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Deposition
Air pollution deposition is measured monthly within the SWETHRO, Swedish Throughfall Monitoring Nework, both in open areas and under tree canopies in the forest. In open areas, deposition is measured using bulk collectors, while deposition in forest ecosystems is measured as throughfall. Some substances interact with the tree canopies, and therefore string samplers are also used to determine the dry deposition of these substances.
The total deposition consists of both wet and dry deposition. The deposition at a specific site is influenced by the amount and composition of precipitation as well as the concentrations of various substances in the air. Land use also affects the magnitude of deposition. For example, deposition conditions are different in a forest compared to adjacent open land, which can be explained by the trees acting as filters for airborne pollutants.
The total deposition of various substances to the forest involves several processes. Part of the deposition occurs as precipitation, known as wet deposition. Another part occurs as gases and particles "sticking" to the tree canopies, known as dry deposition. What has been deposited as dry deposition is washed down to the forest floor with precipitation in the form of throughfall.
Provides a comprehensive measure
Throughfall is therefore considered to provide a comprehensive measure of the total wet and dry deposition. Dry deposition could thus be calculated as the difference between throughfall deposition and precipitation deposition in open areas. However, certain substances can be directly absorbed by, or alternatively leached from, the tree canopies. This means that throughfall measurements provide a good measure of the total deposition only for substances that do not interact with the tree canopies, such as sodium and chloride. For other substances, such as nitrogen and base cations, supplementary measurements with string samplers are required to accurately calculate dry deposition.
All work within SWETHRO, from sampling to chemical analysis, validation, and data processing, is carried out according to well-established procedures, and the laboratories are accredited for the chemical analyses. This ensures high-quality data and guarantees that data from different locations and from different years are directly comparable.
Dry deposition with string samplers
An area within the Swedish Throughfall Monitoring Nework with open field string samplers.
Dry deposition consists of particles, gases, and aerosols that are carried by the winds and adhere to various surfaces. For biologically inert substances, such as sea salt, dry deposition to the forest can be calculated as the difference between throughfall and wet deposition. In open areas, dry deposition is relatively small, while in forested areas it is significant because the forest has a larger amount of surface area available for dry deposition. The dry deposition is usually greater in spruce forests than in pine and deciduous forests, as the canopy density is significantly higher in spruce forests.
However, certain substances can be directly absorbed by or leached from the tree canopies. For these substances, which often constitute nutrients for the trees, dry deposition must be calculated differently using a string sampler. The dry deposition of a specific substance is calculated as the concentration of the substance in the sample from the string sampler, divided by the concentration of sodium (Na+) in the string sample, and then multiplied by the net throughfall of Na+ to the forest at the sampling site. The net throughfall of Na+ is calculated as the difference between throughfall and wet deposition. This method relies on using Na+ as a biologically inert marker.
The calculations above only include the dry deposition of particulate matter and do not account for the uptake of gaseous substances through stomata. The method is currently developed only for application in coniferous forests.
Dry deposition depends on the concentration of substances in the air multiplied by a factor known as the deposition velocity. The deposition velocity for particles to forest ecosystems depends on several factors, such as wind speed, turbulence, surface roughness, and the aerodynamic properties of the particles.
Throughfall
An area within the Swedish Throughfall Monitoring Nework in the woods
Deposition to forests is measured in SWETHRO using throughfall plots in managed forests, typically 30 x 30 square meters. The tree canopies act as collectors, filtering particles, gases, and aerosols from the air (dry deposition). The pollutants are then washed down by precipitation and collected in ten funnel-equipped containers per plot. These ten subsamples, containing both wet and dry deposition, are combined into one composite sample. During the winter period, the containers and funnels are replaced by buckets. The collectors are placed at least 0.5 meters above the ground. To minimize the effect of sunlight, all containers are covered with aluminum foil to reflect the sun's radiation and keep the samples' temperature low.
Deposition in open areas
An area within the Swedish Throughfall Monitoring Nework with bulk samplers in open field.
Deposition in open areas is collected using a bulk sampler (WoF sampler) and primarily consists of wet deposition, i.e., substances dissolved in precipitation. In addition to wet deposition, a small amount of dry deposition is also collected by the sampling equipment. The WoF sampler consists of a tube approximately 1.5 meters high. At the top of the tube, there is a specially designed "crown" that ensures the precipitation amount is as accurate as possible. The equipment also includes a bird ring to minimize contamination from bird droppings. In summer, a net is used to prevent debris from entering the sampler along with the precipitation.