If you've been living in Southern California for more than a couple of years, the memory of how bad the air used to be might bring a tear to your eye. Nowadays? Not so much. Our cars and trucks are still the largest source of the smog-forming chemicals that cause the South Coast airshed to haze over more than we'd like, but air pollution regulations have changed that smog's chemistry. The air doesn't make us cry as often as it used to.
That's according to a team of scientists from the National Oceanic and Atmospheric Administration's Cooperative Institute for Research in Environmental Sciences (CIRES), based at the University of Colorado in Boulder. A CIRES study of half a century's worth of detailed data on air pollution in the greater Los Angeles area has revealed that in addition to just being generally cleaner than it was back in the 1960s, our air is now far less likely to contain a particular pollutant that was the primary reason smog irritated our eyes.
That compound is peroxyacetyl nitrate, a.k.a. PAN. It's what's called a "secondary pollutant": we don't release it directly, at least not usually. Instead, PAN is formed when the chemicals we do release into the atmosphere react with each other in the presence of sunlight.
And along with the major pollutant ozone, which can cause serious respiratory ailments, the amount of PAN in our air has dropped dramatically since 1960. That's due to regulations that limit the amount of volatile organic chemicals and nitrogen oxides we dump into our air, which have caused air pollution to drop since 1960, despite the area's tripling in its numbers of both people and cars in those years. The fewer primary pollutants we emit, the lower the levels of secondary pollutants we eventually have to breathe.
"The emission reductions have 'flipped' some of the chemistry that takes place in the atmosphere," said Ilana Pollack, lead researcher of the CIRES study. "The relevant precursors in the atmosphere now favor chemical pathways that are more likely to produce nitric acid, and less likely to make ozone and peroxyacetyl nitrate (PAN)."
Not that nitric acid in the air is great: it's a major component in acid rain, and acid rain isn't good for ecosystems downwind. It's also not that great for the ecosystem in your lungs. All the more reason to keep on lowering those primary pollutant emissions.
Much of the credit for the reduction since 1960 has to go to the 1981 advent of so-called "three-way" catalytic converters that reduce nitrogen oxide emissions. Aggressive smog certification also played a huge role, and the increasing popularity of hybrid and electric cars in the greater Los Angeles area doesn't hurt.
Tailpipes aren't the only source of Southern California smog, just the largest one. Power plants, industry, natural and introduced vegetation, and everyday activities like starting your charcoal grill with lighter fluid all contribute. Thus the work of reducing emissions continues. The CIRES researchers hope that their study will help regulators continue that work more effectively, by showing how the air's post-emission chemical processes actually work.
"To most people the big deal is that things have got a lot better," Pollack said. "But as scientists we want to know how they have got better."