Gravimetrically Measured Water Content of Filter Sampled Fine Particulate Matter at Low Relative Humidity: Insight from the Surface Particulate Matter Network (SPARTAN)

Abstract

Abstract. Accurate measurements of the composition of fine particulate matter (PM2.5) are important for understanding its sources and health impacts. Water, a portion of PM2.5 mass, is difficult to measure. We describe developments to the Surface PARTiculate mAtter Network (SPARTAN) to better characterize the chemical composition of PM2.5 on polytetrafluoroethylene filters. A robotic weighing facility is used to estimate water content of PM2.5 sampled at three sites (Fajardo, Puerto Rico; Bujumbura, Burundi; Abu Dhabi, United Arab Emirates), and the chemical composition is used to attribute water content to three hygroscopic categories: high growth (sodium chloride), medium growth (ammonium-nitrate-sulfate-potassium), and low growth (organics). The growth rates are tested using two additional sites (Beijing, China; Halifax, Canada). The water content at 35 % relative humidity (RH) is estimated to be 16.5 % (11.6 %–26.5 %, 95 % confidence) for the high growth category, 3.9 % (3.0 %–4.2 %, 95 % confidence) for medium growth, and 1.0 % (0.6–1.3 %, 95 % confidence) for low growth. We calculate the average water content at 35 % RH for 2442 filters from 24 globally distributed sites from December 2019 to September 2024 to be 2.06 % (0.85 %–5.47 %, range), where 58 % of the aerosol water is associated with the medium growth category (22 % low growth, 19 % high growth).