Sodium thiosulfate-coated ceramic denuders for ozone removal in ultrafine particle sampling

Abstract

Ozone (O3) remaining in sampling air can artefactually alter the chemical composition of collected ultrafine particles (UFPs), biasing quantitative analysis of the chemical composition. In this study, we developed and evaluated a sodium-thiosulfate O3 denuder (TSOD) specifically tailored for UFP sampling and assessed its O3 scrubbing efficiency, particle losses, and chemical selectivity. In laboratory tests under controlled relative humidity and inlet O3 levels up to 200 ppbV, the outlet concentration remained consistently below the limit of detection, demonstrating the O3 removal efficiency of the TSOD. During an urban field deployment over 5 d O3 downstream of the TSOD consistently remained below the detection limit while ambient O3 varied between 0 and 65 ppbV. Moreover, for particles with mobility diameters ranging from 10 to 1000 nm, we did not observe any significant losses in particle number concentrations. Using a parallel two-channel UFP sampler (with vs. without upstream TSOD), we quantified O3-driven sampling artefacts in UFP mass focussing on three types of organic markers. (1) Firstly, we targeted polycyclic aromatic hydrocarbons (PAHs), particularly chrysene (Chry), benz[a]anthracene (BaA), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (IcdP), benzo[k]fluoranthene (BkF), and benzo[b]fluoranthene (BbF). Without upstream O3 removal, the individual concentrations of the PAHs were 15 ± 3 %–46 ± 6 % lower. (2) Secondly, for the tire and road wear marker, the antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) and its oxidation product 6PPD-quinone (6PPDq), we observed in-situ ozonation of 6PPD to 6PPDq with transformation yields of about 13 ± 4 % to 20 ± 8 %. (3) In contrast, biogenic organic acids (bOAs) did not show differences when sampled with or without O3, as their O3 reactivity is much lower than the one of the PAHs. Moreover, this test indicated that the TSOD did not perturb the gas–particle partitioning of these semi-volatile species. Our results demonstrate that the TSOD (i) efficiently scrubs atmospheric O3 at relevant mixing ratios, (ii) does not introduce measurable particle losses across 10–1000 nm, and (iii) preserves semi-volatile partitioning.