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Atmospheric Investigations and Measurements (ARIM) Group
ARIM Group Members:
Summary of Activities:
Regional and Global Air Quality
The Atmospheric Investigations and Measurements (ARIM) group deployed the Scanning Actinic Flux Spectroradiometer (SAFS) instruments on the NCAR C-130 and NASA DC-8 for the airborne phases of the Megacities Impact on Regional and Global Environment (MIRAGE http://mirage-mex.acd.ucar.edu/) and Intercontinental Chemical Transport Experiment phase B (INTEX-B http://cloud1.arc.nasa.gov/intex-b/) field intensives. The SAFS instruments determine wavelength dependent actinic flux from 280-420 nm. The actinic flux in combination with the absorption cross section and quantum yield molecular data were used to calculate the photolysis frequencies of multiple photochemically important molecular processes, including O3, NO2, HONO, CH2O, H2O2, CH3OOH, HNO3, PAN, CH3NO3, CH3CH2NO3, and CH3COCH3.
Figure: SAFS ozone and NO2 photolysis frequency measurements and NCAR TUV modeling during MIRAGE while flying over a bright cloud.
The SAFS measurement is based on a 2 p steradian hemisphere hemispherical quartz light collector, a double monochromator , and a low dark current photomultiplier. The monochromator employs dual 2400 G/mm gratings which produce a 1 nm FWHM spectral resolution and very low straylight . The instrument package on the aircraft included two independent, but time synchronized (IRIG-B) spectroradiometer systems to measure the up- and down-welling fluxes in a 10 second scan time. Summing these produced the spherically integrated actinic flux.
Figure: Zenith actinic flux head on the NSF/NCAR C-130 in Veracruz , Mexico during the MIRAGE campaign
The spectral response of the SAFS was determined in the ARIM optical calibration facility equipped with precision radiometric power supplies and multiple NIST traceable 1000W quartz tungsten halogen lamps. Secondary lamp standards were applied in the field to calibrate the systems before each aircraft flight. Mercury line calibrations were also performed to track the wavelength accuracy.
The Mexico City environment contains large concentrations of gas phase pollutants and aerosols that can dramatically perturb the radiation environment and therefore the local photochemistry. The in situ measurements of the photolysis frequencies of photochemically important molecules are critical to understanding the polluted megacity atmosphere and the downstream processing of the plume advected from the city environs. Ozone production in the megacity plume is dependent on the radiation environment encountered by the plume as it is advected out of the city and diluted. Measurements of actinic flux and aerosol size distributions and composition are necessary to determine the impact of the aerosols on photolysis and oxidant production rates. The airborne measurements of radiation and aerosol concentrations, size distributions, and extinctions allow for stringent testing of radiative transfer models used to study regional and global impacts of megacity pollution.
Tropospheric Chemistry-Climate Studies
The NASA Intercontinental Chemical Transport Experiment phase B (INTEX-B http://cloud1.arc.nasa.gov/intex-b/) project, in conjunction with the Intercontinental and Megacity Pollution Experiment ( IMPEX) project endeavored to understand the transport and transformation of gases and aerosols on transcontinental/intercontinental scales and assess their impact on air quality and climate. Central to this understanding are the photochemical changes induced during the transport process. Photochemical reactions provide the driving force for much of the chemistry in the atmosphere. Therefore the in situ rates of photolysis reactions are important in determining production and loss terms for the key atmospheric species, odd hydrogen radicals and ozone.
The ARIM group deployed the Scanning Actinic Flux Spectroradiometers (SAFS) on the NASA DC-8 aircraft for INTEX-B and the NSF/NCAR C-130 for IMPEX, to provide actinic flux measurements in UV and visible wavelengths. The in situ photolysis of 23 important photochemical species, including O3, NO2, CH2O, HONO, HNO3, N2O5, HO2NO2, PAN, H2O2, CH3OOH, CH3ONO2, CH3CH2ONO2, CH3COCH3, CH3CHO, CH3CH2CHO, CHOCHO, CH3COCHO, CH3CH2CH2CHO AND CH3COCH2CH3 was calculated from the actinic flux measurements. IMPEX and INTEX-B were funded by NSF and the Tropospheric Chemistry Program at NASA.
UT/LS and Middle Atmospheric Studies
The Solid state, CCD Actinic Flux Spectroradiometers (CAFS) instruments developed in the NCAR/ARIM laboratory were deployed successfully on the NASA WB-57 Costa Rica Aura Validation Experiment (CR-AVE http://cloud1.arc.nasa.gov/ave-costarica2/). The CAFS measurements of up and downwelling flux were used in conjunction with radiative transfer calculations to obtain the direct solar beam fraction of the measured flux as a function of wavelength. The ozone absorption of the direct beam was determined to obtain the total ozone column abundances above the aircraft.
Figure: The zenith CAFS instrument mounted on the fuselage of the NASA WB-57 aircraft.
This ozone column calculation performed by Irina Petropavlovskikh of NOAA/ESRL/GMD has been extremely useful for the AURA satellite validation, particularly the OMI and MLS instrumentation. An example is shown in the figure below. CAFS retrieved ozone columns above the aircraft level, OMI total column and surface reflectivity are plotted as functions of time. In addition, MLS profiles are selected to match with the WB-57 geo-location. To compare against CAFS data, the MLS profiles are integrated above the altitude of the WB-57 aircraft determined for each co-incidental profile. The agreement between the MLS and CAFS data is within few percent.
Figure A: Time series of ozone above the aircraft level on January 22, 2006 . The CAFS retrieved ozone above the aircraft level (black line), OMI total column (blue line) and surface reflectivity (yellow line, offset by 240 units to fit to the plot area) are shown for comparisons. The spatially co-located MLS data are shown as crosses, where red and green colors indicate zonal distance from the aircraft of greater than or less than 5 degrees respectfully. The green and red vertical lines represent 10 % standard deviation errors on the MLS data.
The summary of comparisons for six of the CR-AVE flights is shown in the figure below. The coincident MLS and CAFS partial ozone columns agree to within +/- 3% for flight altitudes between 16 and 19 km.
Figure: A six day summary of comparisons between co-located CAFS and MLS partial ozone columns during CR-AVE. The differences between MLS and CAFS partial ozone columns as function of altitudes are shown. The green points mark a sub-set of data selected by limiting the WB-57 altitude variations to less than 1 km and instrument differences in geo-location to less than 5 degrees in zonal direction.
Additionally during CR-AVE, CAFS data were used for the first time to validate a new OMI profile product based on the algorithm developed for the SBUV V8 profile retrievals. The figure below presents a time series of the CAFS derived ozone columns and integrated ozone profiles retrieved from co-incidental measurements by the SBUV/2 instrument on board of the NOAA-16 and NOAA-17 satellites, and from the OMI data on board of the Aura satellite or the WB-57 flight on January 22, 2006. The beginning of the time series appears to have some bias possibly related to the high surface reflectivity that represents the presence of the clouds. The SBUV type algorithm does not account for the actual altitude of the cloud; however, ozone profile retrieval is sensitive to the cloud altitude.
Figure: Same as Figure A, but for SBUV type of retrieval in place of the MLS retrieval.
Based on the results provided by the CAFS data, the algorithm for the SBUV-type profile will be re-evaluated and adjusted to account for the altitude of the clouds in the observed scene.
CR-AVE was funded Upper Atmosphere Research Program at NASA.
The ARIM group has similar CAFS instrumentation under development for the NSF/NCAR HIAPER aircraft as part of the HIAPER Aircraft Instrumentation Solicitation (HAIS).
Science Highlight
The ARIM group in ACD/NCAR developed and deployed SAFS instruments on two aircraft during the MIRAGE campaign to measure the total radiation field, or actinic flux, from which photochemical rates are calculated. The project looked at the Mexico City atmospheric environment, which contains large concentrations of gas phase pollutants and aerosols. These can dramatically affect the radiation environment, a driving force of the local atmospheric chemistry. Thus the photochemical rates are critical to understanding the polluted megacity atmosphere and the downstream processing of the plume as it is advected away from the city. In particular, ozone production in the plume is dependent on the radiation environment encountered as it is advected out of the city and diluted. Additionally, these measurements are necessary to determine the impact of the aerosols on the radiation field and on oxidant production rates. The data provides for stringent testing of radiative transfer models used to study regional and global impacts of megacity pollution. The SAFS instruments flew on the NCAR C-130 and the NASA DC-8 which were funded by NSF and the Tropospheric Chemistry program at NASA, respectively.
ARIM group Richard Shetter , Samuel Hall and Kirk Ullmann . For more information contact: Richard Shetter (shetter@ucar.edu) or go to < http://mirage-mex.acd.ucar.edu/> and <http://arim.acd.ucar.edu/>
Figure: Zenith actinic flux head on the NSF/NCAR C-130 in Veracruz , Mexico during the MIRAGE campaign
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