Nocturnal Vertical Profiles of Nitrogen Oxides and Related Species from a 300m Tower at Erie, Colorado

The 300-meter tower in Erie, Colorado, is a platform that provides a unique opportunity to study near-surface vertical distributions of trace gases. The tower has an external, movable carriage with a payload sufficient for multiple instruments and a one-way transit time of 9 minutes. An air quality study took place in July and August 2007. One focus of the study was on nighttime nitrogen oxide chemistry. Concentrations of NO3, N2O5 and NO2 were measured by pulsed cavity ringdown spectroscopy. In addition, measurements of ozone and NO concentrations (chemiluminescence detectors), volatile organic compounds (PTRMS), aerosol size distributions and surface areas were performed. Preliminary analysis of the nighttime data shows distinctive layered structures of nitrogen oxides and VOCs with typical layer thicknesses of 40 to 100m. Trace gas concentrations were highly variable between different layers with peak values of e.g. NO3 mixing ratios of a few hundred pptv and N2O5 mixing ratios of a few ppbv within the altitude range of the tower. In general, a depletion of nitrogen oxide radicals could be observed within the surface layer which was typically less than 20m deep along with strongly enhanced mixing ratios of aromatic VOCs. The results suggest that a representative overview of nighttime nitrogen oxide concentrations requires measurements of the vertical distributions of trace gases throughout and above the shallow nocturnal boundary layer.