Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
Robert M. Hardesty
Jose L. Jimenez
Joost A. de Gouw
Tropospheric ozone (O3) is one of the most prevalent air pollutants affecting human health and environment. It is also a greenhouse gas. O3 is a secondary pollutant formed in the atmosphere by the chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOC) in the presence of sunlight, and a major constituent of photochemical smog. The O3 formation is a complex non-linear process which depends on NOx and VOC reactivity. As a result, reduction in emissions of precursor molecules do not always result in decrease in O3 levels. Occurrence of higher O3 on weekends in an urban environment as a response to reductions in NOx emissions is commonly referred as the weekend O3 effect.
The Airborne Multi-Axis Differential Optical Absorption Spectroscopy (AMAX-DOAS) was deployed aboard the NOAA Twin Otter during the California Research at the Nexus of Air Quality and Climate Change (CalNex) and Carbonaceous Aerosols and Radiative Effects Study (CARES) field campaigns in summer 2010 in California to measure vertical profiles and columns of O3 precursor molecules nitrogen dioxide (NO2), formaldehyde (HCHO) and glyoxal (CHOCHO). Measurements from the two campaigns have been used extensively for evaluation of satellite measurements and atmospheric models over California. This work describes the instrument, retrievals of NO2, HCHO and CHOCHO vertical profiles and columns from the measurements and the validation of the NO2 columns. In addition, application of three remote sensing instruments (AMAX-DOAS, NOAA TOPAZ O3 and Doppler wind lidars) to constrain NO2 and Ox (O3 + NO2) production rates from source regions in Bakersfield, CA is presented. I also present analysis of the aircraft and long term surface network measurements of NO2 and O3 showing recent weakening of the weekend O3 effect in California’s South Coast Air Basin. AMAX-DOAS measurements were also used to confirm the sand signal observed by satellite-borne instruments over deserts.
A mobile solar tracker was developed for direct sun DOAS measurements of NO2 and other trace gases. It features an integrated motion compensation system and an imaging feedback loop allowing for autonomous sun tracking at very high precision from a mobile laboratory. Advantages of direct sun measurements over scattered sunlight include high photon flux, simple conversion of slant columns to vertical columns, and no Ring effect, all of which results in the improvement in measurement precision. This allows for mapping of temporal and spatial variability in NO2 columns with unprecedented detail. The tracker can be simultaneously coupled with UV-vis and FTIR spectrometers. Results from its first deployment during Front Range Air Pollution and Photochemistry Experiment (FRAPPE) in Colorado are presented.
Baidar, Sunil, "Airborne Multi-Axis and Direct Sun DOAS: Development and Urban Air Quality Applications" (2015). Chemistry & Biochemistry Graduate Theses & Dissertations. 163.