To assess the quality of atmospheric air, the authors propose to apply the process of dry deposition of impurities on an artificial underlying surface that binds impurities in contact with it. The mass of these impurities is calculated upon laboratory exposure, after being transferred to an aqueous solution. The ease of absorber fabrication and the low cost facilitate the monitoring of air pollution at various points in woodlands, where the stationary stations for air-pollution-monitoring are very difficult and costly to arrange. A large number of control points makes it possible to identify forest areas with the highest levels of air pollution. A dynamic air-quality study at one of the monitoring points is necessary and sufficient to determine the concentration of impurities. The authors surveyed an urban forest using the proposed method, and the survey results confirmed that areas with an elevated concentration of airborne nitrogen dioxide exist within the woodland. This can lead to soil eutrophication and changes in forest biodiversity at the species and ecosystem levels.
A major problem that must be addressed during environmental monitoring in woodlands, especially on the urbanized areas, is the determination of the time-space distribution of impurities in the air. As it was previously substantiated theoretically and shown experimentally , , , , the content of impurities in the air in different parts of the woodlands is not uniform. In forests, especially in urbanized areas, the spread of impurities in the air with distance from the borders with their greatest concentration is extremely uneven. In some cases, in certain areas of the woodlands, due to the peculiarities of its structure or landscape, a significant accumulation of impurities in the air is possible. This can lead to a change in species diversity. and the emergence of danger to humans during recreational use. In this regard, there is a need to identify such areas and, in the future, take measures to limit the intake of impurities on them. However, due to the significant areas of distribution and characteristics of forest ecosystems in urbanized areas, where they, most often, are part of specially protected natural areas, the organization of stationary posts for monitoring the state of atmospheric air is very difficult and costly. In this case, an alternative can be considered based on diffusion processes passive dosimetry methods.
As far as is known, changes in the prioritization of the presence of the most common impurities in atmospheric air according to their hazards have already occurred, both in many regions and at the global level. The content of sulfur dioxide has decreased, and consequently, its impact on terrestrial ecosystems has decreased significantly. However, the industry-related emission of nitrogen oxides (NOx) into the atmosphere has increased. In general, the anthropogenic impact on the nitrogen cycle in nature has far exceeded the planetary boundaries [5, 6]. The impact of NOx on forest ecosystems is particularly noticeable in urbanized areas and near highway, where vehicles are the main source of pollution. Studies have been conducted on the distribution of NOx in the air of forest ecosystems located within the road traffic pollution footprint. The model area was the spruce forest of the Kuntsevo Dacha—part of the Setun River Valley nature reserve (Moscow). It is a designated conservation area where a forest ecological monitoring network has been organized, and the state of biotic and abiotic components of the forest ecosystem is monitored. In these studies, the uniform network of samplers was arranged within the forest boundaries to cover the entire ecotopic diversity of the area.
To assess the state of the air in different areas within the boundaries of the forest, it is proposed to use the processes of dry deposition of impurities on the artificial underlying surface of the samplers.
Physicochemical basis of the method
Unlike the commonly used method in which the analyzed air is aspirated through a sorbent, the proposed method is based on the molecular diffusion of the analyte through a diffusion resistance to the sorbent surface. Molecular diffusion processes are described by Fick’s diffusion law. The mass of a substance transferred during molecular diffusion per unit of time through a unit of surface is proportional to the concentration gradient of this substance [7, 8].
The proportionality coefficient, or molecular diffusion coefficient (D) is a physical constant that characterizes the ability of a given substance to penetrate through a stationary part of the medium due to diffusion (Eq. (1)). Thus, its value is not dependent on the hydrodynamic conditions observed in the bulk of the medium. The value of the molecular diffusion coefficient (D) is a function of the properties of the substance being distributed, properties of the diffusion medium, temperature, and pressure:
where k is a constant determined by the nature of the substance being distributed and the diffusion medium, T is the temperature, P is the pressure, b is the exponent, which depends on the nature of the gas and temperature and is usually equal to 1.5.
Air pollution assessment methods
One of the features of the proposed air-pollution-determination method is that the absorbing surface is protected from above by a waterproof and airtight material. Therefore, gaseous impurities and fine aerosols subjected to Brownian motion reach the absorbing surface only on one side, passing through a thin layer of air during molecular diffusion (Fig. 1), and are completely absorbed by the sorbent.
The flux of the impurity deposited on the sorbent surface during the dry deposition depends on the sampler design, meteorological conditions, impurity concentration, and nature of the sorbent and impurity.
Upon exposure, the impurities sorbed are transferred (in a laboratory setting) into a solution during sorbent water washing. Then, the concentration of the corresponding impurity in the resulting solution is determined, and the mass of the sorbate is calculated. The ratio of the mass of the impurity absorbed to the exposure time determines the rate of the dry deposition process for a given absorber (Eq. (2)):
where Q i,j is the impurity absorption rate in units of mass of the i-th sorbent-bound substance per unit of time at the j-th monitoring point (µg/*hour); M i,j is the mass of the absorbed i-th impurity at the j-th monitoring point (µg); τ is the exposure time (hours).
The obtained values of the absorption rate are proportional to the airborne impurity concentration averaged over the exposure period, and their links to the relevant monitoring points can determine spots (areas) with an elevated concentration of airborne impurity.
A conversion factor may be used (Eq. (3)) to switch from the impurity absorption rate to the impurity concentration averaged over the exposure period at a given monitoring point:
where С i,j is the value of the i-th impurity concentration averaged over the exposure period at the j-th monitoring point (mg/m3); К is the conversion factor; Q i,j is the i-th impurity absorption rate at the j-th monitoring point.
The value of this coefficient depends on the design features of the sampler, which affect the thickness of the molecular diffusion layer formed near the sorbent surface, and on the meteorological conditions during the sampler exposure. The values of this coefficient for same-design samplers located in an area with the same meteorological conditions will be the same for all monitoring points. Therefore, it is sufficient, at one of the monitoring points within a specific exposure period and simultaneously with the determination of the impurity absorption rate, to determine the impurity concentration averaged over the exposure period using the dynamic method and to evaluate the conversion factor for the given substance (Eq. (4)):
where Q i is the i-th impurity absorption rate; С i is the value of the i-th impurity concentration averaged over the exposure period.
Our samplers were distinguished by simple design, ease of maintenance, compactness, and low cost. The sampler can be of various shapes and was made from special grades of paper or polyethylene.
In woodlands, absorbers are placed directly on trees in blow areas. Sampling can be performed at any time of the year. Boundary walls of samplers prevent the ingress of atmospheric moisture onto the sorption surface (Fig. 2).
In our experiments, we used filter paper impregnated with the appropriate solution as a sorbent. As shown by previous experiments , for the reliable absorption of such impurities as compounds of sulfur, nitrogen, fluorine and chlorine, it is enough to soak the filter paper for 1 min with a 0.5N NaOH solution in a 50 % aqueous solution of ethanol with the addition of 0.5% glycerin. When determining the presence of ammonia and aerosols containing ammonium ions, alkali, alkaline Earth and heavy metals in the air, it is recommended to impregnate the filter paper for 1 min with a 0.5N solution of H2SO4 in a 50% aqueous solution of ethanol with the addition of 0.5% glycerin. With an area of the sorbent equal to 0.0038 m2 and exposure for 20 days in air containing up to 2 MPCo.t corresponding to impurities in the air, the discrepancy in the values of the absorption intensity determined at the same control point on 10 samplers did not exceed 20%.
Results and discussion
The model area in this study was the spruce forest of the Kuntsevo Dacha—part of the Setun River Valley nature reserve (Moscow). It is a designated conservation area where a forest ecological monitoring network has been organized, and the state of biotic and abiotic components of the forest ecosystem is monitored. During the period of our research, samplers were placed within the boundaries of the forest in the form of a uniform network, so as to cover the entire ecotopic diversity of the territory.
Analysis of the results obtained allowed us to reveal an interesting picture of the distribution of nitrogen dioxide in the inner space of the urban forest. First, the average value of NO2 concentration decreased with distance from the edge of the forest, which undoubtedly indicates the presence of a medium-stabilizing effect of the forest ecosystem, but, in general, this decrease is insignificant (Table 1).
|Distance of control points to the edge of the forest, m
|Average concentration NO2 in the zone, μg/m3
|Root mean square deviation of concentration values
|Coefficient of variation of concentration values
|90 ± 6
|71 ± 6
|81 ± 9
Secondly, as the result of processing the obtained data using the Golden Soft Ware Surfer geographic information system (Fig. 3) shows, the zone of localization of the highest NO2 concentrations is confined to a forest area that is quite remote from the marginal zones, but has serious changes in the biogeocenotic structure due to with recreational effects.
Earlier in works [3, 4] we analyzed the ecological reasons for such results. In this article, it is advisable only to designate them. On the one hand, this is the influence on the migration and transformation of atmospheric inputs of nitrogen compounds of a special regime of temperature inversions and illumination under the forest canopy, which forms conditions for the accumulation of impurities in the surface air layer. On the other hand, there is a connection between the anthropogenic transformation of forest ecosystems and the intensification of their nitrogen eutrophication under conditions of uninterrupted supply of nitrogen under conditions of atmospheric pollution.
In this work, it was important to show that this inexpensive and fairly accurate method for assessing the state of atmospheric air, based on an analysis of the intensity of the process of dry deposition of impurities on an artificial underlying surface, makes it possible to determine air quality at a variety of control points located in such an ecologically heterogeneous space as an urban forest. This, in turn, allows visualizing the distribution of impurities in the surveyed area using geographic information systems.
Thus, the method for assessing the state of atmospheric air during the deposition of impurities in the process of dry deposition on an artificial underlying surface allows a reliable assessment of the content of impurities in a large number of control points of the forest and to identify areas with high concentrations of impurities. Such information is necessary for the rational zoning of forest areas, and, especially, in the conditions of urbanized areas, where urban forests simultaneously perform ecosystem, nature conservation, and recreational functions.
 A. Z. Razyapov, S. S. Voronich, V. A. Bagryantsev, V. N. Stepchenko, G. V. Lomakin. Bull. Russ. Acad. Nat. Sci. 6, 14 (2012).Search in Google Scholar
 M. A. Sutton, C. Howard, J. W. Erisman, G. Billen, A. Bleeker, P. Grennfelt, H. Van Grinsven, B. Grizzetti. The European Nitrogen Assessment: Sources, Effects and Policy Perspectives, Cambridge University Press, Cambridge (2011).Search in Google Scholar
 W. Steffen, K. Richardson, J. Rockström, S. Cornell, I. Fetzer, E. M. Bennett, R. Biggs, S. R. Carpenter, W. de Vries, C. A. de Wit, C. Folke, D. Gerten, J. Heinke, G. M. Mace, L. M. Persson, V. Ramanathan, B. Reyers, S. Sörlin. Sci. Exp. (2015).Search in Google Scholar
 V. A. Kuznetsov, N. P. Tarasova. Ecol. Ind. Russia 10, 41 (2008).Search in Google Scholar
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