Barcelona’s Low Emission Zone reduces NO₂ levels

A study estimates a decrease of almost 16% in NO₂ levels between 2020 and 2022, while its effect was smaller in reducing PM10 and PM2.5 particles, which are less directly linked to traffic emissions

Barcelona Institute for Global Health (ISGlobal)

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Barcelona, March 24th, 2026-. According to a study published in the journal Atmospheric Environment: X, the implementation of the low emission zone (LEZ) in the Barcelona metropolitan area significantly reduced concentrations of nitrogen dioxide (NO₂) between 2020 and 2022. However, it had a more limited effect on particulate matter (PM10 and PM2.5), which is more closely related to pollution sources other than traffic. The results indicate that the LEZ contributes to improving air quality, although additional measures are needed to mitigate air pollution. The research was led by the Barcelona Institute for Global Health (ISGlobal), a centre supported by the ”la Caixa” Foundation, in collaboration with the Barcelona Public Health Agency (ASPB), the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) and the Government of Catalonia.

Air pollution poses a major risk to public health, especially in cities, where traffic is one of the main sources. For this reason, many air-quality policies focus on reducing vehicle circulation through temporary restrictions, limitations during high-pollution episodes, or the creation of low emission zones.

Low emission zones (LEZs) aim to improve air quality by restricting the circulation of the most polluting vehicles in specific urban areas. There are currently 363 such zones in Europe and, despite their widespread use, evidence of their effectiveness is still limited, particularly regarding the reduction of fine particles. Barcelona was the first city in Catalonia to implement a permanent LEZ, in 2020. The zone, covering 95 km², banned circulation on weekdays from 07:00 to 20:00 for diesel vehicles EURO 3 or earlier and petrol EURO 2 or earlier, including older motorcycles.

How to assess the effectiveness of the LEZ in a context marked by the pandemic

Assessing the effect of Barcelona’s LEZ was not straightforward, as other factors were at play during the same period (beyond the traffic reduction caused by the LEZ) that influenced pollution levels. These included the gradual renewal of the vehicle fleet, the promotion of sustainable mobility and the COVID-19 pandemic, which reduced mobility and encouraged remote working. In addition, meteorological conditions such as temperature and rainfall also affect the concentration of pollutants in the air.

Pollution measurements were taken between 2015 and 2022 at 29 official monitoring stations in Catalonia, including 11 within the LEZ and 18 in control areas (both nearby and more distant), used as a reference to compare pollution levels before and after the intervention. Stations were classified according to their proximity to traffic, distinguishing between traffic stations and background stations.

“We used an innovative technique, the statistical SC-PI method, which allows us to construct a simulated version of the area without the LEZ, in order to compare what would have happened if the measure had not been implemented. This ‘synthetic zone’ serves as a reference to estimate what levels of NO₂, PM10 and PM2.5 would have been recorded without the measure and to compare them with the actual values,” explains Vanessa N. dos Santos, predoctoral researcher at ISGlobal and UPF and first author of the study. The model also controlled for external factors that could alter the results, such as changes in activity during the pandemic, new international regulations or meteorology, with the aim of isolating the real effect of the LEZ as much as possible.

A 15.8% reduction in NO₂ levels

In the study, the introduction of the LEZ was associated with a decrease of up to 7.6 µg/m³ of NO₂ at traffic stations, equivalent to a 15.8% reduction. By contrast, reductions attributable to the LEZ in PM10 and PM2.5 were small (around 1 µg/m³) and became non-significant with some analytical methods. This may be explained by the fact that a substantial proportion of particulate matter is not emitted directly by vehicles but forms in the atmosphere from pollutant gases originating from different sources, including a contribution from natural sources. This so-called secondary origin, together with natural contributions, makes particles more difficult to reduce through local measures such as the LEZ.

“Our results suggest that the LEZ in the Barcelona metropolitan area is an effective tool to mitigate air pollution, although its effects depend on the type of pollutant,” explains Xavier Basagaña, ISGlobal researcher and coordinator of the study. “Although it clearly reduced NO₂, its impact on particulate matter was limited. To reach the levels recommended by WHO in 2021, additional measures will probably be required, such as further reducing traffic or acting on other emission sources, including agricultural ones,” he notes.

Reference

dos Santos, V. N., Font-Ribera, L., Rico, M., Massagué, J., Nebra, N., Pérez, E., Gómez-Gutiérrez, A., Rivas, I., Querol, X., & Basagaña, X. (2026). Effectiveness of a low emission zone in improving air quality in Barcelona. Atmospheric Environment: X, 29(100428), 100428. https://doi.org/10.1016/j.aeaoa.2026.100428

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Journal

Atmospheric Environment

DOI

10.1016/j.aeaoa.2026.100428 

Method of Research

Data/statistical analysis

Article Title

Effectiveness of a low emission zone in improving air quality in Barcelona

 

Hidden carbon in mangroves: How “black carbon” strengthens coastal climate protection

Maximum Academic Press

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By mapping their spatial distribution and identifying environmental controls, researchers show that plant growth, soil chemistry, and tidal processes jointly regulate these highly stable carbon pools.

BC, produced by incomplete combustion of biomass and fossil fuels, consists of highly condensed carbon structures that can persist for centuries to millennia. Part of it dissolves into water as dissolved black carbon (DBC), a mobile form that transports carbon from land to aquatic systems and influences global carbon cycling. Mangrove ecosystems, known as “blue carbon” habitats, store large amounts of carbon in waterlogged soils that slow decomposition. Despite their major role in coastal carbon storage, the distribution and dynamics of BC in mangrove soils remain poorly understood, limiting accurate assessments of coastal carbon sequestration and transport processes.

A study (DOI: 10.48130/ebp-0026-0001) published in Environmental and Biogeochemical Processes on 28 January 2026 by Junjian Wang’s team, Southern University of Science and Technology, reveals how black carbon dynamics enhance long-term carbon storage and connect terrestrial and marine carbon cycles in mangrove ecosystems.

Using a land–sea transect and a soil-depth profile in Zhangjiang Estuary mangroves, the researchers quantified total organic carbon (TOC), BC, dissolved organic carbon (DOC), and DBC, calculated key stoichiometric ratios, and then combined correlation tests with machine-learning (random forest) and structural equation modeling (SEM) to identify the environmental drivers and pathways behind observed spatial patterns; they also applied BPCA molecular markers to compare the condensation/stability of BC versus DBC. These measurements showed that variability differed strongly among pools: TOC and DOC were highly heterogeneous (CV 42.8% and 53.6%), BC was comparatively uniform (CV 13.0%), and DBC was most variable (CV 63.1%). Mean BC was 1.27 ± 0.16 g/kg (0.95–1.67 g/kg), generally lower than more urban-influenced mangrove/coastal sites but slightly above some regional shelf sediments, while BC/TOC averaged 8.26% ± 3.34% (3.50%–17.41%), a relatively low proportion that is consistent with limited human disturbance and stronger atmospheric/long-range inputs. Along both gradients, BC and DBC concentrations declined seaward and with depth, yet BC/TOC increased, indicating preferential preservation of recalcitrant BC as other organic matter is lost or diluted by fresh surface litter inputs; BC also tracked TOC closely (r = 0.65), consistent with TOC protection/adsorption and shared depositional controls. DBC was more than twice as high in surface soils as in deeper layers, and DBC/DOC remained low (0.36%–3.07%, mean 1.21% ± 0.56%) but rose with DOC (r=0.48), suggesting shared transport plus DOC-facilitated dissolution and microbial processing that releases both DOC and DBC. BPCA results indicated highly condensed aromatic BC (B6CA+B5CA = 78.3% of BC markers) and a higher condensation index in BC than DBC (B6CA/B5CA: 1.44 vs 0.40), supporting selective dissolution of less-condensed BC into DBC. Driver analyses identified plant biomass as the strongest positive control on BC, TN as a key regulator of TOC and DBC (but negatively associated with BC/TOC), and pH and minerals (especially Ca) as major controls on ratios and BC/DBC stability; SEM further showed that landward distance and depth act largely indirectly by reshaping biomass, TN, pH, clay, bulk density, and water content, collectively reinforcing long-term BC persistence in subsoils while constraining DBC production and mobility with depth.

This study highlights black carbon as a stable and essential contributor to mangrove blue carbon storage, linking long-term carbon sequestration with carbon transport to coastal waters. By clarifying the mechanisms controlling BC and DBC dynamics, the findings improve coastal carbon accounting and climate predictions, while emphasizing that conserving vegetation, soil integrity, and hydrological stability can strengthen mangrove-based climate mitigation.

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References

DOI

10.48130/ebp-0026-0001

Original Source URL

https://doi.org/10.48130/ebp-0026-0001

Funding Information

This work was supported by the Natural Science Foundation of China (Grant Nos 42321004, 42192513, and 42477227), the Fundamental and Interdisciplinary Disciplines Breakthrough Plan of the Ministry of Education of China (JYB2025XDXM909), the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2023A1515110245), and the High-Level University Special Fund (Grant No. G03050K001).

About Environmental and Biogeochemical Processes

Environmental and Biogeochemical Processes is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment.

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DOI

10.48130/ebp-0026-0001 

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

Soil black carbon distribution in a mangrove blue carbon ecosystem

 

Turning waste into climate solutions: Meta-analysis reveals how smarter composting cuts emissions and boosts fertilizer quality

Maximum Academic Press

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By analyzing data from more than a thousand experiments, researchers found that targeted composting control measures can simultaneously reduce greenhouse gas emissions, suppress odors, and improve fertilizer quality. The findings demonstrate that composting—often viewed as environmentally friendly but imperfect—can be optimized to retain nutrients while sharply lowering methane, nitrous oxide, and ammonia emissions.

Rapid growth in global consumption is generating unprecedented amounts of organic waste, projected to reach 3.4 billion tons annually by 2050. Much of this waste still ends up in landfills or open dumps, where decomposition releases methane and nitrous oxide—greenhouse gases far more potent than carbon dioxide. Composting has emerged as a key alternative because it converts waste into nutrient-rich fertilizer while reducing landfill pressure. However, poorly managed composting can release large quantities of greenhouse gases and odorous pollutants while losing valuable nutrients, limiting its environmental benefits. Although many mitigation strategies exist, their overall effectiveness has remained unclear due to fragmented and sometimes contradictory experimental results.

A study (DOI: 10.48130/ebp-0025-0022) published in Environmental and Biogeochemical Processes on 27 January 2026 by Dong Liu’s & Fuqiang Yu’s team, Chinese Academy of Sciences, provides evidence-based strategies to make composting more climate-friendly while improving fertilizer quality and resource recycling.

Using a meta-analytic framework, the researchers synthesized results across composting studies and quantified the overall effects of multiple “air-pollution control” interventions (biological, chemical, physical, and mechanical) on process conditions, fertilizer-quality indicators, and gaseous emissions. They calculated response ratios (RR) for core composting variables (temperature, C/N, TOC), maturity/quality metrics (TN, germination index, humic acid), and emissions (CH₄, N₂O, NH₃, CO₂, H₂S, VOCs), and then conducted moderator analyses to test how feedstock type, bulking agents, treatment types (e.g., pressure aeration, biochar), application rate, and composting duration shaped outcomes. The meta-synthesis showed that control measures broadly improved composting performance: temperature increased (RR = 0.48), consistent with intensified and prolonged thermophilic conditions that support pathogen suppression and faster stabilization; simultaneously, C/N decreased (RR = −0.38) and TOC declined (RR = −1.60), indicating more advanced organic matter decomposition and maturation. These process shifts translated into better fertilizer quality, with TN rising sharply (RR = 0.89), GI improving (RR = 0.73) as phytotoxicity fell, and HA increasing (RR = 0.29), reflecting enhanced humification. In parallel, emissions dropped substantially relative to unmanaged composting, including CH₄ (RR = −1.14), N₂O (RR = −1.76), NH₃ (RR = −1.53), CO₂ (RR = −1.51), H₂S (RR = −0.53), and VOCs (RR = −0.54), underscoring dual climate-and-odor benefits. Moderator results highlighted that context often mattered as much as the intervention: feedstock type significantly influenced CH₄ (e.g., sewage sludge showing strong reductions) and CO₂, while bulking agents generally reduced CH₄ (corn straw notably strong) but were less consistently significant across datasets. Treatment type affected CH₄ most clearly, with pressure aeration producing the largest reductions, whereas biochar stood out for suppressing NH₃ and N₂O. Application rate was most consistently linked to lower N₂O, while composting duration showed weaker or mixed relationships across gases. Overall, the analysis suggests optimized controls can improve maturity and nutrient retention while cutting multiple pollutants, but performance depends strongly on feedstock and operating conditions.

This study demonstrates that climate-smart composting can simultaneously reduce emissions and enhance fertilizer quality through strategic feedstock selection, targeted additives, and optimized aeration. By improving nutrient retention and minimizing environmental impacts, optimized composting supports circular bioeconomy principles, promotes soil health, and offers a scalable pathway for climate mitigation in agriculture and waste management systems.

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References

DOI

10.48130/ebp-0025-0022

Original Source URL

https://doi.org/10.48130/ebp-0025-0022

Funding Information

Partial financial support was received from the Caiyun Postdoctoral Project of Yunnan Province, the Yunnan Revitalization Talent Support Program (awarded to Dong Liu), the 'Strategic Priority Research Program' of the Chinese Academy of Sciences (Grant No. XDA26050302), and the Yunnan Technology Innovation Program (awarded to Fuqiang Yu, Grant No. 202205AD160036).

About Environmental and Biogeochemical Processes

Environmental and Biogeochemical Processes is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment.

---

DOI

10.48130/ebp-0025-0022 

Article Title

Synthesis of air pollution patterns and nutrient composition during organic fertilizer production: a meta-analytical study

 

‘Ghost forests’ could be key to understanding coastal resilience to climate change

American Chemical Society

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image: 

Robyn O’Halloran (left) and Ryan Kim (right) collect the rainwater that travels down branches and the trunk of a sweetgum tree to study the rapid changes to coastal forests caused by rising sea levels.

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Credit: Samantha Chittakone

ATLANTA, March 26, 2026 — Rising sea levels are contributing to a phenomenon called “ghost forests,” which are groups of dead and dying standing trees that have been drowned by intruding saltwater. And all along the eastern U.S. where there used to be vibrant green trees, clusters of bare gray trunks appear. Now, researchers say studying how water cycles through ghost forests may hold the key to understanding how coastal forest ecosystems respond to climate change.

Samantha Chittakone, an undergraduate student in environmental engineering at the University of Delaware, will present the research team’s results at the spring meeting of the American Chemical Society (ACS). ACS Spring 2026 is being held March 22-26; it features nearly 11,000 presentations on a range of science topics.

Ghost forests serve as powerful, visible warnings of climate change. Encroaching ocean waters are poisoning salt-intolerant trees, leaving behind eerie skeletal remains. Researchers from the University of Delaware are wading through these surreal landscapes along the mid-Atlantic coastline to better understand the environmental impact of this climate-driven phenomenon.

“Walking through these coastal forests, surrounded by nature, is beautiful,” says Chittakone. “However, it is disheartening to see the healthy trees becoming less prevalent as you approach the shoreline and the effects of rising sea levels become apparent.”

Ghost forests mark hidden shifts in how trees process carbon and nutrients belowground. So, Chittakone, with her supervisors Robyn O’Halloran, Delphis Levia and Yu-Ping Chin, and other colleagues, chose to collect and study sweetgum tree stemflow: the rainwater that travels down branches and the trunk of this common mid-Atlantic coastal tree. They say stemflow is a useful diagnostic tool for understanding rapid changes to forest ecosystems because the water running down the trees acts like a funnel to concentrate water and mobilize nutrients near the roots.

“Stemflow is basically injecting nutrients and really important chemicals into the forest ecosystem so the microbiome there can thrive,” Chin says. He further explains that stemflow color can vary greatly, ranging from intense dark brown, like rich coffee, to pale tan, like weak tea, depending on the bark texture and concentration of nutrients and other substances picked up from the bark.

The researchers collected stemflow from healthy, dead and stressed sweetgum trees to determine if stressed trees could cause cascading impacts in forest ecosystems. For example, changes in stemflow from dead or dying trees may change the soil and consequently impact other organisms such as moss and understory vegetation. When they analyzed their samples, the researchers found significantly less stemflow made it to the forest floor from the dead trees. Another finding was unexpectedly high sugar concentrations in stemflow from dead and stressed trees.

What’s the potential cause and effect of the stemflow changes? “The stemflow’s being absorbed by the dead trees. They’re acting like sponges,” says Chin. “Suddenly you cut off water, nutrients and dissolved organic carbon to the forest floor. Not only is this changing the health of the trees, but it changes the health of the forest floor.” And Levia speculates that the high sugar transport by stemflow from dying trees could alter microbial communities in near-trunk soil.

“Our results signify that the transition from healthy trees to ghost forests changes the magnitude and chemistry of stemflow, leading to pronounced differences in dissolved carbon inputs,” shares Levia. “Further research will better contextualize these changes in stemflow chemistry on the overall cycling of carbon in coastal forests.”

These underground changes ultimately affect how coastal forests store carbon, and understanding these processes could help scientists predict which forests are most vulnerable as sea levels continue to rise.

This research is one piece of the work that the team is doing to investigate stemflow, including how stemflow is impacted by wildfires. “People are beginning to understand the role that stemflow plays in forest floor carbon cycling,” adds Chin. “We’re kind of preaching the gospel, not just to the general community, but our own scientific community.”

“Stemflow is a significant transporter of nutrients and other important chemicals in these coastal forests. It’s something that we should study more and not overlook whenever it comes to carbon cycling, especially in these vulnerable ecosystems,” says Chittakone. 

The research was funded by the U.S. National Science Foundation.

Visit the ACS Spring 2026 program to learn more about this presentation, “Linking stemflow to groundwater in ghost forests: Accessing tracers and impacts of tree health on dissolved organic carbon composition,” and other science presentations.

The quality and quantity of dissolved organic matter changes as a healthy forest transitions into a ghost forest.

Credit

Samantha Chittakone, Delphis Levia and Robyn O’Halloran

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Title
Linking stemflow to groundwater in ghost forests: Accessing tracers and impacts of tree health on dissolved organic carbon composition

Abstract
Coastal margins along the Atlantic seaboard are undergoing profound transformations due to rising air temperatures, sea level rise, and saltwater intrusion. A visible consequence is the emergence of “ghost forests”, characterized by moribund and dead trees that potentially disrupt biogeochemical cycling. Such disruptions would include altering stemflow (the intercepted precipitation that is funneled over tree stems), which serves a critical yet understudied role in carbon cycling. In coastal forests, where the groundwater table is at or near the surface, stemflow may interact with groundwater more directly, possibly introducing previously unaccounted-for organic matter. This study explores the interactions between stemflow and groundwater from moribund and healthy sweetgum trees (Liquidambar styraciflua L.) within a coastal forest experiencing the effects of sea level rise. Groundwater wells were placed near tree trunks and measured water levels for trees with stemflow collection systems, where groundwater is unaffected by stemflow, and trees without collection systems, where stemflow can enter the ground and affect groundwater. Following each rain event, the water table near moribund and healthy tree trunks was consistently higher, suggesting contributions from stemflow. Dissolved organic matter (DOM) fluorescence indices (FI) for stemflow impacted groundwater (median value of 1.46) revealed the presence of allochthonous precursors, while less stemflow influenced groundwater (median of 1.90) revealed DOM that has been microbially processed. Dissolved lignin was also directly measured in these samples for use as a tracer of tree-derived organic matter. The median concentration of total dissolved lignin products in stemflow-influenced groundwater near healthy trees (0.88 µM) was greater than that of unimpacted groundwater (0.54 µM), corroborating results from the FI analysis. By defining the relationship between stemflow and groundwater, the quality and quantity of carbon entering these vulnerable ecosystems can be determined, providing insight into the effects of tree health on carbon cycling.