Intermountain cold pool structure, transport, and mixing as revealed by isotopic trace gas and particulate matter concentrations over the Salt Lake Valley

Carbon dioxide (CO2) is a stable constituent of the atmosphere that has no major terrestrial sinks other than atmospheric transport in the absence of photosynthetic activity by plants. In urban atmospheres, CO2 mixing ratios are often elevated above ambient by large local combustion sources. We measured CO2 mixing ratios and the isotopic composition of CO2 at four locations in the Salt Lake Valley of Utah during a persistent cold pool event in the winter of 2004. The results showed a strong influence of atmospheric stability and the capping inversion height on CO2 mixing ratio and revealed that the cold-pool airmass was initially absolutely stable, but evolved into a cloud-topped boundary layer that was well mixed below the capping inversion. Spatial and temporal patterns in the isotopic composition of CO2 and the relationship between particulate concentrations and CO2 mixing ratio, as well as meteorological observations, support this interpretation. These results illustrate that cloud-topped mixed layers may form within deep stable layers and that CO2, which is abundant and easily measured in urban atmosphere, can provide information about wintertime transport and mixing that may be useful for air quality applications.