Direct sun and airborne MAX-DOAS measurements of the collision induced oxygen complex, O₂O₂ absorption with significant pressure and temperature differences

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 5 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. 10 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 15 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. 20 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 O₂O₂ column densities. Vertical O₂O₂ columns are calculated from the atmospheric sounding temperature, pressure and specific humidity profiles. 25 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 10017 AMTD 7, 10015–10057, 2014 Absorption with significant pressure and temperature differences E. Spinei et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 %. 5 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 10 ) (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 15 ) at 203 and 293 K further improved results at UV and visible wavelengths for AMAX-DOAS.