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The University ofColorado Airborne Multi-Axis Differential Optical Absorption Spectroscopy (CUAMAX-DOAS) instrument uses solar stray light to detect and quantify multipletrace gases, including nitrogen dioxide (NO2), glyoxal (CHOCHO),formaldehyde (HCHO), water vapor (H2O), nitrous acid (HONO), iodinemonoxide (IO), bromine monoxide (BrO), and oxygen dimers (O4) atmultiple 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 ofview 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 samespectrometer/detector system. Sets of solar stray light spectra collectedfrom nadir to zenith scans provide some vertical profile information within2 km above and below the aircraft altitude, and the vertical column density(VCD) below the aircraft is measured in nadir view. Maximum information aboutvertical profiles is derived simultaneously for trace gas concentrations andaerosol extinction coefficients over similar spatial scales and with avertical resolution of typically 250 m during aircraft ascent/descent. The instrument is described, and data from flights over California during theCalNex (California Research at the Nexus of Air Quality and ClimateChange) and CARES (Carbonaceous Aerosols and Radiative Effects Study) airquality field campaigns is presented. Horizontal distributions of NO2VCD (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 ofNO2, CHOCHO, HCHO, and H2O concentrations and aerosol extinctioncoefficients, ε, at 477 nm calculated from O4 measurementsfrom 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 informationapproximately every 250 m. The boundary layer NO2 concentration, andthe integral aerosol extinction over height (aerosol optical depth, AOD)agrees well with nearby ground-based in situ NO2 measurement, andAERONET station. The detection limits of NO2, CHOCHO, HCHO,H2O442, ϵ360, ϵ477 for 30 sintegration 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), and30 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 aerosolsare complimentary to in situ observations, and help bridge the spatial scalesthat are probed by satellites and ground-based observations, and predicted byatmospheric models

The CU Airborne MAX-DOAS instrument: vertical profiling of aerosol extinction and trace gases