PM is mainly the result of anthropogenic processes in cities [27]. In this study, we analysed the accumulation of PM from a city’s exit road and its distribution into the urban forest. The studied urban forest had a zonal structure. The highest number and cover of herbaceous plants was represented by synanthropic and grass species in the first two zones (No. 1 and No. 2), located near the source of transport pollution. The plant species composition in zones 3, 4, and 5 were more typical for fresh coniferous forests, which means that the habitat was more “natural”, with less anthropogenic impacts. When designing vegetation barriers to mitigate near-road air pollution, zones of urban greenery characterised by different structures of vegetation, including herbaceous plants and mosses, are rarely taken into the account; the greatest emphasis is placed only on trees and shrubs [45,46,47,48]. However, for safety (better visibility, avoiding accidents) and space reasons, trees and high shrubs cannot grow in the immediate vicinity of roads and intersections. Consequently, where trees cannot be planted, PM emitted from the road may be dispersed and transferred to new locations [48,49,50,51] or accumulated by herbaceous plants and grasses [39]. The presence of trees can in turn lead to elevated PM concentrations close to the road because high vegetation acts as a dam for polluted air [50,51]. In some cases PM dispersion can be interpreted as a favourable phenomenon, as it leads to lower PM concentration in the air at the place of its emission [50,52]. Conversely, PM that has not been adsorbed on any physical barrier may be relocated to new locations, which are perceived as clean since they are devoid of PM sources. It seems that a much safer solution would be PM accumulation and its permanent retention on plants growing in the immediate vicinity of the road. The critical problem to be addressed is finding plants that will be able to tolerate very difficult growing conditions (salinity, drought, pollution, soil degradation, and compotation), grow along the roads, not pose a threat to road users, and effectively accumulate PM from the air. The results of this work demonstrate that more attention should be paid to herbaceous plants. Many perennial herbaceous plants can tolerate a close proximity to roads. Herbaceous plants, especially those that are relatively big in size, tend to have deep root systems and, as resource-conserving species, invest more in their defence and storage mechanisms [53]. In this study, herbaceous plants were shown to grow in areas where airborne PM concentrations were significantly high (5 and 10 times higher than at the forest edge). These plants were the most exposed to the toxic effects of air pollutants and constituted the first physical barrier (filter for air pollution) between the road and cleaner air. Herbaceous vegetation turned out to be effective at PM accumulation. PM accumulation by herbaceous perennial plants is a consequence of their location, which is as close as possible to emission sources (1–10 m from the road), but also their morphology and canopy structure. The high accumulation of PM by plants growing in the immediate vicinity of the emission sources is a well-described phenomenon [54,55]. Many herbaceous plants are covered with hairs and a thick waxy coating, which are the features that determine the effective accumulation of PM [56,57]. In this work, we analysed the leaves of Achillea millefolium, Plantago major, and Hieracium pilosella, which are covered with hairs. The high proportion of grasses (mowed rarely or not at all) in the assessed plant community was responsible for the high density and porosity of the plant filter. The high efficiency of PM accumulation by herbaceous plants was previously shown by Przybysz et al. [52], Weber et al. [56], Speak et al. [58], and Janhäl [59], while the ability of grasses to accumulate PM from the air was demonstrated by Przybysz et al. [34]. Unmoved herbaceous plant communities are typically tall enough (30–50 cm at least), yet permeable and dense enough to act as an effective air filter near the source of roadway emissions. In order to exploit the phytoremediation potential of herbaceous plants fully, it is worth limiting the mowing of roadside vegetation, especially in locations where aesthetics are less important, as for example, the city exit road studied in this work. It has already been shown that the structure and location of greenery near pollution emission sources has a much greater impact on PM accumulation potential than the biodiversity and species composition of plant communities [34]. This suggests that even a spontaneous roadside herbaceous community can have a positive impact on the air quality near roads.

The first line of trees accumulates (per unit of leaf area, not the whole plant) significantly less PM than herbaceous perennials. This is a surprising result, as until now street-adjacent trees were seen as the only element of urban greenery having a real impact on limiting air pollution with PM [60,61]. In this work, the higher efficiency of PM accumulation by herbaceous plants compared to trees is attributed to their shorter distance from the road, which was the only PM source in the area. Possibly, herbaceous plants filtered the polluted air emitted from the road before it reached the trees. Since the air pollutants were emitted close to the ground, and the herbaceous plants were right next to the road, they could significantly reduce PM dispersion and transport. Conversely, the trees grew just 10 m away from the road, which is relatively close to the emission source; thus, they should accumulate PM efficiently, especially considering their height (at least a few meters) and planting density. The high concentration of PM in the air between the road and the first row of trees shows that, despite the very effective accumulation of PM by herbaceous plants, the air was still polluted, and the amount of PM deposited on shrubs and tree leaves should be higher. Another explanation may be the re-suspension of PM temporarily accumulated on shrubs and trees by wind and rain. Wind and rain can remove significant amounts of PM from the leaf surface [34,51,59,62,63]. Re-suspended PM returns to the environment and can pollute the air again. It seems that herbaceous plants growing under and close to the trees most probably also accumulate PM re-suspended from taller vegetation. It can be assumed that herbaceous plants growing along the road should adhere to the first row of trees, so that they could accumulate PM re-suspended from trees and shrubs. Under trees and shrubs, especially near the roadside, there should be no hardened surfaces (pavements, bicycle paths, parking lots), because they cause further re-suspension of PM. This is another piece of evidence that herbaceous plants (preferably tall and unmown) play a very important role in removing PM from the air and should be a permanent element of urban greenery along roads. If possible, lawns should be replaced by meadows. Lawns offer fewer ecosystem services (biodiversity, food for pollinators, habitat for invertebrates) and require higher maintenance costs than a meadow [64]. In addition, lawns require mowing, which results in additional air pollutant emissions [65].

A new and very interesting finding of this study is the increasing accumulation of PM on leaves inside the forest, despite the very low concentration of PM in the forest (20 and 50 m from the road). Previous studies have indicated that PM accumulation on plants decreases with increasing distance from the edge of the forest/park [55]. Popek et al. [55], however, studied a large city park with lower planting density and the presence of numerous wide footpaths, which resulted in greater ventilation and air movement. The small amount of PM that reached the interior of the park did not have to be accumulated by the plants, but could be carried somewhere else. The trees and shrubs in this park were also exposed to rain and wind. In the present study, the dense canopy of tall trees constituted an insulating layer for air pollutants, including PM. Once PM was transferred deeper into the forest, it was trapped there due to little/no air movement. Most probably, most of the PM that entered the forest was accumulated by forest vegetation and then retained for a long period of time. The plant samples in the forest were collected at a height of 1.5 m; therefore, the analysed plant material was exposed to rain and wind only to a minimal extent. PM was not regularly washed off the surface of the leaves, as happened at the edge of the forest. Moreover, the increased accumulation inside the forest mainly concerns the smallest PM and the most detrimental to health (0.2–10 µm). On the contrary, the accumulation of PM by herbaceous plants and mosses decreased with increasing distance from the road, and it was significantly lower inside the forest. It is probable that before the PM fell by gravity onto lower vegetation, it was accumulated and retained by trees and shrubs. The presence of a dense forest close to the road caused the PM concentration in the air between the road and the first row of trees to be very high, often exceeding the permissible standards. The results obtained in this study support the finding of Vos [66], who showed that trees can increase PM concentrations along the road. Tong et al. [50] argue that, unlike trees, lawns (and possibly other low vegetation) lead to the dispersion of pollutants into the air, consequently lowering the concentration of PM. We are of the opinion that in order to combat air pollution generated by road transport effectively, roadside greenery must be diverse and have a layered structure. A mixture of relatively tall herbaceous plants should be grown along the roadside in areas where the presence of shrubs and trees is not possible. The task of the usually underestimated herbaceous vegetation is to capture PM from the air immediately after its emission. Shrubs and trees, in turn, act as a barrier against the uncontrolled spread and accumulation of PM that has not been deposited on herbaceous plants present in zones 1 and 2. The first rows of trees should be relatively porous so as to enable PM transport into the forest, where it will be permanently accumulated by trees and shrubs. For the above reason, the area between the road and the first rows of trees should be inaccessible to human activity, e.g., bicycle and walking paths should always be at least a few rows of trees away.