Diurnal Variation of Tropospheric Trace Gas Amounts and Aerosol Optical Characteristics
October 26, 2012 15:25:43
Description of Problem
The project involves the development of a new ground-based network of highly accurate spectrometer systems (Pandora and Cleo instruments, Fig 1) and the development, optimization and validation of remote-sensing retrieval algorithms for obtaining new measurements of aerosol optical characteristics and tropospheric trace gas amounts and vertical distribution (NO2, O3, SO2, H2O, HCHO). The resulting data provide a unique dataset for bounding tropospheric photochemical models, and studying the evolution of tropospheric ozone, NO2, other trace gases, and aerosols and their impacts on climate and air quality. Measurements are applied to improve interpretation of current satellite observations and assess more effective design and observing strategies for future NASA satellite missions.
Scientific Objectives and Approach
Our specific objectives are:
• Characterization of the spatial and temporal (diurnal and seasonal) variability of tropospheric trace gas amounts, and O3 and NO2 vertical distribution at both polluted and clean sites. This is essential for improvement of both satellite retrievals and photochemical model predictions.
• Determine the input of trace gas and aerosol pollutants into the atmosphere during extreme events (e.g. extensive forest fires, desert dust events).
• Determine aerosol optical characteristics (optical depth, single scattering albedo and particle size distribution) over UV and visible wavelengths.
• Apply results to improve satellite retrieval algorithms and interpretation of satellite observations to diagnose near‐surface conditions relating to air quality
We use the Multiple-Angle Spectrally Resolved method for measuring tropospheric O3 and NO2 total column amounts and vertical profiles. The measurements are obtained every 10 minutes using measured sky radiances from a small, highly accurate, spectrometer system, Pandora (280 – 525 nm with spectral resolution of 0.5 nm; high signal to noise ratio 1000:1). A new spectrometer version (300 nm to 800 nm; 1 nm resolution) of a shadowband-type instrument (Cleo) has also been developed for measurements of UV aerosol properties.
A network of Pandora spectrometers (both direct and sky-radiance instruments) is currently operating successfully. We have started working on the development of a new modified Pandora instrument that can also operate on coastal platforms and onboard ships, using feedback from a digital camera to the sun-tracker to keep Pandora pointing at the sun on a moving platform.
Measurements of total column NO2 have been validated using MFDOAS spectrometers and the Direct-Sun DOAS Technique. We have studied changes in the diurnal variability of total column NO2 with season and day of the week based on a 2-year time series at Goddard Space Flight Center. Results have been applied to OMI validation [Herman et al., 2009].
We have developed an algorithm for measuring total column ozone (TCO) amounts using direct-sun observations and an independently measured reference spectrum (Kurucz extraterrestrial spectrum, normalized to SUSIM, convoluted with the Pandora slit function). Pandora TCO retrievals were compared with accurate TCO measurements obtained by a modified, Brewer (#171) double monochromator and with OMI satellite total column ozone retrievals at several locations (e.g. NAA/Goddard, NASA/Langley, Cabauw/Netherlands). Measurements were found to be in close agreement throughout the day, including near sunrise and sunset conditions with solar zenith angles up to 80o. Considerable temporal variability was observed in TCO measured by Pandora, with diurnal variability as high as 40 DU in some cases. These short-term (e.g. hourly) temporal changes in ozone amounts cannot be captured by OMI or other satellite instruments in sun-synchronous orbits (daily overpass) [Tzortziou et al., 2010; Tzortziou et al., In Prep.].
We have participated in several air-quality field campaigns (e.g. Cabauw-Netherlands, in Summer 2009, Frostburg MD, in Fall 2010; DISCOVER-AQ campaign, Chesapeake Bay, in Summer 2011), for integration of Pandora and Cleo measurements with other ground-based and aircraft observations.
Measurements of trace gas variability from our network of Pandora spectrometers at 3 locations (soon to be 14) provide a unique dataset for testing and validating model photochemistry and capturing short-term, small-scale variability in atmospheric composition. The data from 14 sites will be particularly useful for measuring satellite sub-pixel and pixel-to-pixel variability, and validating satellite data so that they can be used for monitoring the response of the ozone layer to atmospheric changes and studying impacts on climate and air quality.
We are currently developing a new software tool for deriving tropospheric O3 vertical distribution amounts using VLIDORT (Vector LInearized Discrete Ordinate Radiative Transfer) and our published O3 profile retrieval algorithm [Tzortziou et al., 2008]. Frequent measurements of tropospheric ozone concentrations and vertical distribution are essential for determining short time-scale changes in ozone amounts close to the ground, and quantifying the role of tropospheric ozone on local and regional environmental degradation, tropospheric chemistry, surface UV-B budgets, human health, radiative forcing and climate.
Results from our activities and participation in field campaigns, and comparisons with CMAQ and WRF-Chem model outputs, are currently being used to determine how often and by how much tropospheric column NO2 and O3 deviate from climatology in coastal areas. Information will be used to evaluate whether climatology or OMI data would be sufficient for accurate atmospheric correction of the high resolution ocean color measurements proposed for the NASA GeoCAPE (Geostationary Coastal and Air Pollution Events mission) ocean sensor. Results will be assessed in terms of the proposed characteristics of GeoCAPE and recommendations will be made to meet ocean retrieval requirements.
Refereed Journal Publications
Tzortziou M., N.A. Krotkov, A.Cede, J. R. Herman, A. Vassilkov (2008), A new technique for retrieval of tropospheric and stratospheric ozone profiles using sky radiance measurements at multiple view angles – Application to a Brewer spectrometer. J. Geophys. Res., 113, D06304, DOI:10.1029/2007JD009093.
Tzortziou M., Herman J.R., Cede A., Abuhassan N., 2011, “High Precision, Absolute Total Column Ozone Measurements from the Pandora Spectrometer: Comparisons with Data from a Brewer Double Monochromator and Aura OMI”, JGR-Atmospheres, In Prep.
Herman J., A. Cede, E. Spinei, G. Mount, M. Tzortziou, N. Abuhassan (2009), NO2 Column Amounts from Ground-based Pandora and MFDOAS Spectrometers using the Direct-Sun DOAS Technique: Inter-comparisons and Application to OMI Validation. Journal of Geophysical Research, 114, D13, DOI:10.1029/2009JD011848.
Other Publications and Conferences
Tzortziou M., J. Herman, A. Cede, N. Abuhassan, “New Total Column Ozone and Trace Gas Measurements from the Pandora Spectrometer System”, AURA Science Team Meeting, Boulder, CO, September 27-29, 2010
Fig. 1 – Close agreement throughout the day between Brewer, Pandora and OMI total column ozone amounts over the Goddard Space Flight Center, in Greenbelt, MD.
Fig. 2 – Pandora and Cleo Systems