Trends in Fine Particulate Matter and Source Contributions over the Last Decade in Central Los Angeles (2014-2024): Policy-Driven Declines, COVID-19 Disruptions, and Wildfires.

This study analyzed speciated PM data (2014-2024) from the Chemical Speciation Network in central Los Angeles and used positive matrix factorization (PMF) to identify and apportion sources, and quantified changes in their contributions over time to examine combined effects of regulatory actions, COVID-19 Lockdowns, and Wildfire Episodes over this period. Nine factors were identified, namely vehicular emissions, tire wear, mineral dust, fresh sea salt, aged sea salt + marine combustion, biomass burning, secondary nitrate, secondary sulfate, and secondary organic aerosol (SOA). Over the decade, mean gravimetric PM declined from 15.4 μg/m to 10.5 μg/m (-34 %), driven primarily by a statistically significant 47% decrease in vehicular emissions (from 2.15 to 1.14 μg/m), while secondary nitrate also declined by 61% (from 5.57 to 2.17 μg/m), though this trend was not statistically significant; secondary sulfate remained steady at 0.83-1.38 μg/m. Considering the tightening of the annual PM standard to 9.0 μg/m, central Los Angeles remains in nonattainment, underscoring the need for additional mitigation strategies to drive the annual mean below this revised threshold. Moreover, two significant perturbations interrupted these trends: stay-at-home orders (March-June 2020) suppressed vehicular and nitrate contributions by roughly 65 %, although meteorological conditions and data limitations might introduce some uncertainty, while the Bobcat Fire later in 2020 doubled the annual biomass-burning contribution to 3.64 μg/m (peaking at 6.78 μg/m in September) and raised SOA to 2.47 μg/m. Collectively, the results highlight the interplay between sustained emission controls and natural events, offering guidance for adaptive air-quality management.