WSOC in accumulation mode aerosols: Distribution and relationship with BrC light absorption at an urban background site

Abstract

This paper presents the outcomes of a one-month long campaign (Jan 11, 2021 to Feb 11, 2021) of size segregated particulate matter (PM) in the accumulation mode (AM). Samples were collected at an urban background station (southeastern Iberian Peninsula) using a micro-orifice uniform deposit impactor (MOUDI). PM mass and water-soluble organic carbon (WSOC) were subsequently measured. The main goals of this research were: (1) to explore differences in WSOC mass size distributions under different meteorological conditions and (2) to determine the most efficient size for light absorption by WSOC. For the second objective, measurements of absorption coefficients (σap,λ) were obtained by means of an Aethalometer (AE33). Significant differences in WSOC concentrations and σap,λ values during stagnation episodes compared to non-event days were observed. Increases in WSOC concentrations during these episodes were most likely associated with the photochemical production of secondary compounds (in small-size ranges of the AM) and with water uptake (in large-size ranges of the AM). The average σap,λ value during this type of event was 2.6 higher than for non-event days. No significant increases in WSOC levels were detected under Saharan dust events. The average contribution of BrC to light absorption at 370 nm was ∼35%, revealing a significant influence of BrC on the absorption process. High contributions from BrC to light absorption were associated with AAE values above 1.5 and, in general, with high WSOC concentrations. WSOC in the smallest size mode (0.25 μm) was mainly emitted from biomass burning (BB) and showed the highest light absorption efficiency.