Biogenic volatile organic compounds (BVOCs) emitted from terrestrial vegetation exert a considerable influence on the earth system through their roles in the global carbon cycle and shaping atmospheric composition. Globally, BVOC emissions are approximately 10 times higher than anthropogenic VOC (AVOC) emissions, thus acting as a highly important source for secondary organic aerosol (SOA) and tropospheric ozone (O3) formation, both of which degrade air quality in urban areas. Estimating the impacts of BVOCs on secondary pollutant formation in urban areas holds significant importance due to the detrimental effects of both SOA and O3 on human health and ecosystems. However, quantifying the net effect of BVOCs on urban air quality is challenging because BVOCs can serve as both sources and sinks of aerosol and O3. One specific uncertainty lies in the plant emission profile within urban spaces where plants might experience increased stress, potentially leading to changes in their emission rates and composition. The primary objective of this thesis is to evaluate the impact of urban BVOCs on air quality. This involves evaluating BVOC emission source signatures in urban areas and investigating the SOA formation chemistry from BVOCs emitted by stress plants, as plants are expected to experience more frequent stress conditions in urban areas. Through this research, we aim to understand how changes in BVOC emissions due to factors like future vegetation change, human activities, and climate change will affect the formation of secondary pollutants in urban environments.Chapter 1 provides an overview of the BVOC emissions and their role in regulating urban air quality by influencing secondary pollutant formations. In Chapter 2, the net effect of urban BVOCs in Los Angeles County on secondary pollutant formation has been investigated. Because AVOC reductions resulting from air pollutant regulations improve urban air quality, the increased BVOCs resulting from urban greening program initiatives are expected to degrade it. In this study, we’ve evaluated the impacts of different species of trees used in urban greening programs on urban air quality by quantifying SOA formation potential (SOAP) and O3 formation potential (OFP). The method involved analyzing emission inventory data and employing box modeling. Our findings revealed that overall, the urban greening program could exacerbate air quality issues in Los Angeles and this effect varies greatly depending on the types of trees planted. These results underscore the substantial influence of urban greening programs and the types of plants chosen on urban air quality.
Chapters 3 and 4 tested two analytical methods to improve the development of the emission inventory of an important urban BVOC, limonene by getting more insights of limonene source apportionment in urban areas. Limonene is emitted from fragranced products used by humans (i.e. lotions, shampoos, body washes, etc) and from living plants, but it is unclear how much ambient limonene in urban air is derived from these two different sources. This research evaluated the potential of using enantiomeric analysis (Chapter 3) and stable carbon isotope analysis (Chapter 4) to address this challenge. In Chapter 3, the (+) and (-) - enantiomeric ratios of limonene from the volatile chemical product (VCP) and conifer emission sources were quantified to aid in source apportionment and emission inventory development. Samples were analyzed using a gas chromatograph equipped with a chiral column and mass spectrometry. Our findings revealed distinct enantiomeric ratios for limonene from VCPs compared to conifers, with VCP sources being predominantly composed of (+)-limonene (>97%), contrasting with conifer sources. The results were compared to those of air samples collected outside at two locations and indoors. The levels of (−)-limonene in outdoor air in Irvine and Portland and in indoor air were 50%, 22%, and 4%, respectively. This suggests outdoor limonene had both VCP and plant emission sources while indoor air was dominated by VCP sources. In Chapter 4, we employed a two-dimensional gas chromatograph isotope ratio mass spectrometer (TD-2DGC-IRMS) to measure the stable carbon isotope ratio of limonene from VCP and living plant emissions. Our analysis revealed significant differences in the stable carbon isotopic signature of limonene between VCPs and non-stressed plant emissions, with a p-value of <0.01. This investigation underscores the potential utility of enantiomeric and stable carbon isotope ratio analysis in refining VCP emission estimates and better constraining the impact of BVOCs on urban air quality.
Chapter 5 focused on SOA formation and composition from the photooxidation of acyclic terpenes – a common type of BVOC emitted from plants under stressed conditions, such as drought or pest outbreaks. In this work, SOA was generated from the photooxidation of acyclic terpenes in an oxidation flow reactor and compared to SOA production from a reference cyclic terpene – α-pinene. Results showed that oxidation of all acyclic terpenes has lower SOA yields measured after 4 days of photochemical age, in comparison to α-pinene. Most interestingly, β-ocimene SOA had higher oligomeric content than all the other chemical systems studied. These results contribute to novel chemical insights about SOA formation from acyclic terpenes and relevant chemistry processes, highlighting the importance of improving underrepresented biogenic SOA formation and further, the accuracy of estimating the effects of BVOCs on urban air quality.
Overall, this work has characterized the role of BVOCs on secondary pollutant formations, particularly SOA, developed approaches (technical and conceptual) in facilitating BVOC signatures in urban areas, and elucidated how changes in land use, human activities, and plant stress affect BVOC responses, thereby shaping urban air quality.