Ever notice that weekends are smoggier than weekdays? Probably not, but Robert Harley has. He's one of a growing number of scientists who are studying why smog levels spike on weekends.

Smoggier weekends are becoming more pervasive in California.

"Smog levels always increase in the summer when it's warmer and there is more sunlight," says Harley, with Berkeley Lab's Environmental Energy Technologies Division (EETD) and the University of California at Berkeley's Department of Civil and Environmental Engineering. "But buried in this annual cycle is variability at other time scales, such as the day of the week."

Harley and his colleagues, Linsey Marr of UC Berkeley and Laurent Vuilleumier and Nancy Brown of EETD, aren't the first scientists to report the weekend effect. It's been noticed in urban areas throughout the world. But Marr and Harley were the first to analyze 20 years of air-monitoring data culled from over 100 sites throughout California, and determine that the weekend effect has spread inland. More specifically, they found that this perplexing phenomenon has become more pervasive in California in recent years, spreading from coastal urban areas such as Los Angeles and San Francisco to Sacramento and the northern San Joaquin Valley.

"Weekday-weekend ozone differences have increased over the last twenty years in California," says Harley.

Smog is formed when pollutants such as nitrogen oxides and volatile organic compounds (VOCs) are emitted into the lower atmosphere and react with sunlight to create ozone. At high enough concentrations, ground-level ozone can cause lung and respiratory problems. In California, the main sources of these smog-forming pollutants are cars, which emit both nitrogen oxides and VOCs (primarily as unburned fuel from tailpipes), and diesel trucks, which emit mainly nitrogen oxides (as well as other pollutants such as soot).

Precisely how this combination of car and diesel truck emissions contributes to the weekend effect remains unclear, but there are several theories. One is based on the fact both cars and trucks are on the road during the week, emitting nitrogen oxides. But on the weekend, when work-related diesel truck traffic diminishes, there's an approximately 40 percent reduction in nitrogen oxide emissions. This reduction actually causes smog levels to increase, most likely because nitrogen oxides can suppress ozone-formation under certain conditions.

A small decrease in nitrogen oxide pollution, due to lighter diesel traffic, may lead to an increase in ozone and smog.

"It's counterintuitive, but reducing emissions from diesel trucks may lead to higher ozone levels on weekends," says Harley. "This could mean, at least in urban areas, that making small reductions in nitrogen oxides may cause increases in ozone formation. But larger reductions, such as up to 90 percent less nitrogen oxide emissions, would probably result in a reduction in smog."

According to another theory, the weekend effect stems from a shift in peak driving hours, from morning and late afternoon commute hours on weekdays to noon on the weekends. At this hour, the sun is brightest and conditions are optimal for ozone formation.

"On the weekend, even though the number of cars on the road is almost the same, you get a shift in the timing of emissions, which can contribute to more smog," says Harley.

However, Harley and his colleagues found that this change in driving times is much less important than the decrease in diesel truck traffic in explaining weekday-weekend differences in air pollution. Ultimately, he believes that weekly cycles in emissions and air quality provide a practical way to test scientists' understanding of how the atmosphere responds to changes in pollution-causing activities.