Absorption enhancement of BC particles in a Mediterranean city and countryside: effect of PM chemistry, aging and trend analysis

. Black carbon (BC) is recognized as the most important warming agent among atmospheric aerosol particles. The absorption efficiency of pure BC is rather well known, nevertheless the mixing of BC with other aerosol particles can enhance the BC light absorption efficiency, thus directly affecting the Earth radiative balance. The effects on climate of the BC absorption enhancement due to the mixing with these aerosols is not yet well constrained because these effects depend on the availability 5 of material for mixing with BC, thus creating regional variations. Here we present the mass absorption cross-section, MAC, and absorption enhancement of BC particles, (E abs ), at different wavelengths (from 370 nm to 880 nm for on-line measurements and at 637 nm for off-line measurements) measured at two sites in the Western Mediterranean, namely Barcelona (BCN; urban background) and Montseny (MSY; regional background). E abs values ranged between 1.24 and 1.51 at the urban station depending on the season and wavelength used as well as on the 10 pure BC MAC used as a reference. The largest contribution to E abs was due to the internal mixing of BC particles with other aerosol compounds, on average between a 91 and a 100 % at 370 and 880 nm, respectively. Additionally, 14.5 and 4.6 % of the total enhancement stations from off-line measurements enabled a decade-long trend analysis of E abs at 637 nm, that showed positive statistically 25 significant trends of E abs during the warmer months at MSY station. This s.s. positive trend at MSY mirrored the observed increase of the OC:EC ratio with time. Moreover, in BCN during the COVID-19 lockdown in spring 2020 we observed a sharp increase of E abs due to the observed sharp increase of OC to elemental carbon (EC) ratio. Our results show similar values of E abs to those found in the literature for similar background stations. 2015); al. BCN; and al. Pey et al. al. 2014a, 2016) for MSY. These supersites are part of the Quality Monitoring Network and are part of ACTRIS and GAW networks. Aerosol optical properties at BCN and MSY are measured following

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