Tuesday, 15 January 2002: 8:30 AM
The Influence of Boundary Layer Structure on Rural Ozone Observed during PROPHET 2001 and 2001 Summer Intensives
The Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) has conducted atmospheric chemistry intensives at the University of Michigan Biological Station (UMBS) during July and August of 1998, 2000 and 2001. Observations of ozone and various precursors are made on a 30 meter tower within a mixed hardwoood forest. The site is located near the tip of the Michigan Lower Peninsula. The northern half of the lower peninsula is relatively rural and sits at the northern edge of a summer gradient in ozone; it experiences a range of air-mass types from continental-polar to maritime-subtropical. Published results, based on analyses of back trajectories and 1998 chemical data, have shown the influence of air mass origin on trace gas mixing ratios. In this paper we will demonstrate the relationship between local boundary layer dynamics and the corresponding chemistry. For the past two summers, 2000 and 2001, we have deployed an NCAR (National Center for Atmospheric Research) integrated sounding system consisting of a 915MHz wind profiler, a radio acoustic sounder, and rawinsonde profiles and tethersonde profiles. These systems provide detailed information on boundary layer structure. Episodes of elevated ozone concentrations (>80ppb) have been observed several times during the PROPHET campaign, typically under southerly transport on the back of a surface high pressure system. Considering the number of trajectories with southerly origin and their average duration, we find that southerly transport occurs ~24% of the time at this location. However, not all of these transport events result in extreme ozone levels. In this paper we will demonstrate the role of the local boundary in controlling fluctuations in ozone level observed at the site. The timing of episodes is an important factor; nocturnal boundary layers are typically 200 meters or less, with evidence of an elevated residual daytime boundary layer apparent in the reflectivity data. Intensive sounding profiles have been made for a limted number of events using a tethered rawinsonde system, allowing more detailed information around the transitions of sunrise and sunset when the nocturnal boundary layer breaks down and forms respectively. Using the radar reflectivity we have calculated a range-corrected mixed layer depth in meters. The Richardson number has been extracted from the combined wind profiler and temperature data to provide a measure of stability and the state of turbulence in the lowest layers. These parameters will be used to explain variance in the observed ozone concentrations.