this analysis and the lack of any independent verification data. Any extensive analysis of transport in this region should be done only in conjunction with considerable additional data collection.

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Anne M. Thompson
,
Wei-Kuo Tao
,
Kenneth E. Pickering
,
John R. Scala
, and
Joanne Simpson

Theoretical studies, aircraft, and space-borne measurements show that deep convection can be an effective conduit for introducing reactive surface pollutants into the free troposphere. The chemical consequences of convective systems are complex. For example, sensitivity studies show potential for both enhancement and diminution of ozone formation. Field observations of cloud and mesoscale phenomena have been investigated with the Goddard Cumulus Ensemble and Tropospheric Chemistry models. Case studies from the tropical ABLE 2, STEP, and TRACE-A experiments show that free tropospheric ozone formation should increase when deep convection and urban or biomass burning pollution coincide, and decrease slightly in regions relatively free of ozone precursors (often marine). Confirmation of post-convective ozone enhancement in the free troposphere over Brazil, the Atlantic, and southern Africa was a major accomplishment of the September–October 1992 TRACE-A (Transport and Atmospheric Chemistry near the Equator—Atlantic) aircraft mission. A flight dedicated to cloud outflow showed that deep convection led to a factor of 3–4 increase in upper tropospheric ozone formation downwind. Analysis of ozonesondes during TRACE-A was consistent with 20%–30% of seasonally enhanced ozone over the South Atlantic being supplied by a combination of biomass burning emissions, lightning, and deep convection over South America. With the Tropics the critical region for troposphere-to-stratosphere transfer of pollutants, these results have implications for the total ozone budget. Cloud-scale analyses will guide the development of more realistic regional and global chemical-transport models to assess the full impact of deep convection on atmospheric chemical composition.

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Anne M. Thompson
,
Jacquelyn C. Witte
,
Samuel J. Oltmans
, and
Francis J. Schmidlin

This article describes the Southern Hemisphere Additional Ozonesondes (SHADOZ) network of ozonesonde-radiosonde stations in the southern Tropics and subtropics. SHADOZ was initiated in 1998 by NASA, NOAA, and a team of international meteorological services and space agencies to remedy a paucity of ozone profile data in a region of intense natural variability and anthropogenic change. SHADOZ augments launches at selected sites and provides a public archive of ozonesonde and radiosonde data (see additional information online at http://croc.gsfc.nasa.gov/shadoz). Ozone is important because of its role as an atmospheric UV shield, surface pollutant, oxidant, and greenhouse gas. Ozone profile data are essential for the detection of ozone trends and for verification of satellite ozone retrievals. Instrumentation, data, and a summary of the first scientific findings from SHADOZ are presented. A zonal view shows that troposphere ozone accumulates over the south tropical Atlantic and adjacent continents throughout the year, consistent with large-scale atmospheric motion. At individual stations, week-to-week variations in tropospheric ozone profiles reflect episodic meteorology, for example, convection or advected pollution.

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Emily N. Tinney
,
Cameron R. Homeyer
,
Lexy Elizalde
,
Dale F. Hurst
,
Anne M. Thompson
,
Ryan M. Stauffer
,
Holger Vömel
, and
Henry B. Selkirk

Abstract

Definition of the tropopause has remained a focus of atmospheric science since its discovery near the beginning of the twentieth century. Few universal definitions (those that can be reliably applied globally and to both common observations and numerical model output) exist and many definitions with unique limitations have been developed over the years. The most commonly used universal definition of the tropopause is the temperature lapse-rate definition established by the World Meteorological Organization (WMO) in 1957 (the LRT). Despite its widespread use, there are recurrent situations where the LRT definition fails to reliably identify the tropopause. Motivated by increased availability of coincident observations of stability and composition, this study seeks to reexamine the relationship between stability and composition change in the tropopause transition layer and identify areas for improvement in a stability-based definition of the tropopause. In particular, long-term (40+ years) balloon observations of temperature, ozone, and water vapor from six locations across the globe are used to identify covariability between several metrics of atmospheric stability and composition. We found that the vertical gradient of potential temperature is a superior stability metric to identify the greatest composition change in the tropopause transition layer, which we use to propose a new universally applicable potential temperature gradient tropopause (PTGT) definition. Application of the new definition to both observations and reanalysis output reveals that the PTGT largely agrees with the LRT, but more reliably identifies tropopause-level composition change when the two definitions differ greatly.

Significance Statement

In this study we provide a review of existing tropopause definitions (and their limitations) and investigate potential improvement in the definition of the tropopause using balloon-based observations of stability and atmospheric composition. This work is motivated by the need for correct identification of the tropopause to accurately assess upper-troposphere–lower-stratosphere processes, which in turn has far-reaching implications for our understanding of Earth’s radiation budget and climate. The result of this research is the creation of a new, universally applicable stability-based definition of the tropopause: the potential temperature gradient tropopause (PTGT).

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EXECUTIVE COMMITTEE
,
Robert J. Serafin
,
Richard D. Rosen
,
James F. Kimpel
,
George L. Frederick Jr.
,
Anne Thompson
,
Mary M. Glackin
,
Kenneth C. Spengler
, and
Ronald D. McPherson
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John T. Sullivan
,
Timothy Berkoff
,
Guillaume Gronoff
,
Travis Knepp
,
Margaret Pippin
,
Danette Allen
,
Laurence Twigg
,
Robert Swap
,
Maria Tzortziou
,
Anne M. Thompson
,
Ryan M. Stauffer
,
Glenn M. Wolfe
,
James Flynn
,
Sally E. Pusede
,
Laura M. Judd
,
William Moore
,
Barry D. Baker
,
Jay Al-Saadi
, and
Thomas J. McGee

Abstract

Coastal regions have historically represented a significant challenge for air quality investigations because of water–land boundary transition characteristics and a paucity of measurements available over water. Prior studies have identified the formation of high levels of ozone over water bodies, such as the Chesapeake Bay, that can potentially recirculate back over land to significantly impact populated areas. Earth-observing satellites and forecast models face challenges in capturing the coastal transition zone where small-scale meteorological dynamics are complex and large changes in pollutants can occur on very short spatial and temporal scales. An observation strategy is presented to synchronously measure pollutants “over land” and “over water” to provide a more complete picture of chemical gradients across coastal boundaries for both the needs of state and local environmental management and new remote sensing platforms. Intensive vertical profile information from ozone lidar systems and ozonesondes, obtained at two main sites, one over land and the other over water, are complemented by remote sensing and in situ observations of air quality from ground-based, airborne (both personned and unpersonned), and shipborne platforms. These observations, coupled with reliable chemical transport simulations, such as the National Oceanic and Atmospheric Administration (NOAA) National Air Quality Forecast Capability (NAQFC), are expected to lead to a more fully characterized and complete land–water interaction observing system that can be used to assess future geostationary air quality instruments, such as the National Aeronautics and Space Administration (NASA) Tropospheric Emissions: Monitoring of Pollution (TEMPO), and current low-Earth-orbiting satellites, such as the European Space Agency’s Sentinel-5 Precursor (S5-P) with its Tropospheric Monitoring Instrument (TROPOMI).

Open access
Jack Fishman
,
Kevin W. Bowman
,
John P. Burrows
,
Andreas Richter
,
Kelly V. Chance
,
David P. Edwards
,
Randall V. Martin
,
Gary A. Morris
,
R. Bradley Pierce
,
Jerald R. Ziemke
,
Jassim A. Al-Saadi
,
John K. Creilson
,
Todd K. Schaack
, and
Anne M. Thompson

We review the progress of tropospheric trace gas observations and address the need for additional measurement capabilities as recommended by the National Research Council. Tropospheric measurements show pollution in the Northern Hemisphere as a result of fossil fuel burning and a strong seasonal dependence with the largest amounts of carbon monoxide and nitrogen dioxide in the winter and spring. In the summer, when photochemistry is most intense, photochemically generated ozone is found in large concentrations over and downwind from where anthropogenic sources are largest, such as the eastern United States and eastern China. In the tropics and the subtropics, where photon flux is strong throughout the year, trace gas concentrations are driven by the abundance of the emissions. The largest single tropical source of pollution is biomass burning, as can be seen readily in carbon monoxide measurements, but lightning and biogenic trace gases may also contribute to trace gas variability. Although substantive progress has been achieved in seasonal and global mapping of a few tropospheric trace gases, satellite trace gas observations with considerably better temporal and spatial resolution are essential to forecasting air quality at the spatial and temporal scales required by policy makers. The concurrent use of atmospheric composition measurements for both scientific and operational purposes is a new paradigm for the atmospheric chemistry community. The examples presented illustrate both the promise and challenge of merging satellite information with in situ observations in state-of-the-art data assimilation models.

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