Study suggests that tall mountain ranges may not always act as natural barriers against pollutant transport

Denver Front Range, Credit: James St. John (Flickr, CC BY 2.0)

Scientists curious to find out how far into the Rocky Mountains pollution (tropospheric ozone) could be transported from the Denver Metro area, set out to find answers with a 2008 field campaign called the Front Range Air Quality study (FRAQ). New research led by NOAA and CIRES scientists at the Physical Sciences Laboratory (PSL) and Chemical Sciences Laboratory (CSL), combined FRAQ air quality observations from state, university, and federal agencies (including measurements taken by NOAA instruments), to assess pollution transport along Colorado’s Front Range.

The researchers analyzed two days—one with upslope (easterly) winds and the other with downslope (westerly) winds. For the upslope day, they found high ozone levels, greater than 90 parts per billion by volume (ppbv), were transported from the Denver Metro area into Rocky Mountain National Park and beyond. In fact, airborne measurements documented the transport of ozone across the Continental Divide. Their findings, published in the journal Elementa: Science of the Anthropocene, highlight a process likely to be an important ozone transport mechanism in areas where mountainous terrain is adjacent to pollution sources, when the right circumstances come together. In the case of the Front Range, those circumstances included a very deep afternoon boundary layer in conjunction with deep easterly winds that exceeded mountain-top height. The results suggest that under these conditions, even tall mountain ranges may not act as natural barriers against pollutant transport.

The study shows unequivocally that a deep layer of ozone with values greater than 80 ppbv, and at times up to 100 ppbv, existed over the plains, adjacent to the Rocky Mountains, on both case study days. The transport of ozone-rich air into wilderness areas with vegetation sensitive to ozone was dependent on both the depth of the afternoon boundary layer and easterly winds. The high ozone also was transported to mountain communities that do not regularly monitor ozone.

Measurements taken by NOAA instruments provided crucial information about the transport of ozone pollution from the Denver Metro area into the Rocky Mountains. The study results show that vertical monitoring of pollutants and winds are critical for understanding air pollution transport, and that high ozone air is transported into regions that are not monitored by any regulatory agency. Society may benefit from more geographically extensive air quality monitoring.

Darby, Lisa S., Christoph J. Senff, Raul J. Alvarez II , Robert M. Banta, Laura Bianco, Detlev Helmig, and Allen B. White (May 2021): Spatial and temporal variability of ozone along the Colorado Front Range occurring over 2 days with contrasting wind flow. Elementa Sci. Anthrop., (PSL Authors Bolded)