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Abstract
Knowledge gaps in the optical properties of carbonaceous aerosols account for a significant fraction of the uncertainty of aerosol - light interactions in climate models. Both black carbon (BC) and brown carbon (BrC) can display a range of optical properties in ambient aerosol due to different sources and chemical transformation pathways. This study investigates the optical absorption properties of BC and BrC at an urban and a rural site in the Po Valley (Italy), a known European pollution hot spot. We observed spatial and seasonal variability of aerosol absorption coefficients, with the highest values measured in winter at the urban site of Milan (12 Mm−1 on average) and the lowest values in summer at the rural site of Motta Visconti (3 Mm−1 on average). The average aerosol Absorption Å ngström Exponent (AAE) measured during the two experiments across the 370–880 nm wavelength range was 1.1 and 1.2 at the urban and the rural site, respectively. The observed AAE values in winter (the average AAE during the two winter campaigns was 1.2) are consistent with the contribution of wood burning BrC, as confirmed by macro-tracer analysis. The BC mass absorption cross section (MACBC) did not show a specific seasonal or spatial variability across the two sites and maintained an average value of 10 ± 5 m2 g−1 at 880 nm. The optical properties of BrC, investigated off-line after extraction of organic aerosol (OA) indicate that wood burning was the dominant BrC source in winter, while secondary organic aerosol (SOA) from other anthropogenic emissions was the main source of BrC in summer.
Highlights
Carbonaceous aerosol accounts for more than half of fine atmospheric aerosol mass in urban, rural, and remote locations (Zhang et al, 2007; Bond et al, 2013)
The good agreement between the Cavity Attenuated Phase Shift (CAPS) PMSSA and MAAP babs values suggests that the effects related to the absorption of gas-phase species on the filter and subsequent coating of the filter fibers (Lack et al, 2007; Subramanian et al, 2007), as well as multiple scattering artifacts (Nakayama et al, 2010), can be considered negligible during the present study
We investigated the seasonal and spatial variability of black carbon (BC) and brown carbon (BrC) absorption across the Po Valley
Summary
Carbonaceous aerosol accounts for more than half of fine atmospheric aerosol mass in urban, rural, and remote locations (Zhang et al, 2007; Bond et al, 2013). Part of the discrepancy between the radiative forcing estimated by models and observations is attributed to the variability of MACBC, which depends on intra-particle BC mixing state and particle morphology (China et al, 2013; China et al, 2015; Bhandari et al, 2019). MACBC enhancements observed in laboratory experiments vary from 2 to 3.5 ( Zhang et al, 2008; Cross et al, 2010; Shiraiwa et al, 2010; Bond et al, 2013; Saliba et al, 2016), while field observations are characterized by a larger variability that is difficult to incorporate in models (Gustafsson and Ramanathan, 2016). Limited comparability among measurement techniques contributes to the variability of observed BC optical properties as well (Subramanian et al, 2007; Lack et al, 2008; Moosmuller et al, 2009)
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