The Influence of Synoptic Meteorology on Convective Boundary Layer Characteristics and the Observed Chemical Response during PROPHET

This thesis investigated 1) the observed changes in trace gas mixing ratios driven by large-scale synoptic meteorology and source region, and 2) the impact of synoptic meteorology on the physical properties of the atmospheric boundary layer and the observed chemical response to these changes. The atmospheric boundary layer is the portion of the Earth’s atmosphere that is directly influenced by physical characteristics of the surface (ie. terrain, bodies of water), surface processes (ie. heating/cooling, chemical emissions), and large-scale atmospheric stability. The large-scale origin of an airmass directly impacts the chemical composition and physical meteorological properties (ie. energy partitioning, stability) of a particular airmass. The physical meteorological properties of an airmass influence the evolution of the atmospheric boundary layer, which in turn has a direct impact on surface trace gas mixing ratio measurements. Therefore, continuous measurements of atmospheric boundary layer, surface meteorology, and airmass chemical composition were obtained at the University of Michigan Biological Station (UMBS), a rural, forested site located at the northern tip of Michigan’s lower peninsula, during atmospheric chemistry intensives in July and August of 2000 and 2001. The UMBS experiences a variety of transport regimes; however, the UMBS is influenced by two dominant summertime flow regimes in which the chemical and physical meteorological properties of the airmass are significantly different. A relatively clean, dry Canadian continental-polar airmass is located to the north, while a more polluted, maritime-subtropical airmass sits to the south of the region. The mean peak ozone (O3) mixing ratio within an airmass with northerly origin was 34.1 ppbv, while an airmass with southerly origin had a mean peak O3 mixing ratio of 58.5 ppbv. These two dominant transport regimes comprised sixty-three percent of the total hours that made up the PROPHET intensive measurement campaign. More specifically, northerly flow was observed forty-three percent of the time, while southerly flow was observed twentypercent of the time.

Hourly mixing height estimates were obtained using an automated mixing height algorithm (AMHA), based on Doppler wind profiler measurements, while O3 mixing ratio measurements were made on a 30-m tower within the mixed hardwood forest. The meteorological (ie. mixing height estimates, stability, and energy flux measurements) and chemical analyses were subjected to an airmass categorization in order to preserve the physical meteorological and chemical differences associated with different airmass origins. The mean maximum afternoon mixing height derived during PROPHET was 1.18 km; however, there is a statistically significant difference in this particular measurement when separated according to airmass origin. The mean maximum afternoon mixing height estimate observed while under an airmass with northerly origin is 275-m greater than the estimate observed under an airmass with southerly origin. In addition, hourly afternoon mixing height estimates are on average 250-m higher under northerly flow than southerly. Mean hourly mixing height growth rates (1200Z-1700z) are rapid under the influence of an airmass with northerly origin and correspond to a significantly slower increase in O3 mixing ratios, as a weaker O3 mixing ratio gradient exists between the residual layer and surface. Conversely, mean hourly mixing height growth rates (1200Z-1700Z) are slow under the influence of an airmass with southerly origin and correspond with a steep increase in surface O3, as a strong O3 mixing ratio gradient exists between residual layer air and surface air. Atmospheric stability and partitioning of energy at the surface are the main factors responsible for the differences in hourly mixing height estimates at the PROPHET site.

The goal of this research was to assess the impact that airmass origin has on the physical meteorological properties controlling mixing characteristics and the observed surface O3 mixing ratio response to different mixing characteristics. A conceptual model was developed, which integrates large-scale synoptic meteorology (ie. transport), local mixing characteristics (ie. mixing height, winds), and surface meteorological and chemical measurements, to explain the meteorological and chemical differences observed under opposing airmass regimes.