Observations of Volatile Organic Compounds in the Los Angeles Basin during COVID-19

Volatile organic compounds (VOCs) were measured in the Los Angeles (LA) Basin from mid-April to mid-July 2020 during the COVID-19 pandemic, as a part of the Los Angeles Air Quality Campaign (LAAQC). VOCs were quantified in over 450 samples using one- and two-dimensional gas chromatography with different detectors; mixing ratios were determined for 150 compounds associated with on- and off-road mobile, volatile chemical product, and biogenic sources. During the sampling period, traffic counts increased from ∼55% to ∼80% of pre-COVID levels. While the average afternoon combustion-derived VOCs and carbon monoxide (CO) mixing ratios did not change significantly between April–May and June–July, there was a shift in the distribution to higher mixing ratios in June–July, particularly for VOCs associated with gasoline evaporation. Compared to observations made in the last major air quality campaign in the LA Basin (CalNex-2010), emission ratios for 40 compounds relative to acetylene (VOC/acetylene) have remained similar, while emission ratios relative to CO (VOC/CO) have dropped to ∼60% of their 2010 values. This divergence in trends suggests that whereas mobile sources are still the dominant source of the combustion-derived VOCs measured in the LA Basin, there has been a shift in the mobile source sectors, with a growing contribution from sources that have lower CO/acetylene emission ratios, including off-road equipment and vehicles. In addition to the observed shift in source sector contributions, estimated OH exposure was 70–120% higher than in 2010.