Consider Phoenix’s so-called “brown cloud.” Generally present on still, cloudless days – particularly in the cooler months* – the brown cloud is composed of a combination of pollutants (including many of the constituent components of PM-10).
In the excellent Arizona Republic series mentioned in the previous installment, Shaun McKinnon writes: “fine-particle-pollution levels spike during winter months, when cold weather combines with high fireplace use to thicken the brown cloud.”
Nightly temperature inversions, which are also more prevalent in the winter months, further contribute to the trapping of pollutants that form the brown cloud. In an inversion, ground level temperatures cool faster than the layer of air above, trapping the cooler air below – along with pollutants from fuel combustion, fireplaces, and other ground level sources. While this is occurring, another mass of cooler air slides down from the mountains surrounding metro Phoenix, which pushes the brown cloud of pollution across the Valley.
According to the National Weather Service, areas located in valleys with surrounding mountains and a prevalence of calm winds and clear skies are more likely to experience strong temperature inversions. Geographically-speaking, this is Phoenix.
In addition to temperature inversions and the brown cloud, Phoenix’s air quality is affected by its desert location. In the Arizona Republic series, McKinnon noted that Maricopa County and Arizona Department of Environmental Quality (ADEQ) officials have urged EPA to recognize that most of Maricopa County’s exceedances of the NAAQS (National Ambient Air Quality Standards) are “exceptional” events.* Their argument being: due to Phoenix’s location in the midst of the Sonoran Desert, there are atypical concentrations of naturally occurring dust from the desert; which concentrations of dust are then whipped up by winds – leading directly to the violations that make Maricopa County a nonattainment area.
(*McKinnon writes: “EPA issued the exceptional-events rule in 2007 to cover air-quality monitor readings that exceed the federal standard as a result of conditions that were beyond the control of local air-quality managers – a wildfire, perhaps, or a major dust storm.”)
The Maricopa County/ADEQ argument fingering natural causes for Phoenix’s air quality violations contains a sliver of merit. As Richard Gilman outlines in a post on Thinking Arizona, a study from Arizona State University (ASU) demonstrates that spikes in large particulate readings are connected to heavy wind events. There is also an additional linkage between dust storms and meteorological patterns: in years with below average rainfall, there is more available dust to be carried away during wind episodes. Further, there is some scientific agreement that soil particles in Maricopa County are more susceptible to becoming airborne than are particles in other similarly arid regions.
Notwithstanding the wind/particulate pollution connection, there are additional factors at play that are less helpful for the Maricopa County/ADEQ story. Primarily: the problematic loose dust particles are especially troublesome in areas where human activity has extensively disturbed the earth.
Turning back to McKinnon once more, we find that scientists at ASU dispute claims tying dust storms to the natural state of affairs in the Sonoran Desert. To quote McKinnon (at length this time):
“In its natural state…moisture and other natural elements help form a crust over parts of the desert floor and plants anchor the soil. Scientists say [that the notorious 2011 dust storms] grew so dense and tall because winds scooped up particulate matter that had been loosened by farmers, construction crews, vehicles on dirt roads and other human activity.”
An ADEQ memo to EPA hints at the primary culprit, boasting that Maricopa County has actually been remarkably successful in meeting standards for PM-10 pollution – in light of the unprecedented growth the County experienced from 1990-2009. During this time, the Valley saw its population nearly double.
Contrary to ADEQ’s reading, it is actually inapposite to blame nature for episodic dust pollution; in the face of the unbridled, lightly planned, growth that has occurred in Maricopa County through recent decades.
The Valley’s sprawling development pattern – with minimal planning on where best to channel future development – has often focused on the overdevelopment of outlying unimproved desert areas, instead of closer-in land. As Sandy Bahr of the Sierra Club’s Grand Canyon Chapter told McKinnon, planners have prioritized the growth advantages to be gained from extending freeways to distant areas of the metroplex – in anticipation of future subdivision development.
Thus, the natural state of the desert is disturbed, and more dust – particularly of the large particle, PM-10 variety – is made available to the milieu. This new dust is then in turn accessible to be kicked up again as new traffic begins to fill-in newly-developed areas. Meanwhile, as McKinnon reports, applicable regulations only account for the front-end dust pollution contributed from the construction activities building new developments – ignoring the “long-term consequences of traffic and lengthy commutes.”