Enabling comprehensive wet deposition measurements in remote locations
Atmospheric deposition plays a crucial role in the distribution of pollutants and the cycling of biogeochemically important species like carbon, nitrogen, and sulfur. Monitoring networks have been established to track spatial and temporal patterns of wet and dry deposition, providing critical data for understanding the effectiveness of environmental policies. However, deploying deposition samplers in remote locations with no grid power or regular access remains a challenge.
To address this, we present the design of a customizable, off-grid automatic precipitation sampler that can selectively collect wet deposition samples while operating continuously for extended periods. This system leverages a conductivity sensor to detect the onset of conductive precipitation events, triggering a motor-driven lid to open and close over a funnel, funneling the sample into a collection container. The modular design allows for the deployment of up to six independent units, enabling replicate measurements of incident precipitation (open fall, OF) and canopy throughfall (TF).
We demonstrate the performance of these samplers over 2 years across the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT), a remote region spanning 47-53°N. The collected samples were analyzed for pH, conductivity, and dissolved organic carbon (DOC) flux, with results consistent with previous measurements in this understudied area. The ability to autonomously collect wet deposition samples while preserving biogeochemically relevant analytes showcases the potential of these cost-effective platforms to expand environmental monitoring networks and improve our understanding of atmospheric-terrestrial-aquatic linkages.
Atmospheric deposition: A critical process in global biogeochemical cycles
Atmospheric deposition, the transfer of particles and gases from the atmosphere to the Earth’s surface, is a central process in the distribution of pollutants and the cycling of major nutrients and emerging contaminants of concern. Dry deposition, driven by the direct interaction of gases and particles with boundary layer surfaces, and wet deposition, involving the scavenging of atmospheric species by precipitation, are both globally significant but can vary in relative importance depending on local sources and transport patterns.
Wet deposition, in particular, plays a key role in delivering long-lived atmospheric particles and gases to terrestrial and aquatic environments. The size and number of cloud droplets and ice crystals, as well as the solubility and reactivity of the atmospheric species, govern the rate of wet deposition. As a result, this process is closely linked to regional meteorological conditions.
The recognition of atmospheric deposition’s significance has led to the establishment of long-term monitoring networks, such as the Canadian Air and Precipitation Monitoring Network (CAPMoN) and the National Atmospheric Deposition Program (NADP) in North America. These programs have provided critical data on the spatial and temporal patterns of wet and dry deposition, informing our understanding of the effectiveness of policies aimed at reducing environmental issues like acid rain.
Despite these successes, challenges remain in comprehensively monitoring atmospheric deposition, especially in remote locations lacking grid power or regular access. Innovative sampling strategies and materials are needed to expand monitoring networks and address current and emerging research priorities, such as improving constraints on atmospheric carbon linkages to terrestrial and aquatic processes.
Customizable, off-grid automatic precipitation samplers
To enable more extensive deployment of deposition samplers in the field, we have developed a cost-effective, modular, and autonomous precipitation sampling system that can operate off-grid. The key features of this system include:
Selective precipitation sampling
The system leverages a conductivity sensor to detect the onset of conductive precipitation events, triggering a motor-driven lid to open and close over a funnel, funneling the sample into a collection container. This approach selectively samples precipitation containing dissolved ionic species, rejecting the “ultrapure” portion of rainfall during atmospheric washout.
Modular and customizable design
The sampling assembly consists of modular components, including a water level transmitter, pumping unit, control unit, and sample collection system. This allows the arrangement to be adapted to various field situations, with the ability to deploy up to six independent units for replicate measurements of open fall (OF) and throughfall (TF) precipitation.
Off-grid operation
The system is designed for low power consumption, utilizing a 12V battery-based power supply that can sustain autonomous operation for extended periods. This enables deployment in remote locations without access to grid power.
Robust construction and materials
The system components are housed in weatherproof enclosures, and the sample intake and transmission tubing are selected to minimize photochemical reactions and microbial growth within the collected samples. This ensures the preservation of biogeochemically relevant analytes.
Automated data logging and control
The control unit logs water level data and manages the sampling process, with user-defined programs controlling the onset, duration, and termination of sampling based on the measured precipitation characteristics. This information can be retrieved remotely or during site visits.
The modular design of this system allows for the easy modification of materials, the number of replicate samplers, and the preservation strategies employed, depending on the specific analytes of interest. This versatility makes the platform suitable for a wide range of environmental monitoring applications, from tracking the deposition of persistent organic pollutants to quantifying fluxes of dissolved organic carbon (DOC) and other biogeochemically relevant species.
Performance evaluation in the NL-BELT region
We deployed these automated precipitation samplers across the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT), a remote region spanning 47-53°N, during the snow-free seasons of 2015 and 2016. This allowed us to evaluate the system’s performance and the representativeness of the collected samples for various water quality parameters.
Sample collection efficiency and representativeness
Over the 2-year field deployment, a total of 97 automatic composite runoff samples were successfully collected, along with 28 manual grab samples taken during specific runoff events. The collected sample volumes were compared to theoretical values calculated from the registered pump activity, revealing an overall mean sampling efficiency of 71% with a standard deviation of ±26%.
Variations in sampling efficiency were observed across the different field sites, with some locations exhibiting lower efficiencies due to high loads of coarse suspended sediment that settled within the sample intake units. Regular maintenance, including cleaning the intake and transmission components, helped improve performance in most cases.
To assess the representativeness of the composite samples, we compared the relative concentrations of various pollutants (heavy metals, total petroleum hydrocarbons, and polycyclic aromatic hydrocarbons) between the composite samples, manual grab sample time series, and background conditions. The results showed that the composite samples closely resembled the general patterns observed in the grab sample time series, while both differed significantly from the background conditions.
This demonstrates that the customized precipitation sampling design of these new platforms enables the collection of representative wet deposition samples for a range of urban-derived pollutants and biogeochemically relevant species.
Water quality parameters
The collected precipitation samples were analyzed for pH, conductivity, and dissolved organic carbon (DOC) flux, providing insights into the chemical composition of wet deposition in this remote region.
The pH of the collected precipitation ranged from 4.14 to 5.71 in the open fall (OF) samples and 4.74 to 5.99 in the throughfall (TF) samples, consistent with recent measurements from nearby monitoring sites. The slightly higher pH in TF samples is likely due to the interaction of precipitation with the forest canopy, which can absorb acidic species and release base cations.
Conductivity values ranged from 21 to 166 μS/cm, with no apparent seasonal or spatial trends. These values are comparable to typical surface water conductivities and indicate the presence of dissolved ionic species in the precipitation.
The DOC fluxes in the collected samples ranged from 600 to 4,200 mg C/m²/yr across the transect, with TF fluxes generally higher than OF due to the interception and leaching of organic matter by the forest canopy. These values are similar to those reported for other boreal forest environments, highlighting the potential of these samplers to provide new insights into the atmospheric deposition of this important biogeochemical pool.
Expanding environmental monitoring capabilities
The customized precipitation sampling design of these new platforms enables more universal accessibility of deposition samples for the atmospheric observation community. The modular and cost-effective nature of the system allows for easy deployment in remote locations, overcoming challenges associated with grid power and regular access.
By facilitating the collection of representative wet deposition samples, these samplers can contribute to improving our understanding of atmospheric-terrestrial-aquatic linkages, particularly regarding the transport and cycling of biogeochemically relevant species like dissolved organic carbon. This information is crucial for constraining carbon budgets and reducing uncertainties in Earth system models.
Furthermore, the ability to easily modify the materials and preservation strategies of these samplers opens up possibilities for monitoring a wide range of atmospheric pollutants, from persistent organic compounds to emerging contaminants of concern. As environmental monitoring needs continue to evolve, platforms like these can help expand observation networks and provide valuable data to support policy decisions and scientific research.
Overall, the performance and versatility of these autonomous, off-grid precipitation samplers demonstrate their potential to enhance small-scale environmental monitoring efforts in remote and underrepresented regions, advancing our understanding of atmospheric deposition and its role in global biogeochemical cycles.
