Cost-effective off-grid automatic precipitation samplers for pollutant and biogeochemical atmospheric deposition

Abstract

Abstract. An important transport process for particles and gases from the atmosphere to aquatic and terrestrial environments is through dry and wet deposition. An open-source, modular, off-grid, and affordable instrument that can automatically collect wet-deposition samples allows for more extensive deployment of deposition samplers in fieldwork and would enable more comprehensive monitoring of remote locations. Precipitation events selectively sampled using a conductivity sensor powered by a battery-based supply are central to off-grid capabilities. The prevalence of conductive precipitation – that which initially contains high solute levels and progresses through trace-level concentrations to ultrapure water in full atmospheric washout – depends on the sampling location but is ubiquitous. This property is exploited here to trigger an electric motor (via limit switches) to open and close a lid resting over a funnel opening. The motors are operated via a custom-built and modular digital logic control board, which has a low energy demand. All components, their design and rationale, and their assembly are provided for community use. The modularity of the control board allows the operation of up to six independent wet-deposition units, such that replicate measurements (e.g., canopy throughfall) or different collection materials for various targeted pollutants can be implemented as necessary. We demonstrate that these platforms are capable of continuous operation off-grid for integrated monthly and bimonthly collections performed across the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (47 to 53° N) during the growing seasons of 2015 and 2016. System performance was assessed through the measured power consumption from 115 V of alternating current (VAC; grid power) or 12 V of direct current from battery supplies during operation under both standby (40 or 230 mA, respectively) and in-use (78 or 300 mA, respectively) conditions. In the field, one set of triplicate samplers was deployed in the open to collect incident precipitation (open fall), while another set was deployed under the experimental forest canopy (throughfall). The proof-of-concept systems were validated with basic measurements of rainwater chemistry, which found (i) pH values ranging from 4.14 to 5.71 in incident open fall rainwater, (ii) conductivities ranging from 21 to 166 µS cm−1, and (iii) dissolved organic carbon concentrations in open fall and canopy throughfall of 16±10 and 22±12 mg L−1, respectively, with incident fluxes spanning 600 to 4200 mg C m−2 a−1 across the transect. Ultimately, this demonstrates that the customized precipitation sampling design of this new platform enables more universal accessibility of deposition samples for the atmospheric observation community – for example, those who have made community calls for targeting biogeochemical budgets and/or contaminants of emerging concern in sensitive and remote regions.