Tropospheric Ozone Studies
Background.
Tropospheric ozone continues to be a problem in the eastern U.S. despite
decades of air quality regulation and several generations of legislation. The general problem was
addressed in the recent NSF report Rethinking the Ozone problem in Urban and Regional Air
Pollution; this called for a coordinated multi-agency approach to ozone research. The North American
Research Strategy for Tropospheric Ozone (NARSTO)
resulted from these beginnings. Sponsors of the program include
NOAA, EPA, DOE, EPRI, the American Petroleum
Institute, the Motor Vehicle Manufacturers Association, and many power companies. ARL
scientists from several groups are contributing to the overall goal of learning
how to control surface ozone. ARL has direct interaction with the Ozone Transport Commission
and the Ozone Transport and Assessment Group (set up by the eastern states to address the
issue); in fact ARL models are serving as a basis for OTAG assessments. It is recognized, however,
that these models are likely to be found wanting in some areas, as a result of the studies now
ongoing under NARSTO. It is clear that the regulatory process is marching ahead in advance of the science. The
scientific community, therefore, is preparing its understanding of the problem in anticipation of the
next round of policy debate, early in the next decade.
Why NOAA, why now, and why ARL
NOAA is widely viewed as a unique source of high quality scientific
understanding of the problem.
The NOAA Aeronomy Laboratory serves in a
leadership role in this area; ARL is a partner with AL
and specializes on the measurement and interpretation of fluxes of ozone. The
urgency for information is related to the pressure for new regulations that can solve an
ozone problem that is still growing, and that will expand greatly if/when currently proposed revised ozone
standards are put into place. Many places that are now "in attainment" will lose this status
under the revised standards now being proposed. ARL specializes in measurements of deposition from the atmosphere,
using all of micrometeorological tower-based, aircraft, and inferential methodologies. ARL
scientists (especially at Research Triangle Park) are active
in studies of exchange between the boundary layer and the free troposphere, including the transport of
ozone and precursors by convection in clouds and the interactions at the marine-continental boundary
layer interface. ARL is the lead agency of the
World Meteorological Organization's
Quality Assurance Center for the Americas, a cornerstone activity of the WMO Global Atmosphere Watch with specific
concern related to the measurement of tropospheric ozone.
The many related strengths of ARL are described elsewhere in this document (e.g. the
Twin Otter flux-measuring aircraft). ARL focuses
research attention on improving understanding of the processes that influence the geochemical cycles of ozone
and its co-pollutants. ARL research concentrates on the physical (and frequently biological) mechanisms that
influence ozone concentrations. Results obtained, together with the conclusions drawn from
chemical process studies conducted mainly by other laboratories, are ingested into the comprehensive
models such as are being developed in the Atmospheric Sciences
Modeling Division, Research Triangle Park.
Several of the most relevant ARL activities are discussed below. These relate to surface deposition,
tropospheric ozone transport, and chemistry (including tracer technologies).
The goal of NARSTO is to carry
out a research program which will provide data for the development and evaluation of air quality
control strategies. This relates well with the dominant goal of ARL research. Effective action
to control ozone levels can only be taken if the mechanisms of ozone production and transport are
understood and if the effects of control measures on ozone levels can be measured. Reliable
Air Quality Simulation Models (AQSMs) must be developed and
tested so that the effects of control strategies can be predicted.
A strong team of ARL personnel took part in the NARSTO/Southern
Oxidant Study field program, conducted in central Tennessee (centered on Nashville) in mid-1996. Two ARL
teams operated surface instrumentation, measuring eddy fluxes of ozone and meteorological
quantities. The main ARL focus, however, was on the use of the new flux instrumentation carried
aboard the Twin Otter aircraft. The goal of the ARL aircraft program was to test the ability to
measure spatial average exchange rates between the atmosphere and the surface, in conditions where plume
chemistry complicates the ozone flux divergence field. In good conditions, measurements
of flux divergences can yield in-air determinations of the reaction rates of the ozone production
process. The ARL goal is to quantify these reaction rates, as a field test of results mainly derived
from laboratory and smog chamber studies.
The results obtained in the Nashville study indicated that a statistically
significant flux divergences of ozone could indeed be measured, even in the presence of the plumes. However,
without NO fluxes at the same time interpretation of the divergences that were measured
proved too daunting. At the time of this writing, plans are being made for a follow-up study in
conditions free of plumes (in southern Kansas), leading up to a further test over exceedingly flat land in
central Illinois.
Surface Ozone Research.
Work on ozone deposition is ongoing at the Atmospheric Turbulence and Diffusion
Division, in Oak Ridge, Tennessee, and at the Atmospheric Sciences
Modeling Division, Research Triangle Park, North Carolina.
Much of what is known about the destruction of ozone upon contact with the
surface ("air-surface exchange" of ozone) has been the result of intensive field studies made at
locations that are carefully selected to permit scrutiny of particular processes without confusion from
competing mechanisms. Such sites have been favored in the ARL field program, so far. It is studies of this kind that have
generated the knowledge that is currently integrated into numerical models of
the air-surface chemical exchange mechanism. These models emphasize that the controlling
property is often biological, associated with the stomatal resistance of the vegetation at the
surface and hence strongly a function of the biological species and the prevailing conditions. Recently,
ARL has been moving more towards long-term measurement programs of similar kind, to capture a wide
range of environmental conditions at a single location. This approach is useful, for
example, to evaluate model performance during conditions of short-term water stress, dewfall,
precipitation, etc.
Measurement capabilities include flux-measuring systems for use on towers as
well as aircraft and boats. In general, measurements at fixed locations are used to investigate
temporal variations in fluxes and in the process that control them. Aircraft and boat measurements
have recently been made possible by the development of a Mobile Flux Platform capability, that can
now be used to investigate the spatial distribution of fluxes. In association with the
development of this multi-faceted flux measuring capability, new fast-response
ozone sensors have been developed. Further improvements are being made at this time, to provide more stable and sensitive
response from the instrumentation.
The models developed in this work are used routinely to assess ozone
air-surface exchange at many sites distributed across the U.S., with most in the East. The monitoring of
ozone concentrations remains central to this network operation. Data obtained are used not only to
address research questions related to surface chemical and biological processes, but also to
determine ozone exposure levels within ecosystems, ozone uptake rates by vegetation, and human health
risk.
Current work is directed towards exploring the complex and rapid interactions
among various nitrogen oxides, hydrocarbons, and ozone in the near vicinity of vegetated
surfaces, where the reactions cause fluxes to change with height much more rapidly than for
non-reactive species. Work of this kind requires the use of rapid-response sensors, presently still under
development. Attention will first be paid to developing a fast-response NO detector, additional
methodologies for fast-response measurements of O3 (i.e., reverse NO
chemiluminescence and/or ethylene chemiluminescence) and perhaps exploration of sensitive, fast-response
measurements of biogenic hydrocarbon precursors (isoprene, etc.) to quantify emission/deposition fluxes
of these compounds to/from the atmosphere. Much of this work is in collaboration with the
Department of Meteorology, University of Maryland.
Tropospheric Ozone Research.
There has been a long history of boundary layer research within ARL, in which studies of ozone
profiles have been common. For example, ARL scientists have routinely operated
a tethersonde system to obtain information of this kind, often supported by profile data
derived from use of aircraft.
Several extensive field studies have been conducted. These include the Lake
Michigan Ozone Study, the San Joaquin Ozone Study, and the Arizona Ozone Study. In each case,
the focus was on the causes of ozone anomalies of potential regulatory concern.
The performance of ozone monitors used in tropospheric boundary layer
research was a major worry during the early 1990s. The results of this research are now widely known, and
the studies themselves have been finalized. ARL served as a party to the debate, which centered around the role
of aerosol particles as interferents in the measurement of ozone by UV-absorption.
A central interest at the time of this writing relates to how gas-phase
photochemical reactions and processes interact with the meteorological processes that cause transport and
dispersion. Critical measurements in this effort include O3,
NO/NOx/NOy, H2O2 and ROOH (organic
hydroperoxides), CO, etc. An effort in re-establishing and enhancing ARL measurement capabilities involves the
development of ultra-sensitive and fast response detectors for reactive nitrogen, including the
building and testing of a super-sensitive and fast-response NO detector, and the appropriate
converters for the detection of NOx (photolysis cell) and NOy (molybdenum and/or Au/CO
oxidation). Several ARL divisions (notably at Silver Spring,
Research Triangle Park, Oak Ridge, and Idaho
Falls) are well equipped to measure ozone; effort is now being directed towards measurement of ozone precursors.
An understanding of the effects of meteorological variables on tropospheric ozone levels is essential
to the detection of air quality changes resulting from emissions reductions. Only when these effects
are known can air quality data be normalized for meteorology and changes detected. The effects of
humidity, temperature, clouds, and solar radiation on ozone production must be measured.
Determination of diurnal and seasonal variations in meteorological parameters and the changes in
ozone production and transport they produce are necessary. The changes caused by climate patterns
and resulting from solar radiation changes induced by stratospheric ozone depletion should be
investigated. This meteorological data must have sufficient spatial and temporal resolution to
enable boundary layer characterization including inversion height changes.
From mid-July to mid-August 1996, scientists from NOAA's Air Resources Laboratory participated in the
Project NOVA (Natural emissions of Oxidant precursors: Validation of techniques and Assessment)
field experiment in rural northeastern North Carolina. The primary purpose of the project was to
compare different methodologies for estimating the release of nitric oxide (NO) from agricultural soils.
Secondary goals of the project included the quantification of emission estimates of NO from agricultural
crop land, and characterization of tropospheric photochemistry at a rural site in the southeastern U.S.
Collaborations
The main interactions involved in this work are with the set of universities
involved in the AIRMoN program, plus the University of Maryland.
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