- Author: Kara Manke
Reposted from the UC Berkeley News
The Central Valley's heavy wintertime tule fog – known for snarling traffic and closing schools — has been on the decline over the past 30 years, and falling levels of air pollution are the cause, says a new study by scientists at the University of California, Berkeley.
Tule fog, named for a sedge grass that populates California's wetlands, is a thick ground fog that periodically blankets the Central Valley during the winter months.
To find out why the fog is fading, the researchers analyzed meteorological and air pollution data from the Central Valley reaching back to 1930. They found that while yearly fluctuations in fog frequency could be explained by changes in annual weather patterns, the long-term trends matched those of pollutants in the air.
The results help explain the puzzling decades-long rise and fall in the number of “fog days” affecting the region, which increased 85 percent between 1930 to 1970 and then decreased 76 percent between 1980 to 2016. This up-and-down pattern follows trends in air pollution in the valley, which rose during the first half of the century, when the region was increasingly farmed and industrialized, and then dropped off after the enactment of air pollution regulations in the 1970s.
“That increase and then decrease in fog frequency can't be explained by the rising temperatures due to climate change that we've seen in recent decades, and that's what really motivated our interest in looking at trends in air pollution,” said Ellyn Gray, a graduate student in environmental science, policy and management at UC Berkeley and first author on the paper, which appears online in The Journal of Geophysical Research: Atmospheres. “When we looked at the long-term trends, we found a strong correlation between the trend in fog frequency and the trend in air pollutant emissions.”
The link between air pollution and fog also explains why southern parts of the valley — where higher temperatures should suppress the formation of fog — actually have a higher occurrence of fog than northern parts of the valley.
“We have a lot more fog in the southern part of the valley, which is also where we have the highest air pollution concentrations,” Gray said.
And it makes sense, given what we know about how clouds and fog form, Gray says. Oxides of nitrogen (NOx) react with ammonia to form ammonium nitrate particles, which help trigger water vapor to condense into small fog droplets. Emissions of NOx have declined dramatically since the 1980s, resulting in a decrease in ammonium nitrate aerosols and fog.
“In order to get fog to form, not only do you need the temperature to go down, but there has to be some sort of seed for water to condense around, similar to how you would have a cloud seed in the atmosphere,” Gray said. “Ammonium nitrate happens to make very good fog seeds — water is very attracted to it.”
As a next step, the team plans to take a close look at the association between air pollution, tule fog and traffic safety in the valley.
“When I was growing up in California in the 1970s and early 1980s, tule fog was a major story that we would hear about on the nightly news,” said Allen Goldstein, a professor in the Department of Environmental Science, Policy, and Management, and in the Department of Civil and Environmental Engineering at UC Berkeley and senior author on the paper. “These tule fogs were associated with very damaging multi-vehicle accidents on freeways in the Central Valley resulting from the low visibility. Today, those kind of fog events and associated major accidents are comparatively rare.”
Co-authors of the paper include S. Gilardoni and Maria Cristina Facchini of the Institute of Atmospheric Sciences and Climate in Italy; Dennis Baldocchi of UC Berkeley, and Brian C. McDonald of the University of Colorado, Boulder, and the NOAA Earth System Research Laboratory in Boulder.
This research was supported by a National Science Foundation (NSF) Graduate Fellowship, the California Agricultural Experiment Station and McIntire-Stennis Cooperative Forestry Program of the U.S. Department of Agriculture, and the National Research Council of Italy./span>
- Author: Sarah Nightingale
Reposted from University of California News
When plant matter burns, it releases a complex mixture of gases and aerosols into the atmosphere. In forests subject to air pollution, these emissions may be more toxic than in areas of good air quality, according to a new study by the University of California, Riverside and the U.S. Forest Service's Pacific Southwest Research Station.
The results suggest biomass burning of polluted forest fuels may exacerbate poor air quality—and related health concerns—in some of the world's most heavily polluted areas, among them, the Los Angeles metropolitan area, which is expected to suffer from more wildfires as drought conditions continue.
The study, which was led by Akua Asa-Awuku, a researcher at the Center for Environmental Research and Technology (CE-CERT) at UC Riverside's Bourns College of Engineering, was published online recently (March 2) in the journal Environmental Research Letters.
As people burn fuels—in cars, power plants and factories—nitrogen is released into the atmosphere and absorbed by plants. While essential for plant growth, an over-abundance of this biologically-available nitrogen can result in ‘nitrogen saturation,' a phenomenon previously reported by Forest Service scientists in Riverside. Nitrogen saturation can cause a cascade of adverse effects including a decrease in biodiversity, changes in plant species, soil acidification and water contamination.
In this paper, UCR and Forest Service researchers teamed up to explore a previously unstudied aspect of nitrogen saturation: its effect on the gases and aerosols released during burning of forest fuels from an area experiencing nitrogen saturation.
Polluted sites released up to 30 percent more nitrogen oxides than clean sites
Scientists conducted the study in the San Bernardino Mountains, a 60-mile stretch of federal and private forest land to the east of the Los Angeles metropolitan area. Since the pollution concentration decreases from west to east, as the distance from Los Angeles increases, the forests offered a rare opportunity to compare emissions from wildland fuels subjected to different levels of chronic air pollution. At sites 55 miles apart, the researchers collected recently deposited material from the forest floor, called litter, which is a primary fuel in these forests. Both sites have a similar mixture of conifer tree species, and, at the time of collection, had experienced similar temperatures and rainfall.
As shown in previous studies, the litter from the polluted site, which had endured high levels of atmospheric nitrogen oxides and ozone, had higher nitrogen content than litter from the clean site. The researchers then burned the litter in controlled lab tests, collected the emissions and analyzed them. The results showed:
- Fuel from the polluted site released more nitrogen oxides, which contribute to the formation of smog and ozone. In some cases, polluted fuels released 30 percent more nitrogen oxides than fuels from the clean site.
- Polluted fuels released more small fine particles (PM<2.5), which are known cause of respiratory health problems.
- The composition of the particles from polluted regions were different; they were less likely to evaporate but underwent similar atmospheric processing as emissions from clean fuels exposed to sunlight.
Implications for agencies in charge of controlled burns
Asa-Awuku, an associate professor of chemical and environmental engineering at the CE-CERT, said agencies that oversee prescribed burns should consider these findings when they predict the likely impact of prescribed burning of forest fuels in areas subjected to chronic air pollution.
“The environmental impact of prescribed burns has historically been based on data from clean fuels in areas of good air quality, so we have likely been under-predicting the impact of biomass emissions in polluted areas,” Asa-Awuku said.
She added that the study supports growing evidence that humans need to reduce our pollutant footprint associated with burning fossil fuels.
“This study, and specifically the concern that biomass grown and burned in polluted areas is potentially more toxic to human health, is additional evidence that human activities have consequences not yet explored and therefore not understood,” she said.
The research was conducted by Asa-Awuku and Michael Giordano, at UCR's CE-CERT, and Research Forester David Weise and Physical Science Technician Joey Chong from the Forest Service's Pacific Southwest Research Station.
Headquartered in Albany, Calif., the Pacific Southwest Research Station develops and communicates science needed to sustain forest ecosystems and other benefits to society. It has research facilities in California, Hawaii and the U.S.–affiliated Pacific Islands. For more information, visit www.fs.fed.us/psw/./h3>/h3>