Abstract
The development of a new concurrent multiaxis (CMAX) sky viewing spectrometer to monitor rapidly changing urban concentrations of nitrogen dioxide is detailed. The CMAX differential optical absorption spectroscopy (DOAS) technique involves simultaneous spectral imaging of the zenith and off-axis measurements of spatially resolved scattered sunlight. Trace-gas amounts are retrieved from the measured spectra using the established DOAS technique. The potential of the CMAX DOAS technique to derive information on rapidly changing concentrations and the spatial distribution of NO2 in an urban environment is demonstrated. Three example data sets are presented from measurements during 2004 of tropospheric NO2 over Leicester, UK (52.62 degrees N, 1.12 degrees W). The data demonstrate the current capabilities and future potential of the CMAX DOAS method in terms of the ability to measure real-time spatially disaggregated urban NO2.
Highlights
Zenith-viewing UV/Visible scattered light differential optical absorption spectroscopy (DOAS) has been used over a number of decades for the retrieval of atmospheric concentrations of traces species including O3, NO2, BrO, OClO, IO, HCHO and O4
Off-axis techniques exploit the improved sensitivity of non-zenith measurements to tropospheric trace-gas concentrations owing to the enhanced tropospheric path length of scattered light collected at low elevation angles
The sensitivity of the measurement to stratospheric and tropospheric absorbers is related to the length of the absorption path through the stratosphere or troposphere
Summary
Zenith-viewing UV/Visible scattered light differential optical absorption spectroscopy (DOAS) has been used over a number of decades for the retrieval of atmospheric concentrations of traces species including O3, NO2, BrO, OClO, IO, HCHO and O4. (e.g. ref.[1,2,3,4,5,6,7,8,9,10,11]). The instrument noise results from two sources, photo electron noise from the detector, and electronic noise from the various components between the CCD pixels.[26] The systematic errors result from inaccuracies in the many components used in the DOAS analysis, including the cross sections, the Ring effect, the instrument line shape and the wavelength calibration. Full-width half-maximum (FWHM)) to sub-windows of recorded spectra This procedure permits accurate wavelength calibration, and investigation of the variability of the line shape both within an individual spectrum, and between spectra imaged in different areas of the focal plane. The telescope pointing at 2 degrees elevation consistently receives the lowest light intensity, and the signal has a larger relative contribution from CCD dark current and electronic noise For this reason data from the 2 degree elevation view is only used with a 2 degree reference spectrum
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