High-resolution sampling and chemical characterisation of wet deposition using the low-cost R-WASH system

Wet and dry atmospheric deposition plays a crucial role in the removal of airborne pollutants, being influenced by both meteorology and chemical composition. While these processes occur on intra-event (hourly) time scales, most existing monitoring approaches rely on daily or event-based sampling, which limits our understanding of short-term variations in gas scavenging and pollutant deposition. Here, we present the development and application of R-WASH (RainWater Automatic Sampler Hardware), a low-cost, open-source, and semi-automatic device for high-resolution (hourly) rainwater sampling. The system consists of modules for collection, distribution, and storage (up to 24 bottles). We tested the system in nine rainfall events, in which 91 hourly samples were collected and analyzed for ammonium, nitrite, nitrate (via spectrophotometry), and particulate-bound metals (Cd, Cu, Cr, Ni, Zn) using acid digestion and atomic absorption spectroscopy. Recovery tests confirmed high sampling efficiency and chemical integrity. The data revealed distinct temporal behaviours: particulate metals showed peak concentrations during the initial hours of rain events, while nitrogen compounds displayed quasi constant concentration throughout. Correlation analysis showed that metals were positively associated with PM and negatively with temperature and time step, suggesting strong links to particulate-mediated deposition and early-stage scavenging. In contrast, nitrogen species showed weaker and more variable correlations, likely due to their higher solubility and continuous atmospheric input. The performance of regression models improved with increasing bin width, particularly for metals Cu and Zn (R² > 0.85 under 5-hour binning). These findings highlight the value of high-resolution sampling in understanding the deposition processes.