Automatic sequential rain sampling to study atmospheric particulate and dissolved wet deposition

Abstract

Wet deposition is a key mechanism influencing lifetime of particles in the atmosphere. It allows the deposition of particles via two distinct processes, washout and rainout. Wet deposition of particles is highly variable in space and time depending both on atmospheric aerosol loads and precipitation. This variability can be observed from an event to another as well as within an event. Studying the chemical composition of wet deposition can help us to understand the origin of the aerosols as well as the way they are deposited. Indeed, depending on their intrinsic properties, aerosols have specific chemical signatures. Sequential rain sampling provides a means to monitor the chemical composition of wet deposition throughout a rain event, trace its origin, and discuss the relative contribution of rainout and washout during a single event. In order to address these issues, an automatic sequential rain sampler was specifically developed to collect the rain in consecutive fractions. The sampler has a large sampling area (1 m2) and an automatic distribution system allowing the sampling of wet deposition with high temporal resolution to document intra-event deposition variability and a sufficient volume to analyze the dissolved and particulate phases of the rain. Two rain events collected in July 2021 and February 2022 in an urban site in the surroundings of the Paris agglomeration were studied. Our results allowed us to quantify how the total mass concentration decreases with the precipitation amount during a rain event until reaching a constant level. By combining elemental and ionic analysis, the evolution of particulate and dissolved major and trace element concentrations is documented for these two rainfall events. The particulate phase shows a higher decreasing trend than the dissolved one. Major elements such as Fe, Si and Al are predominantly in the particulate phase while trace metals (TMs), like Mn and Zn, are shared in both dissolved and particulate phases. However, each element shows an increasing ratio of its dissolved to total (dissolved + particulate) fraction as rainfall progresses, especially for TM. The study of dissolved compounds (SO42−, NH4+ and NO3−) confirms that concentrations are decreasing with rainfall depth but that the variations are influenced by changes in rainfall intensity. The rainout and washout relative contributions to wet deposition are discussed regarding the studied chemical species and the rain events.