The infamous "brown cloud" looked thick and ugly hanging over the Valley on Monday, yet its appearance is an imperfect measure of a complex pollution problem.
Experts said weather conditions — including a temperature inversion — may mean the cloud will stick around today. Air-quality levels on the ground, however, are expected to stay in the healthy range all week.
"It looks worse, perhaps, than it is," said Patrick Gibbons, spokesman for the Arizona Department of Environmental Quality. "We have not issued an air-quality advisory, yet it appears pretty hazy out there, pretty dirty."
The cloud can mislead in the other direction, too. Sometimes, "what you’re seeing may not be as bad as what you’re breathing," Gibbons said.
Then there are the days when "what you see is what you breathe," he said.
The brown cloud is composed of fine, floating particles much smaller than the width of a human hair that bond with water molecules and scatter light. Most of it comes from vehicle tailpipes. It’s called PM-2.5 — particulate matter that is less than 2.5 millionths of a meter in size.
Scientists believe those microscopic bits cause lung cancer and other respiratory diseases.
But since Valley air-quality officials began measuring PM-2.5 in earnest about two years ago, they have recorded only one violation of the federal health standard for the pollutant — in 2002 at a west Phoenix monitoring station.
Dirtier-looking air probably means more PM-2.5 at ground level than on days with higher visibility, but it is not enough to cause health issues for anyone but the most vulnerable members of the community, Gibbons said.
A bigger problem are the somewhat larger particles known as PM-10. Also very harsh on the lungs, PM-10 has proved much tougher to reduce than carbon monoxide or ozone pollution, which were dealt with by using cleaner-burning fuels and vehicle engines.
The larger particulates are mostly dust grains thrown into the air each day by the wheels of hundreds of thousands of vehicles. Construction sites and unpaved roads also play a role in the formation of PM-10.
"That pollutant is our worse one," said Lindy Bauer, environmental director for the Maricopa Association of Governments. "We’re still violating that standard."
PM-10 violations can occur on windy days, though, when the brown cloud appears diminished, officials said.
Even if the brown cloud itself is not a health hazard, the state needs to work
hard to erase it, said Joy Herr-Cardillo, a lawyer with the Arizona Center for Law in the Public Interest.
"We all live here in Arizona because of beautiful vistas," Herr-Cardillo said. "That is something worthy of protection, especially because our state relies so much on tourism."
Gibbons said the state is about to launch its "urban visibility index" project, which will include a network of cameras and instruments designed to figure out whether the brown cloud is fading away.
"We hope the good days get better," Gibbons said.