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

Chamber and field experiments have shown that organic aerosol particles (OA) can absorb solar radiation with varying efficiencies depending on their chemical composition and physical properties. This complex collection of light-absorbing organic molecules, often 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 mass absorption cross section (MACOA) or imaginary refractive index (kOA), the climate effect of BrC remains uncertain. The reported high variability of MACOA and kOA of BrC is associated with differences in experiments conducted and analytic methodologies, e.g. the organic mass to which the BrC absorption is attributed, the fuel studied, and combustion conditions. Moreover, experiments designed to study specific BrC particles from specific sources often 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 MACOA of OA and its wavelength dependence (AAEOA), the density of OA (⍴OA) and the MACOA of different primary and secondary OA sources and compare the obtained set of information with recently published estimations. We also propose a series of parameterizations describing the relationships between MACOA, AAEOA and ⍴OA with the ambient measurements of the BC/OA ratio. Similar parameterization is also provided for the kOA that was estimated based on the available ambient data. Given that BC and OA (or OC) are routinely measured at many sites worldwide, these measurements can be used in the framework presented here to estimate the absorptive properties of surface OA particles at ambient conditions.