Mixing and Air Mass Exchange Processes within Salt Lake Valley Persistent Cold-Air Pools and their Effect on Particulate Pollution Concentrations

The Salt Lake Valley and other densely populated topographic basins throughout the world suffer from prolonged pollution episodes during wintertime when high pressure systems and subsidence temperature inversions lead to the development of persistent cold air pools (PCAPS). The feedback between meteorological and chemical processes in PCAPs has received increasing attention in recent years.

In early 2017, a small meteorological research project was conducted to document the evolution of Salt Lake Valley PCAPs, augmenting the major chemistry-focused Utah Winter Fine Particulate Study (UWFPS). A suite of meteorological remote sensing and in situ equipment was deployed for the study. Of special interest were the air mass exchange processes in the Salt Lake City Basin, including (a) thermally driven up- and down-canyon diurnal winds from tributary canyons, and (b) air mass exchanges between the Salt Lake Valley and the Great Salt Lake, which can be thermally-forced (lake breeze circulations) or synoptically driven. Selected locations at the mouth of tributary canyons and at the land-lake interface were instrumented with mini-SoDARs, ceilometers, and PM_2.5 and ozone (O₃) monitors. Further, a Doppler Wind LiDAR and ceilometer were co-located with the Utah Department of Air Quality’s Hawthorne Elementary monitoring site at the basin center, collecting profiles of wind, aerosol backscatter, and turbulence. The temperature structure of the basin was observed with a line of inexpensive temperature data loggers deployed along the valley sidewall transect and with 6-hourly radiosondes (National Weather Service 00 and 12 UTC, and additional soundings at 06 and 18 UTC).

The analysis of data collected during three intensive observational periods covering three PCAP and particulate pollution events that will be presented reveal (1) the influx of air with reduced concentrations of PM_2.5 and higher concentrations of O₃ within nocturnal down-canyon flows and (2) occurrences of lake breezes affecting concentrations of both PM_2.5 and O₃ that can either transport cleaner (clean breeze) or more polluted air (dirty lake breeze) into the populated areas. Furthermore, we found that remnants of a PCAP with high PM2.5 concentrations can survive a partial mix-out of the cold pool and can advect into the lowest elevations of the Salt Lake Valley, leading to very pronounced spatial gradients in pollution concentrations.