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You are here: Home / SARP / SARP 2015 / 2015 Student Presentations / Whole Air Sampling / Comparing Vertical Profiles of Trace Gases to Weather Model Planetary Boundary Layer Height

Comparing Vertical Profiles of Trace Gases to Weather Model Planetary Boundary Layer Height

Thomas Mazzetti, Florida State University
00:12:01
2015

Abstract: Trace gases, including criteria pollutants and their chemical precursors, can be horizontally confined by mountains and vertically confined by the planetary boundary layer (PBL); this is common in the LA basin. The PBL is the turbulent, well-mixed lowest layer of the atmosphere. PBL depth varies and ranges from approximately 1-3 km, thickest in the afternoon. Gases emitted from Earth’s surface are trapped in the PBL because the free troposphere above is stably stratified and resists vertical motion. Knowing PBL height is necessary for models to calculate the volume which pollutants will dilute into. Using UC Irvine’s whole air sampler onboard the NASA DC-8, samples were collected during the Student Airborne Research Program. High-frequency discrete whole air sampling was performed when the DC-8 executed vertical profiling maneuvers (missed approaches, spirals, and other low-level objectives). Sample analysis was performed at UC Irvine which quantified volatile organic compounds (VOCs) with gas chromatography. Vertical profiles of selected hydrocarbons and other anthropogenic trace gases were used to calculate local height of the PBL. PBL height was indicated by abrupt decrease in concentration with altitude. These calculated PBL heights were compared to outputs from the NOAA North American Mesoscale Forecast System (NAM). The precision of the trace gas PBL height is dependent on the vertical spacing of samples. In regions where ground elevation rapidly changes, the NAM failed to accurately compute the PBL height, and large discrepancies between the model and observations were observed. Chemical tracers are a robust way to identify the PBL height even in regions where numerical models struggle. Trace gas PBL height can be used to make parameterization of numerical model PBL calculations more accurate.