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The collision-induced O2 complex, O2O2, is a very importanttrace gas for understanding remote sensing measurements of aerosols, cloudproperties and atmospheric trace gases. Many ground-based multi-axis differential optical absorption spectroscopy(MAX-DOAS) measurements ofthe O2O2 optical depth require correction factors of 0.75 ±0.1 to reproduce radiative transfer modeling (RTM) results for a nearly pureRayleigh atmosphere. One of the potential causes of this discrepancy isuncertainty in laboratory-measured O2O2 absorption cross sectiontemperature and pressure dependencies due to difficulties in replicatingatmospheric 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 underactual atmospheric conditions in two wavelength regions (335–390 and435–490 nm). DS irradiance measurements were made by the Washington StateUniversity research-grade Multi-Function Differential SpectroscopyInstrument instrument from 2007 to 2014 at sevensites with significant pressure (778 to 1013 hPa) and O2O2profile-weighted temperature (247 to 275 K) differences. Aircraft MAX-DOASmeasurements were conducted by the University of Colorado (CU) AMAX-DOASinstrument on 29 January 2012 over the Southern Hemispheric subtropicalPacific Ocean. Scattered solar radiance spectra were collected at altitudesbetween 9 and 13.2 km, with O2O2 profile-weighted temperatures of231 to 244 K and nearly pure Rayleigh scattering conditions. Due to the well-defined DS air-mass factors during ground-based measurementsand extensively characterized atmospheric conditions during the aircraftAMAX-DOAS measurements, O2O2 "pseudo" absorption cross sections,σ, are derived from the observed optical depths and estimated O2O2 column densities. Vertical O2O2 columns arecalculated from the atmospheric sounding temperature, pressure and specifichumidity profiles. Based on the ground-based atmospheric DS observations, there is no pressuredependence of the O2O2 σ within the measurement errors(3%). Two data sets are combined to derive the peak σ temperaturedependence 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 bedescribed by a quadratic function at 360 nm and linear function at477 nmwith about 9% ± 2.5% per 44 K rate. Recent laboratory-measured O2O2 cross sections by Thalman andVolkamer (2013) agree with these "DOAS apparent" peak σ( O2O2) at233, 253 and 273 K within 3%. Changes in the O2O2 spectral band shape at colder temperatures are observed forthe first time in field data. Temperature effects on spectral band shapescan introduce errors in the retrieved O2O2 column abundances if asingle room temperature σ( O2O2) is used in the DOASanalysis. Simultaneous fitting of σ( O2O2) at temperaturesthat 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 forobservations over a wide range of atmospheric conditions. Column densitiesderived using Hermans (2011) σ at 296 K require very smallcorrection factors (0.94 ± 0.02 at 231 K and 0.99 ± 0.02 at 275 K)to reproduce theoretically calculated slant column densities for DSand AMAX-DOAS measurements. Simultaneous fitting of σ( O2O2)at 203 and 293 K further improved the results at UV andvisible wavelengths for AMAX-DOAS

atmospheric measurement techniques

Ground-based direct-sun DOAS and airborne MAX-DOAS measurements of the collision-induced oxygen complex, O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, absorption with significant pressure and temperature differences