Abstract
Abstract. The University of Colorado Airborne Multi-Axis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument uses solar stray light to detect and quantify multiple trace gases, including nitrogen dioxide (NO2), glyoxal (CHOCHO), formaldehyde (HCHO), water vapor (H2O), nitrous acid (HONO), iodine monoxide (IO), bromine monoxide (BrO), and oxygen dimers (O4) at multiple wavelengths (absorption bands at 360, 477, 577, 632 nm) simultaneously in the open atmosphere. The instrument is unique as it (1) features a motion compensation system that decouples the telescope field of view from aircraft movements in real time (<0.35° accuracy), and (2) includes measurements of solar stray light photons from nadir, zenith, and multiple elevation angles forward and below the plane by the same spectrometer/detector system. Sets of solar stray light spectra collected from nadir to zenith scans provide some vertical profile information within 2 km above and below the aircraft altitude, and the vertical column density (VCD) below the aircraft is measured in nadir view. Maximum information about vertical profiles is derived simultaneously for trace gas concentrations and aerosol extinction coefficients over similar spatial scales and with a vertical resolution of typically 250 m during aircraft ascent/descent. The instrument is described, and data from flights over California during the CalNex (California Research at the Nexus of Air Quality and Climate Change) and CARES (Carbonaceous Aerosols and Radiative Effects Study) air quality field campaigns is presented. Horizontal distributions of NO2 VCD (below the aircraft) maps are sampled with typically 1 km resolution, and show good agreement with two ground-based MAX-DOAS instruments (slope = 0.95 ± 0.09, R2 = 0.86). As a case study vertical profiles of NO2, CHOCHO, HCHO, and H2O concentrations and aerosol extinction coefficients, ε, at 477 nm calculated from O4 measurements from a low approach at Brackett airfield inside the South Coast Air Basin (SCAB) are presented. These profiles contain ~12 degrees of freedom (DOF) over a 3.5 km altitude range, an independent information approximately every 250 m. The boundary layer NO2 concentration, and the integral aerosol extinction over height (aerosol optical depth, AOD) agrees well with nearby ground-based in situ NO2 measurement, and AERONET station. The detection limits of NO2, CHOCHO, HCHO, H2O442, &amp;varepsilon;360, &amp;varepsilon;477 for 30 s integration time spectra recorded forward of the plane are 5 ppt, 3 ppt, 100 ppt, 42 ppm, 0.004 km−1, 0.002 km−1 in the free troposphere (FT), and 30 ppt, 16 ppt, 540 ppt, 252 ppm, 0.012 km−1, 0.006 km−1 inside the boundary layer (BL), respectively. Mobile column observations of trace gases and aerosols are complimentary to in situ observations, and help bridge the spatial scales that are probed by satellites and ground-based observations, and predicted by atmospheric models.
Highlights
CARES (Carbonaceous Aerosols and Radiative Effects Study) air quality field campaigns is presented
The footprint of CU AMAX-DOAS is comparable to air quality models, and smaller than that of current solar stray light satellite observations, which measure vertical column density (VCD) of trace gases; this makes this data set an excellent opportunity to evaluate emissions in air quality models and validate satellite observations
A first application of CU AMAX-DOAS to test NASA NO2 VCD retrievals from the OMI/AURA satellite instrument is currently under review (Oetjen et al, 2013)
Summary
CARES (Carbonaceous Aerosols and Radiative Effects Study) air quality field campaigns is presented. First retrievals of trace gas concentrations close to the aircraft altitude were reported by Petritoli et al (2002) for stratospheric ozone (O3) These studies were followed by the application of the AMAX-DOAS technique to obtain tropospheric columns for NO2 (Melamed et al, 2003; Heue et al, 2005; Wang et al, 2005) and sulfur dioxide (SO2) (Wang et al, 2006; Melamed et al, 2008) over polluted regions. Pitch and roll information from the aircraft is used during postprocessing to calculate the true viewing angle at the time of measurement during the flight This often leads to a range of EAs assigned to measurements, and results in a loss of sensitivity to a given layer in the atmosphere compared to when the EAs are actively controlled in order to retrieve vertical profile information of trace gases. Appendix A shows the glossary of frequently used abbreviations in the paper
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