Weather patterns and local sources of pollution combined last week to cause the return of Denver's notorious “brown cloud,” the layer of smog over the city that reached its peak in the 1970s and ’80s. And if we're not careful, its return could become a regular event.
Denver’s Air Quality Index (AQI) reached a high of 154 on Wednesday, March 6, exceeding the threshold for unhealthy levels of pollution on the scale set by the Environmental Protection Agency. It’s highly unusual for Denver to have such high levels of smog at this time of year; Wednesday’s AQI was the highest recorded in the month of March since 1983, according to EPA data.
Denver’s air will be in better shape today, though it may not look like it. In an advisory issued yesterday, March 10, state officials said they expect air quality to remain at moderate levels on Monday, but declared an “Action Day for Visibility,” meaning that Denver-area residents are prohibited from indoor burning and asked to limit their driving until at least 4 p.m. today.
Last week’s smog was composed primarily of fine particulate matter, a category of pollutants including dust, soot and other tiny airborne particles that negatively impact human health. Most particle pollution isn’t emitted directly, but forms through chemical reactions in the atmosphere between precursors like nitrogen oxides and sulfur dioxide.
In the winter months, smog is typically caused by an inversion, a weather phenomenon in which cold air near the Earth’s surface is trapped by a warmer layer above it. Last week’s inversion lasted for days, causing the brown cloud to settle and gradually get worse — but even so, such high concentrations of particle pollution are rare. Wednesday was just the ninth day in the past thirty years that Denver's AQI for fine particulate matter reached unhealthy levels, according to EPA data.
So why is the brown cloud able to make a comeback at all? In short: We’re driving too much, and we have too many sources of industrial pollution, including oil and gas facilities.
Like every other major city around the world, Denver is polluted on a daily basis by exhaust from cars, trucks and other gas-powered vehicles and machinery. Anything with an internal combustion engine contributes to smog, and reducing driving rates is among the best ways to achieve cleaner air. Mayor Michael Hanco*ck’s administration has set a goal of reducing the number of commutes made in single-occupant vehicles to 60 percent by the end of next year and to 50 percent by 2030. But the most recent data available shows that figure stuck near 70 percent, and many public-transit initiatives remain unfunded while car-centric projects like the expansion of I-70 have moved forward.
One thing Denver has that many other cities don’t, however, is a major oil and gas patch right in its back yard. Studies have consistently shown that oil and gas facilities along the Front Range are major contributors to the region’s high levels of ozone, responsible for as much pollution as all its cars and trucks put together. In addition to releasing ozone-forming pollutants, oil and gas sites are known emitters of the chemical precursors for fine particulate matter.
Drilling has been inching steadily closer to Denver for years, with active fracking projects under way in communities like Thornton, Commerce City and Aurora. There are nearly a thousand new wells pending approval in Adams, Arapahoe and Boulder counties alone, according to state data. The fate of those applications, as well as any emissions regulations and inspection regimes they might be subject to, hangs in the balance as state legislators consider Senate Bill 181, a package of health and safety reforms proposed by Democrats and bitterly opposed by the oil and gas lobby.
The bad news is that if we manage not to make Denver’s smog problem worse on our own, climate change might do it for us. In the winter months, the world's warming climate could make pollution-trapping temperature inversions stronger, scientists found in a study published last month in the journal Climate Dynamics. And in a report released last year by the American Lung Association, researchers concluded that rising temperatures have already led to increases in high ozone days, threatening millions of people around the world with unhealthy levels of air pollution.
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FAQs
In short: We're driving too much, and we have too many sources of industrial pollution, including oil and gas facilities. Like every other major city around the world, Denver is polluted on a daily basis by exhaust from cars, trucks and other gas-powered vehicles and machinery.
What causes Denver brown cloud? ›
As a result, state agencies in many parts of the country oxygenate automobile fuels during winter months with additives like MTBE in an attempt to reduce the harmful effects of trapped pollution. This trapped pollution is what causes the “brown cloud” effect.
Why is the sky brown in Denver? ›
It's so bad that it has a name – the Brown Cloud. Denver's location at the foot of the Rocky Mountains make it prone to temperature inversions in which warm air traps cooler air near the ground, preventing pollutants from rising into the atmosphere. What is a temperature inversion? Warm air is lighter than cold air.
What causes brown clouds? ›
Atmospheric brown clouds are caused by emissions associated with the combustion of fossil fuels and biomass. The brown colour of the clouds results from the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide.
Why is Denver's air quality so bad? ›
Denver's air quality is primarily challenged by ozone pollution formed from precursor pollutants emitted by motor vehicles (mobile sources) and the oil and gas industry (stationary sources).
What causes a brown haze in the air? ›
Nitrogen dioxide and sulfur dioxide gases from burning of fossil fuels also contribute to the brown cloud. Nitrogen dioxide gas is brown, giving that color to the haze. Chemical reactions in the atmosphere convert these gases to fine particles. Dust, principally from driving on paved roads, is also a contributor.
What causes brown smog? ›
The brown tinge is caused by very small liquid and solid particles scattering the light. Chemicals such as nitrogen oxides, ozone and peroxyacetyl nitrate (PAN) can have harmful effects on plants. These substances can reduce or even stop growth in plants by reducing photosynthesis.
What causes the sky to be brown? ›
The color of the sky is a result of light bouncing off gasses in the atmosphere in what is called Rayleigh scattering, named after physicist Lord Rayleigh. The atoms and molecules comprising the atmospheric gasses are much smaller than the wavelengths of light emitted by the sun.
Why is the air different in Denver? ›
High-Altitude Tips. Denver really is a mile high, but most people don't even notice the altitude difference. The air is just thinner and dryer. In fact, many people with respiratory problems move to Denver for the benefits of the dry air.
Why does the sky look different in Colorado? ›
Ng explained that the blue light that is scattered is reflected into the skies better than other colors, creating the blue skies everyone is familiar with. “Here, the sky looks bluer, due to the fact that we're a little higher up in the atmosphere possibly,” Ng said. Denver sits at 5,280 feet above sea level.
White is how our eyes perceive all wavelengths of sunlight mixed together. When it's about to rain, clouds darken because the water vapor is clumping together into raindrops, leaving larger spaces between drops of water. Less light is reflected. The rain cloud appears black or gray.
What are brown clouds called? ›
The Indian Ocean brown cloud or Asian brown cloud is a layer of air pollution that recurrently covers parts of South Asia, namely the northern Indian Ocean, India, and Pakistan.
When should I be worried about clouds? ›
Wall clouds that rotate are a warning sign of very violent thunderstorms. They can be an indication that a tornado will touch down within minutes or even within an hour. A funnel cloud is a rotating column of air (visible due to condensation) that does not reach the ground.
How to improve Denver air quality? ›
How You Can Help
- Don't Be an SOV (Single Occupant Vehicle) Changing how you commute just one day a week saves you time and money, and improves Denver's air quality!
- When You Stop, Turn Your Engine Off! ...
- Keep Your Car Maintained. ...
- Mow When the Sun is Low. ...
- Conserve Energy at Home. ...
- Burn Wisely. ...
- Follow Simple Steps, Better Air.
Baldwin Park, California
Baldwin Park in San Gabriel Valley, California, was the most polluted city in the 2023 report. The city is well known for its public parks and diverse range of restaurants. At the time of writing, PM2.5 levels in the city are 2.2 times the WHO annual air quality guideline.
Why is Denver hard to breathe? ›
The higher you go, the lower the oxygen levels get. When you breathe, your brain gets fewer oxygen molecules. If this happens suddenly, without gradually getting used to the rise in altitude, fluid may begin to accumulate in the brain and lungs. The Denver, CO elevation can definitely play a role in altitude sickness.
Why does smog look like a brown cloud from a distance? ›
Imparting the clouds' brownish color are nitrogen dioxide gas and aerosols including fly ash, soil particles that are suspended in air, and black carbon, a product of incomplete combustion. These particulates are linked to respiratory problems.
What is the brown haze that develops in sunny cities called? ›
The brown haze that develops in sunny cities is called photochemical smog. Photochemical smog is formed by the reaction of sunlight to pollutants such as hydrocarbons and nitrogen oxides.
Where are atmospheric brown clouds found? ›
Global Distribution of Atmospheric Brown Clouds
Hotspots of ABCs were identified in several regional areas (Ramanathan et al., 2008): East Asia, the Indo-Gangetic plains in South Asia, Southeast Asia, southern Africa, and the Amazon Basin.
What causes air pollution in Colorado? ›
There are many sources of air pollution in Colorado, but the main ones are thought to be from factories, vehicles, buildings, homes and fossil fuel power generators.
