Implications of light absorbing organic aerosol - “brown carbon” - on satellite retrievals of atmospheric trace gases

Satellite instruments like GOME, GOME-2, SCIAMACHY and OMI, measure backscattered solar radiation to retrieve atmospheric trace gas columns. In order to quantify these columns, Radiative Transfer (RT) computations of Air Mass Factors (AMFs) are needed to account for radiation scattering and absorption processes. Any RT modeling requires a priori information on the state of the atmosphere such as trace gas/aerosol vertical profiles and aerosol properties (e.g. size parameter and Single Scattering Albedo (SSA)). This a priori input is crucial for the correct quantification of the measured trace gas columns. Recent field studies of urban aerosol show that the Organic Carbon (OC) component contributes highly to the absorption of short wave incoming solar radiation. This leads to measured Single Scattering Albedo (SSA) values as low as 0.78 below 420 nm. Here, the effect of SSA on retrieved AMFs over organic aerosol hotspots, e.g. the urban polluted boundary layer and biomass burning plumes is studied. It is demonstrated that over such hotspots trace gas columns can be underestimated up to a factor of three when neglecting the additional light absorption by OC aerosol. This applies to all trace gases that absorb light at wavelengths shorter than 420 nm, such as HCHO, SO2, BrO, HONO and possibly NO2, if evaluated below 420 nm. The sensitive coupling between trace gas retrievals and OA optical properties that we quantify here also means that any trends in OA optical properties can cause artificial trends in trace gas retrievals; decoupling of both trends requires additional information about the variability of OA optical properties to avoid bias in the long-term trends in trace-gases.