Application of an ultra-low-cost passive sampler for light-absorbing carbon in India and Mongolia

Abstract

BACKGROUND AND AIM: We tested in India and Mongolia an ultra-low-cost passive sampling method for long term average light-absorbing carbon (LAC) air pollution. METHODS: The passive LAC method estimates the change in reflectance based on digital images of a passively-exposed paper filter. Previous tests in polluted indoor environments in India suggested that the passive LAC method has robust reproducibility and high precision; the present research aimed to calibrate the method and thereafter test its accuracy. We deployed three methods (five monitoring devices) to each of 10 households in Ulaanbaatar, Mongolia: one PurpleAir (PA); two quartz filters for EC-OC (elemental carbon; organic carbon) using a UPAS; and two passive LAC samplers. We compare multiple rounds of 3-week-average measurements from the three methods. The EC- OC filter analysis is the “gold standard” / reference; PA and LAC are proxy-measurements for PM2.5 and BC, respectively. RESULTS:In pilot testing in Mongolia, average concentrations were 266 µg of PM2.5 /m³ (PA; uncalibrated) and, 47 µg of BC /m³ (UPAS, EC). LAC measurements in their native, uncorrected, units reflect the rate of change in filter color (pixel intensity/month), which depends on LAC deposition per time; the average value was 2590 PI/month. Preliminary analyses calibrating the LAC to the EC measurement suggest, as a conversion factor, that 1 µg of BC /m³ corresponds to 22 PI/month, on average. Applying that conversion to all measurements, to predict BC concentrations from passive LAC measurements, yields a root- mean-square-error of 16 µg of BC /m³, or ~25% of the average BC concentration. CONCLUSIONS:Measurements are continuing past this initial pilot stage and likely will shed improved light on the accuracy and precision of the novel passive LAC method employed here. KEYWORDS: low cost sensors, black carbon, passive black carbon sensor, light absorbing carbon