Early Morning Ventilation of a Gaseous Tracer from a Mountain Valley

Abstract

An important component of a joint Environmental Protection Agency?Department of Energy field experiment in Brush Creek Valley, Colorado in July?August 1982, was an aircraft sampling task to help verify the early morning ventilation of a gaseous tracer from the valley. The aircraft sampling supplemented a surface sampling network in the 650 m deep northwest?southeast oriented valley. A 100 m elevated tracer release 11 km up Brush Creek Valley from its confluence with Roan Creek Valley was sampled using an airborne real-time tracer analyzer supplemented with periodic grab bag samples. Three experiments were conducted in the early morning hours (0400?1000 MST) of 31 July, 4 August, and 6 August. The tracer plume during the drainage wind period was observed to be confined below 400 m in the valley and was transported beyond 25 km down-valley from the source into Roan Creek Valley. Maximum tracer concentrations were observed on the lower east-sidewall during the early morning but moved to the lower west-sidewall following sunrise. Sixty to 90 minutes after sunrise, the tracer was detected by the aircraft's sampling instruments along the upper west-sidewall. Average concentrations were between 100 ad 200 ppt with peaks above 1000 ppt. Average scaled concentrations (?macr;/Q?) were between 0.4 and 0.7 ? 10?6 s m?3. Concentrations over the shady east sidewall remained near background levels until about 2.5 hours after sunrise on the west-sidewall and 2 hours after the development of the up-valley winds. These observations indicate that in draining valleys such as Brush Creek Valley, tracer and pollutant material transported down the valley at night is ventilated from the valley following sunrise by upslope and up-valley winds which develop within convective boundary layers over the sunny sidewall and valley. The results from the aircraft sampling verified the existence of the ventilated tracer in higher regions of the valley sidewall, will aid in the study of the physical transport mechanisms operating during the morning transition period, and should assist in the evaluation of valley dispersion models.