Understanding Sources and Trends of Volatile Organic Compounds in the Los Angeles Basin across Gradients of Human Activity and over Decadal Timescales

Historically, on-road vehicles have been the major source of anthropogenic emissions in the Los Angeles (LA) Basin. However, over the past few decades, tailpipe emissions from on-road vehicles have been reduced due to the implementation of stringent emission control technologies. This has resulted in reductions in nitrogen oxides (NOx) and on-road combustion related volatile organic compounds (VOCs). Despite NOx and VOCs reduction, levels of ozone (O3) and secondary organic aerosol (SOA) have not decreased commensurately and have been flattening in recent years. To achieve ambient air quality standards, it is necessary to identify sources of VOCs and to elucidate the underlying chemistry that drive the O3 and SOA production in the LA Basin. While recent research has shown that non-combustion and off-road combustion sources are significant contributors to VOC emissions and chemistry, there are outstanding questions including the magnitude of off-road combustion and non-combustion sources and their relative importance for O3 and SOA formation. To address these questions, we convened the Los Angeles Air Quality Campaign (LAAQC) in which samples were collected in Pasadena, CA from April-July 2020, covering the COVID-19 shelter-in-place orders, and again from April-July 2022. The research presented here focusses on VOC speciation and quantification in gas-phase samples and application of traditional and statistical data analysis approaches, to better understand how changes in human activity can be attributed to changes in anthropogenic VOCs and associated secondary pollutant levels over both short and long timescales. The results and discussion will address: (1) identification of source fingerprints using two-dimensional gas chromatography and statistical analysis; (2) comparisons of emission profiles over human activity gradients; and (3) evaluation of emission profiles, source contributions, and chemistry during LAAQC-2020, LAAQC-2022 (post COVID-19 restrictions), and the California Research at the Nexus of Air Quality and Climate change (CalNex-2010) field study with the goals of quantifying contributions of source sectors to reactivity and secondary pollutant formation and understanding the relative importance of differing sources and changing chemistry in observed air quality trends.