If you had to guess what part of the the U.S. has the very worst air pollution–where winds and topography conspire with fumes from gasoline-chugging vehicles to create an aerial cesspool–places like Los Angeles, Atlanta and as of late, Salt Lake City, would probably pop to mind. The reality may come as a bit of a surprise. According to the Environmental Protection agency, California’s bucolic San Joaquin Valley is “home of the worst air quality in the country.”

Not coincidentally, the San Joaquin Valley is also the most productive agricultural region in the world and the top dairy-producing region in the country. Heavy duty-diesel trucks constantly buzz through the valley, emitting 14 tons of the greenhouse gas ozone daily, and animal feed spews a whopping 25 tons of ozone per day as it ferments, according to a 2010 study. In addition, hot summertime temperatures encourage ground-level ozone to form, according to the San Joaquin Valley Air Pollution Control District. Pollution also streams down from the Bay Area, and the Sierra Nevada Mountains to the east help to trap all of these pollutants near the valley floor. Particulate matter that creates the thick greyish-brown smog hanging over the valley is of paramount concern–it’s been linked to heart disease, childhood asthma and other respiratory conditions.

So when NASA devised a new, five-year air quality study to help fine-tune efforts to accurately measure pollution and greenhouse gases from space, it targeted the San Joaquin Valley. “When you’re trying to understand a problem, you go where the problem is most obvious,” the study’s principal investigator, Jim Crawford, said in an interview. To Crawford, the dirty air over the valley may be important to evaluating how human activities contribute to climate change. “Climate change and air quality are really traced back to the same root in the sense that air quality is the short term effect of human impact and climate change the long term effect,” Crawford said.

In January and February, NASA sent two research planes into the skies above San Joaquin Valley to collect data on air pollution. One plane flew at high altitude over the valley during the daytime, armed with remote sensors, while the second plane cruised up and down the valley, periodically spiraling down toward the ground to compare the pollution at higher and lower altitudes. Weather balloons were used for ground-level measurements as well.

The data NASA collected in the experiment was similar to what satellites can see from space: the presence of ozone, fine particulates, nitrogen dioxide and formaldehyde (precursors to pollution and ozone) and carbon monoxide (which has a median lifetime of a month and can be used to watch the transport of pollution). But satellites are limited in their air-quality-sensing abilities. “The real problem with satellites is that they’re currently not quantitative enough,” Crawford told Surprising Science. “They can show in a coarse sense where things are coming from, but they can’t tell you how much there is.”

Nor can satellites distinguish between pollution at the ground level and what exists higher in the atmosphere. Also, they circle just once a day, and if it isn’t in the early morning, when commuters are busily burning fossil fuels, or in the late afternoon, when emissions have festered and air quality is at its worst, scientists don’t have a clear picture of just how bad pollution can get. Monitoring stations on the ground are likewise limited. They provide scientists with a narrow picture that doesn’t include the air farther above the monitoring station or an understanding of how the air mixes and moves. The research from the NASA study, specifically that collected by the spiraling airplane, fills in these gaps.

Data from the flights will also be used in conjunction with future satellites. “What we’re trying to move toward is a geostationary satellite that will stare at America throughout the day,” Crawford told Surprising Science. Geostationary satellites–which will be able to measure overall levels of pollution–can hover over one position, but like current satellites, researchers need ancillary data from aircraft detailing how pollution travels above the Earth’s surface, like that retrieved from the San Joaquin Valley, to help validate and interpret what satellites see. “The satellite is never going to operate in isolation and the ground station isn’t going to do enough,” Crawford said.

But first, the research will be plugged into air-quality computer models, which will help locate the sources of emissions. Knowing how sources work together to contribute to poor air quality, where pollution is and exactly what levels it’s hitting is a priority for the EPA, which sets air-quality regulations, and the state agencies that enforce them, according to Crawford. The data will inform their strategies on reducing emissions and cleaning the air with minimal impact to the economy and other quality-of-life issues. “Air quality forecasts are great,” Crawford says. “But at some point people will ask, ‘Why aren’t we doing something about it?’ The answer is that we are.” The researchers have conducted similar flights over the Washington, D.C. area and are planning flyovers of Houston and possibly Denver in the years to come.

One thing’s for sure: Data to inform action is sorely needed. In 2011, Sequoia and Kings Canyon National Park, on the eastern edge of the valley, violated the EPA’s national ambient air quality standard a total of 87 days of the year and Fresno exceeded the standard 52 days. Pinpointing exactly where pollution originates and who’s responsible–a goal of the study–will go a long way to clearing the air, so to speak.