Ground-based direct-sun DOAS 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 for understanding remote sensing measurements of aerosols, cloud properties and atmospheric trace gases. Many ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of the O₂O₂ optical depth require correction factors of 0.75 ± 0.1 to reproduce radiative transfer modeling (RTM) results for a nearly pure Rayleigh atmosphere. One of the potential causes of this discrepancy is uncertainty in laboratory-measured O₂O₂ absorption cross section temperature and pressure dependencies due to difficulties in replicating atmospheric conditions in the laboratory environment. This paper presents ground-based direct-sun (DS) and airborne multi-axis (AMAX) DOAS measurements of O₂O₂ absorption optical depths under actual atmospheric conditions in two wavelength regions (335–390 and 435–490 nm). DS irradiance measurements were made by the Washington State University research-grade Multi-Function Differential Spectroscopy Instrument instrument from 2007 to 2014 at seven sites with significant pressure (778 to 1013 hPa) and O₂O₂ profile-weighted temperature (247 to 275 K) differences. Aircraft MAX-DOAS measurements were conducted by the University of Colorado (CU) AMAX-DOAS instrument on 29 January 2012 over the Southern Hemispheric subtropical Pacific Ocean. Scattered solar radiance spectra were collected at altitudes between 9 and 13.2 km, with O₂O₂ profile-weighted temperatures of 231 to 244 K and nearly pure Rayleigh scattering conditions. Due to the well-defined DS air-mass factors during ground-based measurements and extensively characterized atmospheric conditions during the aircraft 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. Based on the ground-based atmospheric DS observations, there is no pressure dependence of the O₂O₂ σ within the measurement errors (3 %). Two data sets are combined to derive the peak σ temperature dependence of the 360 and 477 nm dimer absorption bands from 231 to 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 function 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, 253 and 273 K within 3 %. Changes in the O₂O₂ spectral band shape at colder temperatures are observed for the first time 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 show that laboratory-measured σ(O₂O₂) (Hermans, 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 (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 slant column densities for DS and AMAX-DOAS measurements. Simultaneous fitting of σ(O₂O₂) at 203 and 293 K further improved the results at UV and visible wavelengths for AMAX-DOAS.