Gas-Phase Organic Carbon and Tropospheric Pollution: Sources, Emissions, and Implications for Air Quality
The troposphere is comprised of a complex mixture of molecules with trace amounts of over 10,000 organic compounds in the gas and particle phase. These compounds play important roles in the chemistry of the troposphere and the formation of detrimental secondary air pollution that impacts human health, climate, and the environment. This dissertation describes the development of novel ambient measurement techniques and statistical modeling methods, and their use to provide in-depth characterization of emissions from several prominent anthropogenic and biogenic sources. These results are used to assess the potential of the studied sources to form secondary organic aerosol (SOA) and tropospheric ozone. The objectives of this dissertation are accomplished using data from 6 measurement campaigns in the state of California, which includes some of the worst regions for air quality in the United States.
An automated in situ instrument with a gas chromatograph coupled to a mass spectrometer and a flame ionization detector was modified to measure a broad range of gas-phase organic compounds. This included a mixture of traditionally measured chemical species and numerous compounds for which no previous in situ measurements exist and have otherwise been relatively unstudied. Many of these compounds were in the intermediate-volatility range (i.e. IVOCs), which have previously been hypothesized to have a considerable effect on the formation of SOA. The rest of the compounds measured were in the volatile organic compound (VOC) range, but many of the least volatile compounds in this range had not been sufficiently studied, such as C10 aromatic hydrocarbons from motor vehicles.
Source receptor modeling techniques with chemical mass balancing were developed in several forms and used in this dissertation to assess emissions of gasoline exhaust, non-tailpipe gasoline, diesel exhaust, and unrefined petroleum gas emissions from petroleum operations. A statistical analysis using meteorological data (Flexpart) and ambient ground site measurements was developed to examine the spatial distribution of emissions in a region and is used in this dissertation to examine emissions from several point and area sources. Techniques are also developed to estimate bulk SOA yields of the complex mixtures in gasoline and diesel emissions.
A comparison of gas-phase organic carbon emissions from gasoline and diesel vehicles concludes that diesel emissions form 15 times more SOA than gasoline exhaust per liter of fuel burned, but given the extensive use of gasoline and varied fuel use depending on region, diesel is responsible for 65-90% of vehicular SOA. The non-tailpipe gasoline and unrefined petroleum gas sources examined in this dissertation are significant in abundance, but are comprised largely of relatively small hydrocarbons and thus form negligible amounts of SOA. Biogenic emissions of terpenoid and benzenoid compounds were measured in this work and are highly reactive. In regions with extensive agricultural operations, such as California's San Joaquin Valley, summertime emissions of biogenic compounds have the potential to form a similar amount of SOA and ozone as motor vehicle emissions. Additionally, seasonal emission events, such as flowering, produce an order of magnitude increase in emissions of ozone and SOA precursors. In all, the advancements in source characterization and secondary pollution formation potential in this dissertation provide important insights for future studies, models, and air pollution control policies.