Origin of the Cleanest Air Parcels: Investigating Ozone Sources and Trends across the Eastern Pacific and Western North America

Tropospheric ozone exhibits global and regional variations due to many influencing factors at various scales. A recent study demonstrates an increasing trend in free tropospheric ozone, coupled with a declining trend in surface ozone across western North America (WNA). This increase in free tropospheric ozone is likely linked to changes in transport associated with topographically complex WNA, increases in long-range emission transport originating from Asia and tropical regions, or increases in photochemistry in remote regions.

This study aims to dissect the underlying contributors that dictate the levels of tropospheric ozone in the WNA, encompassing a range of emission types and identifying the source regions. First, we show the predominant source regions driving low ozone levels over the WNA and how these contributions change with altitude. Second, we present how ozone transport patterns shift with changing seasons, offering insights into the impact of seasonal transport patterns on ozone variability. Third, we delve into the types of primary emission sources across various regions, including aircraft, shipping, and lightning emissions, that significantly impact ozone levels over WNA.

We employ a Lagrangian particle dispersion model (FLEXPART v10.4-ERA5) with high-resolution simulations spanning 28 years (1994-2021). Through this receptor-source approach, we compute the surface influence factors originating from diverse source regions that impact ozone amounts (ranging from 900 hPa to 300 hPa pressure levels, obtained from multiple aircraft campaigns and ground measurements) over WNA. The results reveal that ozone concentrations at the lower end (< 5th percentile relative to baseline (2004-2014) values) are primarily influenced by sources emanating from the tropical Pacific Ocean. This pattern is most pronounced during the boreal winter (December, January, and February) and has become increasingly evident in recent years.

Furthermore, by utilizing data from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, Modern-Era Retrospective analysis for Research and Applications, Version 2 - Global Modeling Initiative (MERRA2-GMI), and the Tropospheric Chemistry Reanalysis version 2 (TCR-2) top-down emission estimates, we demonstrate the global and regional ozone trends among the identified source regions. Additionally, we elucidate the relationships between these trends and other processes, such as stratospheric ozone with loss in the troposphere, ozone production rates, and NOx emission due to lightning.