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Southern Oxidant Study (SOS)


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.


  • 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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