The CU 2-dimensional MAX-DOAS instrument – Part 1: Retrieval of NO<sub>2</sub> in 3 dimensions and azimuth dependent OVOC ratios
Journal Article
Abstract. We present an innovative instrument telescope, and describe a retrieval method to probe 3-D distributions of atmospheric trace gases that are relevant to air pollution and tropospheric chemistry. The University of Colorado (CU) two dimensional (2-D) Multi-AXis-Differential Optical Absorption Spectroscopy (CU 2D-MAX-DOAS) instrument measures nitrogen dioxide (NO2), formaldehyde (HCHO), glyoxal (CHOCHO), oxygen dimer (O2-O2, or O4) and water vapor (H2O); also nitrous acid (HONO), bromine monoxide (BrO), iodine monoxide (IO) among other gases can in principle be measured. Information about aerosols is derived through coupling with a radiative transfer model (RTM). The 2-D telescope has 3 modes of operation: (mode 1) measures solar scattered photons from any pair of elevation angle (−20° < EA < +90° or zenith; zero is to the horizon) and azimuth angle (−180° < AA < +180°; zero being North), (mode 2) measures any set of AA at constant EA (almucantar scans); and (mode 3) tracks the direct solar beam via a separate view port. Vertical profiles of trace gases are measured, and used to estimate planetary boundary layer height (PBL). Horizontal distributions are then derived using PBL and parameterization of RTM (Sinreich et al., 2013). NO2 is evaluated at different wavelengths (350, 450, and 560 nm), exploiting the fact that the effective path length varies systematically with wavelength. The area probed is constrained by O4 observations at nearby wavelengths, and has an effective radius of 7.5 to 20 km around the instrument location; i.e., up to 1250 km2 can be sampled near-instantaneously, and with high time resolution. The instrument was deployed as part of the Multi Axis DOAS Comparison campaign for Aerosols and Trace gases (MAD-CAT) in Mainz, Germany from 7 June to 6 July 2013. We present first measurements (modes 1 and 2 only) and describe a four-step retrieval to derive (a) boundary layer vertical profiles of NO2 and PBL; (b) near-surface horizontal distributions of NO2; (c) range resolved NO2 horizontal distribution measurements using an "onion peeling" approach; and (d) the ratios HCHO-to-NO2 (RFN), CHOCHO-to-NO2 (RGN), and CHOCHO-to-HCHO (RGF) at 14 pre-set azimuth angles distributed over a 360° view. 2D-MAX-DOAS provides an innovative, regional perspective about trace gases, their spatial and temporal concentration gradients, and maximizes information to compare near-surface observations with atmospheric models and satellites.;
