Absorption enhancement of BC particles in a Mediterranean city and countryside: effect of PM chemistry, aging and trend analysis
Authors: Yus-Díez Jesús , Via Marta , Alastuey Andrés , Karanasiou Angeliki , Minguillón María Cruz , Perez Noemí , Querol Xavier , Reche Cristina , Ivančič Matic , Rigler Martin , Pandolfi Marco 
Published: March 2022

Abstract. 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 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, (Eabs), 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). Eabs values ranged between 1.24 and 1.51 at the urban station depending on the season and wavelength used as well as on the pure BC MAC used as a reference. The largest contribution to Eabs 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 at the short-UV (370 nm) was due to externally mixed BrC particles during the cold and the warm period, respectively. On average, at MSY station, a higher Eabs value was observed (1.83 at 637 nm) compared to BCN (1.37 at 637 nm), which was associated to the higher fraction of organic aerosols available for BC coating at the regional station, as denoted by the higher OC : EC ratio observed at MSY compared to BCN. At both BCN and MSY Eabs showed an exponential increase with the amount of non-refractory (NR) material available for coating (RNR–PM). The Eabs at 637 nm at MSY regional station reached values up to 3 during episodes with high RNR–PM, whereas in BCN Eabs kept values lower than 2 due to the lower relative amount of coating materials measured at BCN compared to MSY. The main sources of organic aerosols influencing Eabs throughout the year were HOA and COA (primary OA from traffic and cooking emissions, respectively) at both 370 nm and 880 nm. At the short-UV wavelength (370 nm), a strong contribution to Eabs from BBOA (biomass burning OA) and LO-OOA (less-oxygenated OA) sources was observed in the colder period. Moreover, we found an increase of Eabs with the aging state of the particles, especially during the colder period. This increase of Eabs with particle aging was associated to a larger relative amount of secondary organic aerosols (SOA) compared to primary OA (POA). The availability of a long dataset at both stations from off-line measurements enabled a decade-long trend analysis of Eabs at 637 nm, that showed positive statistically significant trends of Eabs 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 Eabs due to the observed sharp increase of OC to elemental carbon (EC) ratio. Our results show similar values of Eabs to those found in the literature for similar background stations.