The use of NO2 absorption cross section temperature sensitivity to derive NO2 profile temperature and stratospheric/tropospheric column partitioning from visible direct sun DOAS measurements
Authors: Spinei E. , Cede A. , Swartz W. H. , Herman J. , Mount G. H. 
Published: June 2014

Abstract. This paper presents a TEmperature SEnsitivity Method (TESEM) to accurately calculate total vertical NO2 column, atmospheric slant NO2 profile-weighted temperature (T), and to separate stratospheric and tropospheric columns from direct-sun (DS) ground-based measurements using the retrieved T. TESEM is based on Differential Optical Absorption Spectroscopy (DOAS) fitting of the linear temperature-dependent NO2 absorption cross section, σ (T), regression model (Vandaele et al., 2003). The direct result of the DOAS spectral fitting retrieval is NO2 differential slant column density (Δ SCD) at the actual atmospheric NO2 T. Atmospheric NO2 T is determined from the DOAS fitting results after SCD in the reference spectrum is estimated using the Minimum Langley Extrapolation method (MLE). Since NO2 is mostly distributed between the lower troposphere and middle stratosphere and direct sun measurements have almost equal sensitivity to stratospheric and tropospheric absorption at solar zenith angles < 75° with a well known photon path, we assume that the retrieved total column NO2 T can be represented as a sum of the NO2 stratospheric and tropospheric Ts multiplied by the corresponding stratospheric and tropospheric fractions of the total SCDNO2. We use Global Modeling Initiative (GMI) chemistry–transport model (CTM) simulations to evaluate diurnal and seasonal variability of stratospheric and tropospheric NO2 T over two northern middle latitude sites in 2011. GMI simulations reveal that stratospheric NO2 T over northern middle latitudes can be estimated with an error of less than 3 K by the simulated temperature at 27 km from April to October. During November–March months the error can reach as high as 10 K. The tropospheric NO2 T can be approximated by the surface temperature within 3–5 K according to GMI simulations. Traditionally, either σ (NO2) is fitted at a single estimated NO2 T, or two predetermined (stratospheric and tropospheric) temperatures. Use of a single T requires prior knowledge of the tropospheric–stratospheric NO2 columns partitioning in the measurement. In addition, it assumes that this partitioning is constant throughout the measurement period (sometimes months). Fitting of two σ (NO2) at fixed temperatures, typically 220 and 298 K, assumes constant stratospheric and tropospheric NO2 T as a function of time. Neither assumption is correct, except as a convenient approximation. TESEM does not require prior knowledge of NO2 effective temperatures during the DOAS fitting stage and retrieves T from the DOAS fitting results themselves. TESEM was applied to the Washington State University Multi Function DOAS instrument (MFDOAS) measurements at four mid-latitude locations with low and moderate NO2 anthropogenic emissions: (1) Table Mountain – Jet Propulsion Laboratory (JPL-TMF) facility, CA (34.38° N/117.68° W); (2) Pullman, WA (46.73° N/117.17° W); (3) Greenbelt, MD (38.99° N/76.84° W) USA; and (4) Cabauw, the Netherlands (51.97° N/4.93° E) during summer months (July 2007, June–July 2009, July–August 2011, May 2013). NO2 T, total, stratospheric and tropospheric NO2 vertical columns were determined over each site.