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Leaf wax hydrogen isotope proxies have been widely used to reconstruct past hydrological changes. However, published reconstructions have given little consideration for the potentially variable hydrogen isotopic fractionation relative to precipitation (ε_wax‐p) under different climate and environmental settings. Chief among various potential factors controlling fractionation is relative humidity, which is known to strongly affect oxygen isotopic ratios of plant cellulose, but its effect on hydrogen isotopic fractionation of leaf waxes is still ambiguous. Analyses of lake surface sediments and individual modern plants have provided valuable information on the variability of ε_wax‐p, but both approaches have significant limitations. Here, we present an alternative method to obtain the integrated, time‐resolved ecosystem‐level ε_wax‐p values, by analyzing modern aerosol samples collected weekly from arid (Arizona lowlands …