Combination of PM optical and chemical properties to estimate the contribution of non-BC absorbers to light absorption at a remote site

Abstract

Values of particulate matter (PM) concentrations, chemical composition and aerosol optical parameters have been measured with a high time resolution (1-hour) for thirteen days at the end of summer 2014. Sampling was performed at a remote site (1558 m a.s.l) located in the southeast of the Iberian Peninsula, close to the Mediterranean coast. According to the optical and chemical characteristics of PM, four periods were differentiated using Cluster analysis. One cluster (C4) was clearly associated with Saharan dust inputs. During this period, mineral dust (MD) concentrations were highest. Values of the intensive optical parameters for C4 confirmed that MD was the dominant type of aerosol: AAE (1.58) was the highest of the four periods, while SAE (−0.58) was the lowest. C4 was selected to apply a methodology capable of discriminating the contribution from Black carbon (BC) and Non-BC absorbers (NBC), that is Brown carbon (BrC) and MD, to the light absorption process. With the objective of determining the contribution of NBC to light absorption, the spectral differences between BC and NBC were used. The value of the absorption coefficient for NBC (σap-NBC) was plotted versus the concentration of MD. The approach consists on drawing a line including points aligned in the lower end of the chart. The equation of this line can be used to estimate the MD contribution (σDust) to light absorption. For λ = 520 nm, around 6% of the absorption was due to MD and ~ 8% to BrC. This methodology allowed the determination of the MAE (Mass Absorption Efficiency) value for Fine-MD, (0.032 ± 0.004 m2 g−1 for λ = 520 nm).