Nighttime chemistry in the Houston urban plume

A late afternoon polluted air parcel transported from the Houston metropolitan area was monitored by an instrumented aircraft throughout the night of 21–22 July, 2005. Sampling was conducted during three flight segments over several downwind areas that were identified by a controllable meteorological balloon released from the Houston area at sundown. Samples were taken for approximately 2 h over each area. Using carbon monoxide as a tracer of the urban plume, it was revealed that the dilution inside the plume was relatively small. Ozone levels of up to 120 ppb were found in the plume at the furthest downwind distance, some 250 km northwest of Houston, with plume transport in the direction of the Dallas metropolitan area. The data further suggest that the nighttime conversion of NOx to NOz was very rapid, with complete (∼100%) conversion by the end of the night. At two locations the urban plume mixed with fresh emissions from power plants. At these sampling points ∼50% of the NOy had already been converted to NOz, thus indicating very rapid oxidation at night.