2021 News & Events

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

21 May 2021
adapted from the story by NOAA PSL Communications

Denver Front Range
Denver Front Range. Photo: James St. John, Flickr

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 the Front Range Air Quality study (FRAQ) in 2008. New research led by NOAA and CIRES scientists at the Physical Sciences Laboratory (PSL) and 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, L.S., C.J. Senff, R.J. Alvarez II, R.M. Banta, L. Bianco, D. Helmig, and A.B. White, Spatial and temporal variability of ozone along the Colorado Front Range occurring over two days with contrasting wind flow, Elementa: Science of the Anthropocene, doi:10.1525/elementa.2020.00146, 2021.


Transport of pollution into pristine wilderness areas is of concern for both federal and state agencies. Assessing such transport in complex terrain is a challenge when relying solely on data from standard federal or state air quality monitoring networks because of the sparsity of network monitors beyond urban areas. During the Front Range air quality study, conducted in the summer of 2008 in the vicinity of Denver, CO, research-grade surface air quality data, vertical wind profiles and mixing heights obtained by radar wind profilers, and ozone profile data obtained by an airborne ozone differential absorption lidar augmented the local regulatory monitoring networks. Measurements from this study were taken on 2 successive days at the end of July 2008. On the first day, the prevailing winds were downslope westerly, advecting pollution to the east of the Front Range metropolitan areas. On this day, chemistry measurements at the mountain and foothills surface stations showed seasonal background ozone levels of approximately 55–68 ppbv (nmol mol–1 by volume). The next day, upslope winds prevailed, advecting pollution from the Plains into the Rocky Mountains and across the Continental Divide. Mountain stations measured ozone values greater than 90 ppbv, comparable to, or greater than, nearby urban measurements. The measurements show the progression of the ozone-enriched air into the mountains and tie the westward intrusion into high-elevation mountain sites to the growth of the afternoon boundary layer. Thus, under deep upslope flow conditions, ozone-enriched air can be advected into wilderness areas of the Rocky Mountains. Our findings highlight a process that is likely to be an important ozone transport mechanism in mountainous terrain adjacent to ozone source areas when the right circumstances come together, namely a deep layer of light winds toward a mountain barrier coincident with a deep regional boundary layer.