Sequential rainwater chemistry constrains in-cloud and below-cloud scavenging at a high-elevation site under contrasting transport regimes

Abstract. High-elevation receptors in monsoon Asia intercept cloud-bearing regional air, yet the event-scale transition from below-cloud removal to a cloud-dominated precipitation signature remains poorly constrained. We collected 203 sequential rainwater fractions from 55 precipitation events in 2023 at the Atmospheric Boundary Layer Eco-Environment Shanghuang Observatory (ABLES; 1128 m a.s.l.) in southeastern China, where complex terrain and frequent cloud occurrence expose the site to contrasting continental and marine transport. Twelve events with at least six fractions were analysed using major-ion chemistry, two independent late-stage baseline estimates, backward trajectories, and reanalysis fields, with near-surface meteorological and turbulence observations available for R3–R6. Anthropogenic-type events began with the largest ionic loads and strongest early decreases, dominated by sulfate, nitrate, and ammonium. Marine-type events had lower but more variable concentrations, whereas calcium persisted relative to secondary inorganic ions in crustal-type events. Median estimated in-cloud contributions were 51.1 % for sulfate, 58.4 % for nitrate, and 49.2 % for ammonium, with event values exceeding 80 %. These fractions exceeded compiled urban medians but overlapped remote-environment ranges. Although not a direct cloud-water measurement, the 0.32 mg L−1 late-event sulfate baseline was one to two orders of magnitude below historical Chinese mountain cloud-water observations and reached the lower range of global mountain records, consistent with sulfur controls extending beyond surface air. By resolving source-dependent within-event transitions hidden by bulk sampling, this study establishes an event- and ion-specific observational benchmark for wet scavenging at an inland mountain receptor and clarifies how continental–marine transport conditions the precipitation signature.