Southern Oxidant Study (SOS)
Background
O3 is a secondary pollutant, formed from precursor
species NOx and NMHCs (VOC), represented here as RH species
RH + OH --> R + H2O
R + O2 + M --> RO2 + M
RO2 + NO --> RO + NO2
RO --> products
NO2 + hv --> NO + O
O + O2 + M --> O3 + M
NET RH + OH + 2 O2 + hv -->H2O + O3 + products
Southern Oxidants Study
SOS was initiated in 1988, and was an outgrowth of 1988
Workshop on Photochemical Oxidants: a Southern Perspective.
Additional impetus was provided by the landmark National
Academy of Sciences report, Rethinking the Ozone
Problem in Urban and Regional Air Pollution.
Motivation
- Background O3 concentrations are about 2-3
times higher than they were 100 years ago.
- Despite progressively tighter controls on VOC emissions
since 1970's, there has been no documented decrease in
O3 concentrations. Complexity and uncertainties
arise from:
- Non-linear/complex chemistry (NOx vs VOC control)
- Natural VOC sources of reactive biogenic hydrocarbons
- Importance of regional O3 distributions,
long-range transport in determining local ozone concentrations
- The strong influence of meteorology
- Uncertainties in emission inventories of precursors
- Lack of suitable measurement techniques for all relevant
species
- Lack of comprehensive meteorological and chemical databases
for model development and evaluation
Why the South as an Area of Study?
- There are 28 O3 non-attainment areas in
a 10 state region of the South
- Land use in the South is 60-70% forested, 20-25% agricultural;
this is distinctly different from land usage in the Northeastern
U.S.
- The South is thus an ideal region to study the effects
of biogenic emissions upon O3 generation.
- The South is characterized by ideal summertime meteorology
for efficient O3 formation: Hot, sunny, stagnant
conditions inhibit pollutant dispersal and effectively
generate O3. High temperatures lead to high
emissions of reactive biogenic NMHCs
Why Nashville?
- Nashville is largely isolated from large upwind urban
centers, and can be considered to be embedded in "regional
background" air.
- Nashville is a large and growing population center;
typical of the South
- The presence of large emission point sources (power
plants) makes Nashville an ideal place to study the embedded
plume phenomenon which can lead to high O3 concentrations:
Goals of ARL Involvement in 1995 SOS Intensive
A total of six instrumented research aircraft participated
in the study. Over 100 Level I O3 measurement
stations were deployed in a 12-state region (State Regulatory
Sites), along with 7 Level II enhanced chemistry sites
(O3, NO, NO2, NOy, CO, SO2,
VOCs, Met) and 2 Level III chemistry sites (Level II plus
PAN, HNO3, Oxygenated HCs, etc.) around Nashville.
Radar profilers; rawinsondes; ozonesondes; emissions inventory
assessments (anthropogenic and biogenic precursors); land
use classification studies; and a variety of meteorological
and computational (numerical models) resources were also
deployed for the intensive. The NOAA Twin Otter was operated
in Nashville by ARL scientists in June-July, 1995 as part
of the 1995 field intensive of SOS. ARL's research goals
included:
- The measurement of surface fluxes of sensible and latent
heat, momentum, CO2, and O3
- Measurements of flux divergence, esp. of ozone, in
the mixed layer
- Simultaneous measurements of ozone and ozone precursors
using sensitive trace gas analyzers. The coupling of
trace gas chemistry and flux measurement capabilities
constituted the first ever such pairing on a U.S. research
aircraft. Simultaneous measurements of ozone fluxes and
precursor species are essential for more reliable numerical
simulation of ozone photochemistry and surface deposition.
- Characterization of vertical profiles of ozone and
key precursors in the mixed layer.
- Characterization of near-field chemistry and emissions
of ozone precursors.
For more information regarding some preliminary results from the Twin Otter
measurements, click here.
For more information about the structure and organization
of SOS; its goals; participants; and observational and
computational resources devoted to SOS, please visit the
SOS
home page.
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