Elevated aerosol layers modify the O₂-O₂ absorption measured by ground-based MAX-DOAS

The oxygen collisional complex (O₂-O₂, or O₄) is a greenhouse gas, and a calibration trace gas used to infer aerosol and cloud properties by Differential Optical Absorption Spectroscopy (DOAS). Recent reports suggest the need for an O₄ correction factor (CF_O4) when comparing simulated and measured O₄ differential slant column densities (dSCD) by passive DOAS. We investigate the sensitivity of O₄ dSCD simulations at ultraviolet (360 nm) and visible (477 nm) wavelengths towards separately measured aerosol extinction profiles. Measurements were conducted by the University of Colorado 2D-MAX-DOAS instrument and NASA's multispectral High Spectral Resolution Lidar (HSRL-2) during the Two Column Aerosol Project (TCAP) at Cape Cod, MA in July 2012. During two case study days with (1) high aerosol load (17 July, AOD~0.35 at 477 nm), and (2) near molecular scattering conditions (22 July, AOD<0.10 at 477 nm) the measured and calculated O₄ dSCDs agreed within 6.4±0.4% (360 nm) and 4.7±0.6% (477 nm) if the HSRL-2 profiles were used as input to the calculations. However, if in the calculations the aerosol is confined to the surface layer (while keeping AOD constant) we find 0.53<CF_O4<0.75, similar to previously reported CF_O4. Our results suggest that elevated aerosol layers, unless accounted for, can cause negative bias in the simulated O₄ dSCDs that can explain CF_O4. The air density and aerosol profile aloft needs to be taken into account when interpreting the O₄ from ground-based MAX-DOAS. Opportunities to identify and better characterize these elevated layers are also discussed.