Why are the clouds brown? Light scattering is one way in which hazes impair visibility; light absorption is another. Increasing the light absorption in a haze makes it appear darker and tends to give it a brown appearance – an effect that owes a lot to the subjective properties of human vision. The human brain tends to perceive the brightest element of a scene as being white (which is why water clouds look white, even when they’re slightly blue). Clouds of a darker, more neutral color will look brown or yellow.
Elemental carbon – which has a chemical composition similar to pencil lead and exists in the atmosphere almost entirely as PM2.5 – is particularly efficient at absorbing light. It accounts for most of the “brown” in a brown cloud. Another contributor is the brown gas nitrogen dioxide, which is formed in the atmosphere from the nitrogen oxides emitted by combustion sources. In fact, nitrogen oxide is the only pollutant gas we can actually see. Other common ones, like carbon monoxide and ozone, are invisible.
Generally speaking, the primary sources of PM2.5 in urban areas are combustion sources, mostly gasoline and diesel engine exhaust. Combustion contributes to particulate pollution both directly, by emitting a variety of particles into the air, and indirectly, by emitting gases that are then oxidized in the atmosphere to create additional particles. Combustion is also the major source of elemental carbon. Across the West, our vehicles are the No. 1 cause of brown clouds.
Scientists can apportion blame for the brown cloud in a particular area by performing chemical mass balance calculations – mathematically computing the combination of emission sources that best account for the pollutants observed in the atmosphere. According to the 1999 MAG report, gasoline engine exhaust accounts for about half of the ambient PM2.5 in Maricopa County, and diesel engine exhaust accounts for about 15 percent. Although soil dust pollution is a huge problem in the Valley, accounting for roughly half of our total PM pollution, its contribution to the visible brown cloud is less significant, because most soil dust particles are too large to efficiently scatter light. Our brown cloud exists not because we live in the desert, but because we drive in it.
Clear Weather Warning
Learning about weather is an important part of a pilot’s training. In the air, you’re not just a weather observer – you’re a participant. Pilots learn early in their careers to distinguish between stable and unstable air; the former makes for a smoother ride, but it also makes it difficult to see.
Obviously, the brown cloud is a lot worse on some days than others. But (with the possible exception of the weekends) that’s not because we’re driving less, or have noticeably changed the amount or type of pollutants we’re pumping into the atmosphere. The one real variable is the weather, and weather is what determines the day-to-day clarity of our air.
As a general rule, brown clouds are most common on calm, cloudless mornings. Here’s why:
On clear nights, with no cloud cover to trap it, the Earth’s surface radiates its residual daytime heat into space. If the air is calm, that layer of air closest to the surface tends to stay there. As the surface cools down, the air next to it does, too.
This creates what’s known as an inversion. Typically, air gets thinner and colder as you gain altitude (which is why ski resorts tend to be in the mountains rather than at sea level). In an inversion, the air near the surface is cooler than the air above it. Because cold air sinks, it tends to stay there – and this stagnant air traps pollutants, creating the familiar brown cloud.
As the day progresses, the sun heats the Earth’s surface and the air next to it. This warm air rises and joins the wind flow aloft, dispersing any pollutants that go with it. For a pilot – and his or her passengers – the updrafts and downdrafts that result from convective heating make for a bumpy ride, but they improve visibility in the cockpit as well as on the ground. Regional weather systems with strong winds can also improve visibility by simply blowing pollutants away (although in Phoenix, they also can whip up some wicked dust storms).
In 1993, the Arizona Department of Environmental Quality (ADEQ) began measuring “light extinction” in Phoenix as a way of quantifying the effects of the brown cloud on visibility. Data from that program indicate that severe hazes occur most frequently from late September through February. There are a couple of reasons why the brown cloud is worse in the fall and winter. Shorter days give less opportunity for convective heating and the mixing action that comes with it. And since more morning commuting is done in the dark, before that convective mixing is active, brown clouds tend to become even denser and more persistent.
The Valley’s geographical features also play a role in the persistence of brown clouds. Night-time airflows tend to run downhill, as air that is cooled more rapidly by radiation on the exposed upper reaches of a slope sinks to replace the warmer air below it. Observations made at Phoenix Sky Harbor International Airport show that in the mornings, winds are typically light and come from the east, the result of cool air draining down the Salt River Valley.