Abstract
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.
Highlights
The O2O2 collision complex has been widely used in remote sensing to retrieve aerosol and cloud information from spectroscopic measurements using ground-based (Wagner et al, 2002, 2004; Frieß et al, 2006; Irie et al, 2008, 2009; Clémer et al, 2010) and space instruments (Acarreta et al, 2004; Sneep et al, 2008)
The main scope of this study is to evaluate O2O2 absorption optical depths (335–390 and 435–490 nm) under wellcharacterized atmospheric conditions using differential optical absorption spectroscopy (DOAS) measurements with well-understood observation geometry
The data were evaluated to understand the temperature and pressure dependence of the O2O2 molecular absorption cross section using vertical O2O2 column densities calculated from atmospheric sounding, in situ data and/or model temperature and pressure profiles adjusted by the surface observations
Summary
The O2O2 collision complex has been widely used in remote sensing to retrieve aerosol and cloud information from spectroscopic measurements using ground-based (Wagner et al, 2002, 2004; Frieß et al, 2006; Irie et al, 2008, 2009; Clémer et al, 2010) and space instruments (Acarreta et al, 2004; Sneep et al, 2008). Thalman and Volkamer (2013) made σ (O2O2) laboratory measurements at a pressure close to ambient (825 hPa). Their σ (O2O2) at 293 K agree with the Hermans σ (O2O2) at 296 K within the instrumental measurement errors. The main confusion arises from the fact that under low aerosol conditions which approach a nearly pure Rayleigh atmosphere, some ground-based multi-axis (MAX) DOAS measurements of O2O2 differential slant column density, SCD, require a “correction factor” (CF) of about 0.75–0.89 to reproduce the SCD(O2O2) modeled by various radiative transfer algorithms using Hermans σ (O2O2) at 296 K (Table 1)
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