Biomonitoring of Air Pollution

The World Health Organization reported that air pollution in 2012 caused the death of about 7 million people worldwide (WHO, 2014), confirming air pollution as one of the principal environmental health risks in the world, and indicating its reduction as an urgent mission to save millions of lives. An effort should be made to assess the presence in the atmosphere of long-known pollutants and to bring to light emerging ones. The monitoring of air quality is currently done through automatic devices, but they can provide data on a limited number of pollutants; furthermore, their high costs prevent the development of dense monitoring networks. Thanks to their innate characteristics, plants are particularly suited to describing the spatial-temporal trends of airborne pollutant depositions and their effects on living organisms and ecosystems, allowing us to forecast environmental changes from a small to a large scale. Biomonitoring with plants is considered an adequate alternative technique to acquire data about pollution, but to date there are still some open issues that need to be explored by the scientific community. In addition to conventional air monitoring systems, it is necessary to set up and implement new, cost-effective, robust, and flexible tools for monitoring air quality using plants.

This special issue is a collection of studies based on new methods in air biomonitoring by plants, or the implementation of already established techniques. Seven articles are included in this special issue, concerning different aspects of biomonitoring techniques using cryptogams and higher plants.

Most articles from the literature are based on the comparison between pollutant concentrations, measured in biomonitors collected or exposed in affected areas and proximal natural areas. In the article by Drava et al. (2020) [1] a new approach was proposed, consisting of a direct comparison between data obtained by bioindicators and parameters measuring population health. The authors found significant correlations between mortality and morbidity due to cardiac and respiratory diseases and several heavy metals found in holm oak bark of trees sampled in urban parks and streets of an Italian city. The results of this work offer a preliminary indication of the potential of this approach for implementing strategies aimed at reducing environmental and human health risks.

In the article by De Agostini et al. (2020) [2], moss transplants (Hypnum cupressiforme Hedw. moss bags) were used to assess the air quality around an oil refinery located in Sardinia island (Italy) over a period of 16 years, by means of three monitoring surveys per year. The authors found that both the distance from the contamination source and rainfall could influence the element content in the biomonitor, depending on the considered element and the exposure conditions. They confirmed the capacity of H. cupressiforme moss bags to provide relatively stable measurements and pointed out that adopting similar exposure conditions is of paramount importance in order to lower the variability of the accumulation values.

Hong Kong is among the most densely populated cities in the world, with millions of people exposed to severe air pollution. The work by Leung et al. (2020) [3] deals with the problem of tropospheric ozone, known to be harmful for both humans and vegetation, inducing foliar damage in sensitive species. In this work, the authors used ozone-sensitive and tolerant genotypes of bush bean Phaseolus vulgaris L. to investigate the impacts of ambient ozone on the leaf traits and plant development in the city of Hong Kong. The results showed that the ozone-sensitive plants suffered as a result of this pollutant and developed ozone-induced foliar damage as expected, but unexpectedly, this genotype produced more pods and beans than the ozone-resistant genotype. To explain these results, the authors postulated that ozone, like other stresses, could shorten the phenological cycle in ozone-sensitive plants, anticipating and enhancing flower production, finally resulting in a higher bean yield. It is probable that a higher concentration of ozone was needed to negatively affect the yield of the ozone-sensitive plants. Meanwhile, ozone-induced foliar damage showed a clearly graduated pattern, which could be useful for estimating the levels of tropospheric ozone.

In the article by Ristorini et al. (2020) [4] the chemical contents of washed and unwashed leaves of Arundo donax L. samples collected along a river in an urban and industrial hot spot in Central Italy were evaluated. The results were compared with element concentrations measured in the river water and in the particulate matter (PM) collected in filters in the same area. The aim of this study was to identify the role of the two environmental matrices on leaf chemical composition. Element contents in washed and unwashed leaves were compared in order to differentiate between the surface depositions and element uptake by roots. The results highlighted that atmospheric concentrations of PM10 and linked elements had a similar trend for Ni, Mo, Cr, Ti, and Fe, indicating a common source for these elements—possibly a steel plant and vehicular traffic. Soluble elements seemed to be mainly bound within leaf tissues (i.e., they were likely taken up by roots), whereas insoluble elements were deposited on the leaf surface. Element concentrations in washed A. donax leaves were poorly correlated with those measured in river water samples, suggesting a limited contribution of the river water to the plant nutrient supply.

The study by Kepusta and Godzik (2020) [5] reports a biomonitoring survey with the native moss Pleurozium schreberi (Brid.) Mitt. to assess relatively recent (2015) and past (2001) levels of air pollution in four regions of Poland. The study area included copper- and zinc-lead industrial districts, large urban agglomerations and an area away from pollution sources. The authors found elemental profiles of native moss samples to be in good agreement with the industrial activities occurring over the study area. The methodology also highlighted that significant changes in air pollution occurred between the 2001 and 2015 surveys, though their magnitude and direction were both metal- and region-dependent. Indeed, while the levels of some metals (e.g., Cd and Pb) decreased, other metals (e.g., Cr and Ni) showed the opposite trend. The results suggest that air quality has not significantly improved recently, despite the adoption of legislation aimed at environmental protection, but the type of emissions has changed, with an increase in non-industrial pollution sources.

Capozzi et al. (2020) [6] carried out a biomonitoring survey of airborne priority pollutants (i.e., elements of environmental concern) using leaves of native black locust trees and moss bags filled with Hypnum cupressiforme. The aims of the work were (i) to evaluate the possible correlation between the data obtained by mosses and leaves and (ii) to evaluate whether leaf traits affect the uptake mechanisms. The results showed that H. cupressiforme shoots transplanted in bags had generally higher elemental contents than R. pseudoacacia leaves, despite the shorter exposure time. Accordingly, the deposition flux for each element was lower in R. pseudoacacia L. for most elements—except Cr, Mo, and Zn—indicating for these elements a two-way uptake mechanism through deposition on the leaf surface and absorption via the root. This work highlighted that leaf traits (the micromorphology of the surface) play an important role in the interception and retention of PM and linked elements. H. cupressiforme transplanted in bags was confirmed to be a powerful bio-accumulator of airborne elements; by contrast, R. pseudoacacia, with a smooth leaf surface and scarce trichomes, showed a limited ability in airborne element retention.

In the last article included in this special issue, Godzik (2020) [7] reported the results from several decades of bioaccumulation campaigns in Poland. The first studies using native mosses as indicators of the heavy metal air pollution level were carried out in national parks of Polonia in the 1970s. When Poland joined the European Heavy Metals Deposition Program, the entire area of the country was included in this research and moss biomonitoring surveys were repeated at five-year intervals using native Pleurozium schreberi. Changes in the level of heavy metals were recorded in the period from 1975 to 2014 in the Niepołomice Forest, located near Kraków. Concentrations of ten heavy metals were constantly monitored. Additionally, in some moss surveys, other elements and compounds, such as ¹³⁷Cesium, PAHs (Polycyclic Aromatic Hydrocarbons), and nitrogen were quantified. Since the 1990s, in all European countries, there was a significant decrease in the level of heavy metals. In Poland, the spatial pattern of metal accumulation in mosses was similar throughout the entire study period. The southern part of the country, which is more industrialized and densely populated, is still the most polluted, whereas the north-eastern part is the cleanest region. In the Niepołomice Forest, the emissions from the large steel mill and from the Kraków agglomeration had the greatest impact on pollution from the 1970s to the 1990s, but lately the impact of local emission sources has become more evident. Compared to other European countries, Poland represents the forefront of the most polluted areas in Europe.