Heat rising

Sun-heated streets can lead to air pollution spikes — study

The asphalt covering our roads and roofs is releasing potentially harmful particles into the atmosphere. Switching to green space can help.

Evening city panorama, vector, sunset. Flat anime style

On a sweltering day, parking lots and city streets are not ideal locations to find yourself. Beating Sun on blacktop can make you feel encased in heat, as fumes rise from below.

Beyond the discomfort, these areas also pose a risk to human health.

The stuff that makes up roads and rooftops, asphalt, also releases potentially harmful substances into the air, new findings show. The damage is spurred by hot days and exposure to the Sun.

A study published Wednesday in the journal Science Advances reveals the dangerous side-effect of this surprisingly ubiquitous substance. Researchers discovered that asphalt gives off a complex mix of organic compounds, which includes dangerous pollutants, especially on hot and sunny days.

Asphalt produces something called secondary organic aerosol (SOA), which contributes to particulate matter, a type of air pollution. Specifically, it creates PM2.5, which refers to particles smaller than 2.5 micrometers in diameter.

PM2.5 is tiny. (For reference, a strand of human hair has a diameter of about 50 micrometers.) That means the substance can easily get into the human respiratory system. PM2.5 is associated with heart and lung problems, acute and chronic bronchitis, asthma attacks, and even premature death.

To understand how asphalt contributes to this dangerous air pollutant, researchers heated the substance to different temperatures and exposed it to varied levels of solar radiation. They found that temperature and other environmental conditions are strongly related to the potency of the chemicals that asphalt products let off.

Solar radiation in particular — even at a moderate level — had a big effect. Moderate sun caused emissions to jump by up to 300 percent.

The findings can help determine the effects of asphalt on air pollution in US cities like Los Angeles, which was used as a case study in the new research. The chemical properties of asphalt make them comparable to emissions from vehicles in LA, the authors say, making analyzing their potential risks key to environmental and public health.

Asphalt is also strikingly common across the United States. Paved surfaces cover 45 percent of US city surfaces, while roof make up 20 percent, the study authors say.

Greening up cities can help — Despite the risks associated with asphalt, its expulsion of chemicals is rarely taken into account when considering air quality plans in cities, researchers say.

The new findings could help to inform policies surrounding air pollution. The research adds to previous studies have looked at what the replace paved areas with. Several data set point to the same conclusion: Having more green spaces in cities is a boon to public health.

In a study published in 2019, scientists designed a "vegetation index" to show how living near parks and other green spaces can add years to people's lives. As that index increases, with proximity to nature, premature mortality dropped, according to data from the United States, Canada, Spain, Italy, Australia, Switzerland, and China.

A separate finding in 2019 showed big mental health benefits from spending time in nature. In particular, being near spaces with a significant tree canopy — compared to grass or low shrubs — improved people's health.

Taken together, research surrounding asphalt and green spaces makes a case for swapping out portions of pavement with public parks. Reconsidering city design plans, in turn, could benefit both human health and the natural environment.

Abstract: Asphalt-based materials are abundant and a major nontraditional source of reactive organic compounds in urban areas, but their emissions are essentially absent from inventories. At typical temperature and solar conditions simulating different life cycle stages (i.e., storage, paving, and use), common road and roofing asphalts produced complex mixtures of organic compounds, including hazardous pollutants. Chemically speciated emission factors using high-resolution mass spectrometry reveal considerable oxygen and reduced sulfur content and the predominance of aromatic (~30%) and intermediate/semivolatile organic compounds (~85%), which together produce high overall secondary organic aerosol (SOA) yields. Emissions rose markedly with moderate solar exposure (e.g., 300% for road asphalt) with greater SOA yields and sustained SOA production. On urban scales, annual estimates of asphalt-related SOA precursor emissions exceed those from motor vehicles and substantially increase existing estimates from noncombustion sources. Yet, their emissions and impacts will be concentrated during the hottest, sunniest periods with greater photochemical activity and SOA production.
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