As air quality improves, the invisible chemistry happening in the air around us is changing. Skies should clear up as emissions drop, but recent results suggested that declining nitrogen oxides can create an environment where airborne carbon-containing compounds more easily convert into small particles that harm human health.

Regulators can now breathe easier. A University of Washington-led study, published in March in the Proceedings of the National Academy of Sciences, provides a fuller picture of the relationship between nitrogen oxides — the tailpipe-generated particles at the center of the Volkswagen scandal, also known as NOx, — and PM2.5, the microscopic particles that can lodge in lungs.

Results show that declining NOx due to tighter standards does ultimately lead to cleaner air — it just might take longer.

A key finding is how the concentration of NOx affects the formation of PM2.5, found in smog, by changing the chemistry of the hydrocarbon vapors that transform into the particles less than 2.5 microns across, or about 3 percent the width of a human hair.

“We found that there are two different regimes of PM2.5 formation,” said first author Joel Thornton, a UW professor of atmospheric sciences. “One where adding NOx enhances PM2.5, and one where adding NOx suppresses PM2.5.”

The findings could help explain why air quality appears to have stagnated in recent years over some parts of North America, even as emissions of all types have been dropping. Regulators are concerned because air pollution is a leading human health hazard, especially among children, the elderly and those with respiratory or heart problems.