Understanding Trends of Volatile Organic Compounds in the Los Angeles Basin across Gradients of Human Activity

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 of nitrogen oxides (NO_x) and on-road combustion related volatile organic compounds (VOCs). Despite NO_x and VOCs reduction, the levels of ozone (O₃) and secondary organic aerosol (SOA) have not decreased commensurately and have plateaued in recent years. The role of atmospheric chemistry of VOCs that drive the O₃ and SOA production in the LA Basin under the low NO_x conditions needs to be studied further to better understand these recent trends and hence to meet ambient air quality standards. While recent research has shown that non-combustion and off-road combustion sources are significant contributors to VOC emissions and their chemistry, there are outstanding questions including 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-August 2022 accounting for the post COVID-19 restrictions. The work presented here focuses on speciation and quantification of gas-phase VOCs 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 and quantification of ambient gas phase organic compounds using two-dimensional gas chromatography and traditional data analysis; (2) comparisons of emission profiles over human activity gradients; and (3) evaluation of emission profiles and chemistry during LAAQC-2020, LAAQC-2022 (post COVID-19 restrictions), with the goals of quantifying contributions of source sectors to reactivity and secondary pollutant formation and understanding the relative importance of changing chemistry in observed air quality trends.