Estimation of organic positive artefacts on Quartz filters and volatilization loss from Teflon filters at a COALESCE network site - Bhopal, India

Measurement of ambient particulate organic carbon (OC) collected on quartz filters is susceptible to net positive artefacts (overestimation of particulate OC due to adsorption of volatile and semi-volatile organic compounds) while that collected on Teflon filters is susceptible to net negative artefacts (loss of particle OC due to volatilization). In this study, QbQ (Quartz behind Quartz) filter configuration was used for estimating positive artefact, while, QbT (Quartz behind Teflon) filters in conjunction with the QbQ were used to estimate OC volatilization from Teflon filters over a two-year (2019 and 2020) period in Bhopal, one of the eleven COALESCE (Carbonaceous Aerosol Emissions, Source Apportionment, and Climate Impacts) network sites in India. OC and EC measurements by thermal-optical carbon analyses on 748 samples (349 bare quartz (Q), 349 QbQ, and 50 QbT; 24 hours time-integrated) were used in this study. The results showed that the average adsorbed gaseous OC contribution to total OC measured on quartz filters was 17 % (0.9 µg m^-3) and 11 % (0.6 µg m^-3) during 2019 and 2020, respectively. Organics volatilization loss from Teflon filters as a fraction of measured PM_2.5 mass were estimated by applying organic matter (OM)/OC ratios ranging between 1.7 and 2.0 to the OC measured on QbT filters. The annual mean volatilized OC  that was likely re-captured by bare quartz but lost from Teflon filters were 27 % (1.6 µg m^-3) and 21 % (1.1 µg m^-3) of the total OC measured during 2019 and 2020, respectively. Also, the average PM_2.5 lost due to OM volatilization was 8 % (± 4 %) and 6 % (± 5 %) during 2019 and 2020, respectively. Our work shows that organic volatilization artefacts from Teflon filters are likely to be substantial at most locations in India, where temperatures exceed 30 °C for most of the year, and should be accounted for in assessments of gravimetrically determined PM_2.5 mass closure using chemical species measured on multiple filter substrates.