The Ups(lope) and Downs(lope) of Pollution Transport along the Colorado Front Range

Hourly-averaged surface ozone reached 91.8 ppb at the NOAA/Global Monitoring Division’s (GMD) Tundra Lab (TL) observation site on 31 July 2008, the highest hourly averaged ozone measurement of the year at this site. At 3523 m ASL, this remote, high-elevation site, northwest of the Denver Metropolitan area, is not within an ozone source region. Under what conditions did such a high concentration of ozone reach this, and other, mountain stations?

Fortunately, in the summer of 2008 scientists from NOAA and the University of Colorado collaborated to enhance summertime air quality and meteorological measurements along the Colorado Front Range and in the mountains west of Denver, including the Continental Divide and beyond. Long-term federal- and state-government surface meteorological and air quality monitoring sites were augmented by a NOAA/PSD 10-m meteorological tower on the Continental Divide and a University of Colorado surface ozone measurement array along mountain slopes west of Boulder, CO. The wind measurements from the NOAA/PSD tower station documented flow characteristics over a portion of the Rocky Mountains.

In addition to the augmented surface measurements, researchers obtained ozone profiles measured by a NOAA/CSD airborne ozone and aerosol profiling DIfferential Absorption Lidar (DIAL). Installed in a NOAA Twin Otter aircraft, this lidar obtained ozone profiles from near the surface to ~4500 m MSL that filled measurement gaps in the horizontal and vertical distribution of ozone along the Colorado Front Range, in the mountains west of the Continental Divide, and over the nearby plains. NOAA/PSD deployed two radar wind profilers on the plains and one in the Rocky Mountains. These profilers produced hourly-averaged wind profiles. For this study we also incorporated hourly convective mixing heights calculated from wind profiler Signal-to-Noise ratio, and vertical velocity variances and spectral width.

With the ozone and wind profiles extending from the surface to ~4500 m MSL, we will show how ozone was transported away from and toward the mountainous terrain of the Colorado Front Range on two consecutive days. While both days had similar convective mixing heights, on 30 July 2008 the prevailing winds were downslope (westerly), transporting pollution to the east of Denver, and the next day, 31 July 2008, the afternoon boundary-layer winds on the plains transitioned to upslope (easterly), transporting pollution into the mountains, aided by convective mixing heights greater than 3500 m MSL.

By combining upper-air data (the airborne ozone lidar profiles, radar wind profiler winds, and the convective mixing heights) with the hourly surface measurements (ozone, wind speed, and wind direction) we characterize and contrast the transport of ozone near the Front Range on these two days, and the resulting impacts on surface ozone distribution in the mountains. A unique result of this small measurement campaign was that the airborne ozone lidar documented the spatial extent (both horizontal and vertical) of ozone mole fractions > 80 ppb over the Denver Metropolitan area, as well as over eastern rural Colorado and over mountainous terrain, including Rocky Mountain National Park.