Asphalt has been found to be an underestimated source of air pollution: a study has been published on its emissions, oxidation, and the formation of ultrafine particles.

The Journal of Hazardous Materials published a paper, "VOC Emissions from Asphalt: Laboratory Oxidation, Ultrafine Particle Formation, and Urban Air Quality Implications." The paper appears in volume 509 of the journal, dated May 15, 2026.

The main idea of the study is that road asphalt is not simply a passive surface, but a source of non-methane volatile organic compounds, which, after chemical oxidation in the atmosphere, can transform into ultrafine secondary organic aerosols. In other words, the problem isn't just that asphalt evaporates something, but that these vapors then become particles capable of degrading urban air quality.

Particularly important is the conclusion that this chemistry operates both day and night, but in different ways. According to the publisher's highlights, during the day, oxidation occurs primarily through the hydroxyl radical, which causes widespread depletion of a mixture of organic compounds, while at night, the nitrate radical plays a significant role, more selectively attacking primarily phenolic compounds.

The work is also important because emissions from asphalt surfaces are still often omitted from standard air quality assessments. Researchers and accompanying materials from Arizona State University explicitly emphasize that if only vehicle emissions are considered and the chemically active road surface is ignored, the urban pollution picture may be incomplete, especially in hot and densely paved areas.

Table 1. Key points about the publication

Parameter	Data
Magazine	Journal of Hazardous Materials
Volume	509
Date	May 15, 2026
Name	VOC emissions from asphalt: Laboratory oxidation, ultrafine particle formation, and urban air quality implications
DOI	10.1016/j.jhazmat.2026.141713
Main theme	Emissions of volatile organic compounds from asphalt and their transformation into ultrafine aerosols
Key takeaway	Asphalt emissions can produce secondary particles both day and night.

Source for table: article metadata and publisher highlights. [1]

How the study was conducted

The study was laboratory-based, but clearly focused on urban atmospheres. The researchers used the THALAMOS atmospheric simulation chamber to study how volatile organic compounds emitted from bitumen and asphalt behave after oxidation and how this affects the formation of secondary organic aerosols. This design allows for not only measuring emissions but also tracking their subsequent chemical fate.

The authors separately analyzed the influence of meteorological factors, primarily humidity. This is important because the behavior of volatile organic compounds in the air depends not only on their composition but also on how easily they transition to the particulate phase, when and how nucleation is initiated, and which oxidation products are retained in the gas and which are released into aerosol.

To analyze the gas and particulate phases, the researchers used multiple methods: GC-EI-MS, SIFT-MS, LC-ESI-QToF-MS/MS, SMPS, and mini-WRAS. This means the study combined chemical identification of molecules and physical measurement of particles, allowing them to assess not only the composition of the original emissions but also the actual formation of the aerosol phase after atmospheric aging.

According to the publisher's highlights, one of the focuses was on low-volatile oxygen-containing and phenolic compounds, as they were the most significant contributors to particle formation. The study thus shifts attention from the overall emissions to their chemical quality: for air, it's not just the amount of organic matter released from asphalt that matters, but which molecules predominate and how actively they participate in subsequent particle formation.

Table 2. How the study was designed

Component	What did the authors do?
Type of work	Laboratory study of atmospheric aging of asphalt emissions
Basic installation	THALAMOS Atmospheric Simulation Chamber
What was studied?	Volatile organic compounds from asphalt and their oxidation
Atmospheric conditions	Day and night scenarios, including the impact of humidity
What was assessed?	Formation of ultrafine secondary organic aerosols
Methods	GC-EI-MS, SIFT-MS, LC-ESI-QToF-MS/MS, SMPS, mini-WRAS

Source for table: institutional description of the study and publisher highlights. [2]

What the results showed

The first major finding is that asphalt surfaces emit non-methane volatile organic compounds, which can indeed transform into ultrafine particles, and do so in both daytime and nighttime conditions. This is important because secondary aerosol formation is often associated primarily with transport, solvents, or biogenic emissions, while road surfaces are perceived as a secondary chemical component. The study demonstrates that this notion is outdated.

The second important result concerns the differences between daytime and nighttime chemistry. During the day, the hydroxyl radical causes widespread oxidation of a mixture of volatile organic compounds, while at night, the nitrate radical acts more selectively, particularly against phenolic compounds. In practical terms, this means that the hazardous chemistry of asphalt doesn't disappear after sunset, but simply changes its mechanism.

The third result is the high potential of individual compounds to form particles. The article's highlights indicate that particle yields from low-volatile asphalt volatile organic compounds can reach 52-130% on a catechol basis. For a scientific audience, this indicates high aerosol-forming efficiency, and for the general reader, it signals that some components of asphalt emissions are particularly susceptible to conversion into new fine pollutants.

Finally, the study demonstrated that humidity is not just a background parameter, but an active process modifier. According to the publication's description, higher humidity delays nucleation and alters the gas-particle distribution of asphalt volatile organic compounds in mixtures. This means that actual asphalt pollution will depend not only on temperature and sunlight but also on weather conditions, and therefore will manifest itself differently in dry and humid cities.

Table 3. Key results

Find	What does it mean
Asphalt emits non-methane volatile organic compounds	The road itself is a source of chemically active aerosol precursors
Particles are produced both day and night.	Asphalt chemistry is not limited to sundials
During the day, the hydroxyl radical is important	Extensive oxidation of a mixture of organic compounds occurs
At night, nitrate radicals are important.	Selective oxidation of phenolic compounds occurs
Particle yield 52-130% on catechol	Individual molecules are particularly effective at forming new aerosols
Humidity delays nucleation	Weather significantly changes the nature of pollution

Source for table: publisher's highlights article. [3]

Why is this important for cities?

The practical implications of this work extend far beyond the laboratory. If the road itself becomes a source of secondary aerosols, then as exhaust emissions decrease, for example through the electrification of transport, the relative role of the surface itself may increase. This doesn't mean that asphalt is already more important than cars, but it does mean that there's a missing element in the purely transport-related logic of urban air quality: the surface chemistry of streets and parking lots.

The study also helps explain why hot cities and areas with large proportions of paved surfaces may experience underreported pollution. Accompanying materials from Arizona State University emphasize that such emissions become particularly noticeable on hot, sunny days, meaning they may be amplified by the urban heat island. In news terms, this shifts the issue of asphalt from an engineering topic to one of urban ecology and public health.

The conclusion regarding the composition of emissions is particularly important. The study suggests that the priority may not be abstract measures to "reduce all evaporations," but a more targeted strategy: reducing the specific proportion of phenolic and low-volatile oxygen-containing compounds in the binder. This means that the path to cleaner air may lie not only through traffic reduction, but also through reformulating road materials.

Finally, this study supports a broader thesis of recent years: non-emission and unobvious sources of pollution are increasingly playing a role in cities. Asphalt, household chemicals, coatings, solvents, and other volatile chemicals are becoming increasingly visible in the air balance as traditional emissions are better regulated. The new paper fits neatly into this shift in the environmental agenda.

Table 4. Possible implications for urban policy

Direction	What the research suggests
Emissions inventory	Asphalt surfaces should be better taken into account in air quality assessments.
Monitoring	Measurements are needed not only near highways, but also in areas with large asphalt coverage.
Materials	It is promising to reduce the proportion of the most reactive organic components in asphalt
Urban studies	Hot and heavily paved areas may be more vulnerable
Environmental strategy	As emissions decline, the relative importance of non-emission sources will increase.

Source for table: interpretation of results from article and accompanying institutional materials. [4]

What is important not to overestimate

Despite the significance of the results, this is still a laboratory study, not direct epidemiological evidence of risk for specific residents of a specific city. The study convincingly demonstrates the chemical mechanism and potential for particle formation, but does not directly answer the question of what concentrations are most often reached in real life on the street, near one's home, or in the workplace.

Furthermore, the article demonstrates the importance of humidity and differences between daytime and nighttime chemistry, meaning that the actual effect will depend on climate, season, illumination, asphalt composition, and pavement age. Simply put, the same mechanism may manifest itself differently in a dry, hot city, a humid coastal town, and a cool, northern metropolitan area.

It's also important to remember that the user is viewing the summary of the news, not the full text of the article. Some methodological and quantitative details in accessible sources are presented through publisher highlights, abstract-style snippets, and institutional summaries. This is sufficient to reliably convey the main idea of the work, but for a highly accurate analysis of the kinetics and a complete set of experimental conditions, the full text of the article itself is required.

But even with this caveat, the article's conclusion remains compelling: asphalt can no longer be viewed simply as a building material. From an atmospheric chemistry perspective, it acts as an active source of organic precursors capable of forming new aerosols and affecting urban air quality. For science, this is already a significant result, and for cities, it's a clear reason to take a closer look not only at what travels on the road but also at what the road itself is made of.

News source: Bej PK, Shariati S, Lostier A, Solaiman S, Chen H, Tomas A, Houzel N, Danjou PE, Coeur C, Romanias MN, Fini EH. VOC emissions from asphalt: Laboratory oxidation, ultrafine particle formation, and urban air quality implications. Journal of Hazardous Materials. 2026;509:141713. DOI: 10.1016/j.jhazmat.2026.141713.