Simulating aerosol hygroscopicity: Water retention behavior of particulate matter constituents and its implications for gravimetric PM measurements.

Gravimetric quantification of atmospheric particulate matter (PM) can be biased by particle-bound water persisting after filter conditioning, with magnitude depending on chemical composition, mixing state, and the protocol's operational definition of "water." Here, particle-bound water was measured by Karl Fischer (KF) coulometric titration with stepwise heating to distinguish weakly adsorbed from strongly retained fractions. Thirty-five single-component chemicals and certified reference materials were analyzed at two loadings (0.6 and 2.0 mg; summer and winter loads), alongside a powder mixture imitating PM (artificial PM, aPM), examined as sequential-addition and bulk systems of relevant composition. Bulk KF measurements showed class-dependent differences: deliquescent inorganic salts retained most water, whereas most metals, metal oxides, PAHs, and graphitic carbons approached background. Water-to-solute molar ratios (W/S) enabled cross-compound comparison and scaled non-linearly with loading-absolute water rose with mass, but W/S fell, consistent with reduced vapor accessibility in compacted powders. Clustering of temperature-resolved release signatures resolved three material groups differing chiefly in strongly bound (250 °C) water. Sequential aPM experiments showed secondary inorganic species dominate retained water; bulk aPM release profiles matched ambient PM₁ filters. PM-bound water releases across a continuum of thermal fractions, governed by composition, modulated by loading-informing mitigation of humidity-related gravimetric uncertainty.