Direct sun and airborne MAX-DOAS measurements of the collision induced oxygen complex, O2O2 absorption with significant pressure and temperature differences
Authors: Spinei E. , Cede A. , Herman J. , Mount G. H. , Eloranta E. , Morley B. , Baidar S. , Dix B. , Ortega I. , Koenig T. , Volkamer R. 
Published: September 2014

Abstract. The collision induced O2 complex, O2O2, is a very important trace gas in remote sensing measurements of aerosol and cloud properties. Some ground based MAX-DOAS measurements of O2O2 slant column density require correction factors of 0.75 ± 0.1 to reproduce radiative transfer modeling (RTM) results for a near pure Rayleigh atmosphere. One of the potential causes of this discrepancy is believed to be uncertainty in laboratory measured O2O2 absorption cross section temperature and pressure dependence, due to difficulties in replicating atmospheric conditions in the laboratory environment. This paper presents direct-sun (DS) and airborne multi-axis (AMAX) DOAS measurements of O2O2 absorption optical depths under actual Earth atmospheric conditions in two wavelength regions (335–390 nm and 435–490 nm). DS irradiance measurements were made by the research grade MFDOAS instrument from 2007–2014 at seven sites with significant pressure (778–1013 hPa) and O2O2 profile weighted temperature (247–275 K) differences. Aircraft MAX-DOAS measurements were conducted by the University of Colorado AMAX-DOAS instrument on 29 January 2012 over the Southern Hemisphere subtropical Pacific Ocean. Scattered solar radiance spectra were collected at altitudes between 9 and 13.2 km, with O2O2 profile weighted temperatures of 231–244 K, and near pure Rayleigh scattering conditions. Due to the well defined DS air mass factors and extensively characterized atmospheric conditions during the AMAX-DOAS measurements, O2O2"pseudo" absorption cross sections, σ, are derived from the observed optical depths and estimated O2O2column densities. Vertical O2O2 columns are calculated from the atmospheric sounding temperature, pressure and specific humidity profiles. Based on the atmospheric DS observations, there is no pressure dependence of the O2O2 σ, within the measurement errors (3%). The two data sets are combined to derive peak σ temperature dependence of 360 and 477 nm absorption bands from 231–275 K. DS and AMAX derived peak σ(O2O2) as a function of T can be described by a quadratic function at 360 nm and linear at 477 nm with about 9 ± 2.5% per 44 K rate. Recent laboratory measured O2O2 cross sections by Thalman and Volkamer (2013) agree with these "DOAS apparent" peak σ(O2O2) at 233 K, 253 K and 273 K within 3%. Changes in the O2O2 spectral band-shape at colder temperatures are for the first time also observed in field data. Temperature effects on spectral band shapes can introduce errors in the retrieved O2O2 column abundances if a single room temperature σ(O2O2) is used in the DOAS analysis. Simultaneous fitting of σ(O2O2) at temperatures that bracket the ambient temperature range can reduce such errors. Our results suggest that laboratory measured σ(O2O2) (Hermans et al. (2011) at 296 K and Thalman and Volkamer (2013)) are applicable for observations over a wide range of atmospheric conditions. Column densities derived using Hermans et al. (2011) σ at 296 K require very small correction factors (0.94 ± 0.02 at 231 K and 0.99 ± 0.02 at 275 K) to reproduce theoretically calculated SCDs for DS and AMAX-DOAS measurements. Simultaneous fitting of σ(O2O2) at 203 and 293 K further improved results at UV and visible wavelengths for AMAX-DOAS.