Lichens serve as important bioindicators of air pollution in cities. Here, we studied the diversity of epiphytic lichens in the urban area of Munich, Bavaria, southern Germany, to determine which factors influence species composition and diversity. Lichen diversity was quantified in altogether 18 plots and within each, five deciduous trees were investigated belonging to on average three tree species (range 1–5). Of the 18 plots, two were sampled in control areas in remote areas of southern Germany. For each lichen species, frequency of occurrence was determined in 10 quadrats of 100 cm² on the tree trunk. Moreover, the cover percentage of bryophytes was determined and used as a variable to represent potential biotic competition. We related our diversity data (species richness, Shannon index, evenness, abundance) to various environmental variables including tree traits, i.e. bark pH levels and species affiliation and air pollution data, i.e. NO₂ and SO₂ concentrations measured in the study plots. The SO₂ levels measured in our study were generally very low, while NO₂ levels were rather high in some plots. We found that the species composition of the epiphytic lichen communities was driven mainly by NO₂ pollution levels and all of the most common species in our study were nitrophilous lichens. Low NO₂ but high SO₂ values were associated with high lichen evenness. Tree-level lichen diversity and abundance were mainly determined by tree traits, not air pollution. These results confirm that ongoing NO₂ air pollution within cities is a major threat to lichen diversity, with non-nitrophilous lichens likely experiencing the greatest risk of local extinctions in urban areas in the future. Our study moreover highlights the importance of large urban green spaces for species diversity. City planners need to include large green spaces when designing urban areas, both to improve biodiversity and to promote human health and wellbeing.

In this study, the co-pyrolysis of food waste with lignocellulosic biomass (wood bark) in a continuous-flow pyrolysis reactor was considered as an effective strategy for the clean disposal and value-added utilization of the biowaste. To achieve this aim, the effects of major co-pyrolysis parameters such as pyrolysis temperature, the flow rate of the pyrolysis medium (nitrogen (N₂) gas), and the blending ratio of food waste/wood bark on the yields, compositions, and properties of three-phase pyrolytic products (i.e., non-condensable gases, condensable compounds, and char) were investigated. The temperature and the food waste/wood bark ratio were found to affect the pyrolytic product yields, while the N₂ flow rate did not. More non-condensable gases and less char were produced at higher temperatures. For example, as the temperature was increased from 300 °C to 700 °C, the yield of non-condensable gases increased from 6.3 to 17.5 wt%, while the yield of char decreased from 63.6 to 30.6 wt% for the co-pyrolysis of food waste and wood bark at a weight ratio of 1:1. Both the highest yield of hydrogen (H₂) gas and the most significant suppression of the formation of phenolic and polycyclic aromatic hydrocarbon (PAH) compounds were achieved with a combination of food waste and wood bark at a weight ratio of 1:1 at 700 °C. The results suggest that the synergetic effect of food waste and lignocellulosic biomass during co-pyrolysis can be exploited to increase the H₂ yield while limiting the formation of phenolic compounds and PAH derivatives. This study has also proven the effectiveness of co-pyrolysis as a process for the valorization of biowaste that is produced by agriculture, forestry, and the food industry, while reducing the formation of harmful chemicals.

In this study, we analyzed 30 legacy and emerging poly- and perfluoroalkyl substances (PFASs) in paired atmospheric particulate and bark samples collected around a Chinese fluorochemical manufacturing park (FMP), with the aim to explore the sources of PFASs in tree bark. The results showed that PFASs in atmospheric particulate and tree bark samples were consistently dominated by perfluorooctanoate (mean 73 ng/g; 44 pg/m³), perfluorohexanoate (47 ng/g; 36 pg/m³), perfluorononanoate (9.1 ng/g; 8.8 pg/m³), and 10:2 fluorotelomer alcohol (10:2 FTOH; 5.6 ng/g; 12 pg/m³). Spatially, concentrations of C₈–C₁₂ perfluoroalkyl carboxylates (PFCAs) and 10:2 FTOH all showed a similar and exponentially decreased trend in both bark and atmospheric particulate samples with the increasing distance from the FMP. For the first time, we observed strongly significant (Spearman’s correlation coefficient = 0.53–0.79, p < 0.01) correlations between bark and atmospheric particulate concentrations for C₈–C₁₂ PFCAs and 10:2 FTOH over 1–2 orders of magnitude, suggesting that the continues trapping of atmospheric particulates resulted in the accumulation of these compounds in bark. Overall, this study provides the first evidence that atmospheric particulate is an obvious source of C₈–C₁₂ PFCAs and 10:2 FTOH in tree bark. This result may further contribute to the application of tree bark as an indicator of certain PFASs in atmospheric particulate.

Polycyclic aromatic hydrocarbons (PAHs) remain a focal concern of the air pollution in China. To discriminate the sources of airborne PAHs in Chinese rural regions, a national-scale tree bark sampling campaign and emission inventory estimation were conducted. The concentrations of the sum of 16 U.S. EPA priority PAHs in rural bark ranged from 6.30 to 3803 ng/g, with the dominance of 3- and 4-ring PAHs. Bark residual PAH concentration correlated significantly with emission flux rate, bark lipid content, ambient PM₂.₅, precipitation and sampling location. Based on the information of emission data, bark PAH congener profiles, principal component analysis, diagnostic ratios and compound-specific isotope analysis, solid fuel combustion was identified as the major source and could explain 40.3%–46.4% of bark PAH residues in rural China. The δ¹³C values of most individual PAHs were more negative at sites with lower longitude and latitude, suggesting a greater contribution of biomass combustion to PAH residues. Our results suggest the importance of regulating solid fuel combustion to significantly improve the air quality in China, and bark samples can provide a wealth of information on effectively monitoring and controlling the sources of PAH emission in rural China.

We report for the first time the Os isotopic composition of tree bark samples from a steel town. Osmium concentrations and ¹⁸⁷Os/¹⁸⁸Os isotopic ratios of ashed bark samples range from 1.40 to 24 ppt and 0.70 to 1.54, respectively, with the lowest ¹⁸⁷Os/¹⁸⁸Os recorded in samples close to the steel plant. Compositional variations in the bark samples can be explained by mixing between at least two sources with different Os isotopic signatures: a radiogenic source consistent with crust-derived materials and a relatively less radiogenic source consistent with mantle-derived chromite. The exact origin of the radiogenic Os component cannot be constrained, as background signatures and crustal materials used in the steel industry (e.g., coal and iron ore) likely have overlapping radiogenic signatures. Cr shows a similar distribution pattern to Os, indicating that both metals have a common origin, which provides further evidence that the Os budget in the bark samples is controlled primarily by the chromite used in the steel manufacturing. This study shows that Os isotopes are an effective tool for tracing steel production-related emissions.

Emission from manufacturing facilities to the surrounding environment is one of the important input source of pollutants. However, no information on the levels of organophosphate esters (OPEs) contamination in the environmental media around the manufacturing facility is available to date. In this study, samples from various environmental media, including sediments, water, surface soils, and tree bark, were obtained near an OPE manufacturing plant in Hengshui, Hebei Province, North China. The three main congeners, detected were tris(2-chloroethyl) phosphate (TCEP), tris(2-chloroisopropyl) phosphate (TCIPP), and triphenyl phosphate (TPHP), with the summed OPE concentrations (∑OPEs) ranging from 340 to 270,000 μg kg⁻¹ dry weight (d.w.), 7100 to 33,000 ng L⁻¹, not detected (N.D.) to 14,000 ng kg⁻¹ d.w., and 5300 to 19,000 ng g⁻¹ lipid weight in the sediments, water, soils, and tree bark, respectively. These findings suggest that point sources of OPEs could have widespread effects on its surrounding environments. Sediment and water concentrations of TCEP and TCIPP measured in this study were among the highest concentrations yet reported in the world. Meanwhile, the concentration ranges of TCEP and TCIPP in surface soils were significantly lower than those in the sediment and water, and among the lowest concentrations yet reported in soil data worldwide. This suggests that the manufacturing facility influenced the OPE distribution in different environmental media in different ways. Furthermore, TCEP and TCIPP might have been transported within the water stream from roots into the aboveground plants and then accumulated in tree barks.

Moss (as a reference material) and camphor (Cinnamomum Camphora) leaf, branch bark and bark samples were systematically collected across an urban-rural gradient in Guiyang (SW China) to determine the efficacy of using these bio-indicators to evaluate nitrogen (N) pollution. The tissue N concentrations (0.13%–2.70%) and δ¹⁵N values (−7.5‰ to +9.3‰) of all of these bio-indicators exhibited large spatial variations, as they recorded higher values in urban areas that quickly decreased with distance from the city center; moreover, both soil N concentrations and soil δ¹⁵N values were found no significant differences within each 6 km from the urban to the rural area. This not only suggests that the different N uptake strategies and variety of N responses of these bio-indicators can be reflected by their different susceptibilities to variations in N deposition but also reveals that they are able to indicate that urban N deposition is mostly from traffic and industry (NOₓ-N), whereas rural N deposition is mainly from agriculture (NHₓ-N). Compared to previously collected urban moss and camphor leaf samples, the significantly increased δ¹⁵N values in current urban moss and camphor leaf samples further indicate a greater contribution of NOₓ-N than NHₓ-N to urban N deposition. The feasibility of using the N concentrations and δ¹⁵N values of branch bark and bark as biomarkers of N deposition thus was further confirmed through the comparative use of these bio-indicators. It can be concluded that vascular plant leaves, branch bark and bark can be used as useful biomonitoring tools for evaluating atmospheric N pollution. For further study, quantitative criteria for the practical use of these bio-indicators in response to N deposition should be developed and the differences in the δ¹⁵N values of different plant parts should also be considered, particularly in urban environments that are severely disrupted by atmospheric pollution.

Natural levels of heavy metals (HM) have increased during the industrial era to the point of posing a serious threat to the environment. The use of tree species to record contamination is a well-known practice. The objective of the study was to compare HM levels under different pollution conditions: a) soil pollution due to mining waste; b) atmospheric pollution due to coal-fired power plant emissions. We report significant HM enrichment in Pinus halepensis tissues. Near a burning power plant, Pb content in a tree wood was 2.5-fold higher that in natural areas (no pollution; NP). In mining areas, Cd content was 25-fold higher than NP. The hypothesis that HM contents in tree rings should register pollution is debatable. HM uptake by pines from soil, detoxification mechanisms and resuspended local soil dust is involved in HM contents in wood and bark.

Tree bark is considered as an effective passive sampler for estimating the atmospheric status of pollutants. In this study, we conducted a national scale tree bark sampling campaign across China. Concentration profiles revealed that Eastern China, especially the Jing-Jin-Ji region (including Hebei Province, Beijing and Tianjin) was a hot spot of bark DDT pollution. The enantioselective accumulation of o,p’-DDT was observed in most of the samples and 68% of them showed a preferential depletion of (+)-o,p’-DDT. These results suggest that DDTs in rural bark are likely from combined sources including historical technical DDTs and fresh dicofol usage. The tree bulk DDT levels were found to correlate with soil DDT concentrations, socioeconomy and PM2.5 of the sampling sites. It thus becomes evident that the reemission from soils and subsequent atmospheric deposition were the major pathways leading to the accumulation of DDTs in bark. Based on a previously established bark-air partitioning model, the concentrations of DDTs in the air were estimated from measured concentrations in tree bark, and the results were comparable to those obtained by the use of passive sampling with polyurethane foam (PUF) disks. Our results demonstrate the feasibility of delineating the spatial variations in atmospheric concentration and tracing sources of DDTs by integrating the use of tree bark with enantiomeric analysis.

To compare three biomonitoring techniques for assessing nitrogen (N) pollution in Germany, 326 lichen, 153 moss and 187 bark samples were collected from 16 sites of the national N deposition monitoring network. The analysed ranges of N content of all investigated biomonitors (0.32%–4.69%) and the detected δ15N values (−15.2‰–1.5‰), made it possible to reveal species specific spatial patterns of N concentrations in biota to indicate atmospheric N deposition in Germany. The comparison with measured and modelled N deposition data shows that particularly lichens are able to reflect the local N deposition originating from agriculture.