Constraining the intensive absorption properties of ambient organic aerosol particles based on pan-European observations.

Both chamber and field experiments have shown that organic aerosol particles (OA) can absorb light with varying efficiencies depending on their content of light-absorbing organic molecules. This complex collection of light-absorbing organic molecules, referred to as brown carbon (BrC) cannot easily be represented in climate models. Indeed, despite the many laboratory and field studies seeking to constrain BrC absorption through its imaginary refractive index (k), the climate effect of BrC is still uncertain. The reported orders-of-magnitude variability of the k of BrC is associated with differences in the conducted experiments, analytic methodologies, fuel studied, and combustion conditions. Often, experiments designed to study specific BrC particles from specific sources fail to fully represent the absorptive properties of bulk OA at ambient conditions. Thus, a better understanding of real-world absorption properties of OA particles is essential. Here we studied the light absorption properties of surface ambient OA at 17 measurement sites in Europe. We present the k, the mass absorption cross section (MAC) and the k wavelength dependence (ω) of OA particles, and compare the obtained set of information with recent published estimations. We propose a series of parameterizations describing the relationships between k, MAC, ω and OA density and the ambient measurements of the BC/OA ratio. Given that BC (and EC) and OA (and OC) are routinely measured and their mass concentrations are available worldwide in international databases, these can be used in the framework presented here to estimate the absorptive properties of surface OA particles at ambient conditions.