PM2.5 Impacts from the Historic Ship Backlog at the Ports of Los Angeles and Long Beach

XIANG LI, Ranil Dhammapala, Melissa Maestas, Nico Schulte, Scott A. Epstein, South Coast Air Quality Management District

     Abstract Number: 136
     Working Group: Urban Aerosols

Abstract
The South Coast Air Basin (Basin), which includes the greater Los Angeles metropolitan area, has had significant challenges meeting federal PM2.5 standards. In 2021, the basin-maximum PM2.5 concentration exceeded the 24-hour PM2.5 federal standards on 18 days in November and December, the highest number of days recorded in those months in the past ten years. Preliminary studies suggest that both meteorological conditions and additional emissions may have played a role. In November 2021, because of the congestion caused by cargo backlogs, over 150 container ships were present off the coast and at berth in the Port of Los Angeles and the Port of Long Beach each day. To reduce the impact of ship emissions on the Basin, the Marine Exchange of Southern California modified the queuing process to encourage ships to wait more than 50 or 150 miles from the coast depending on latitude. However, the authors are not aware of any studies that have been conducted to quantify the potential effectiveness of this new queuing process in reducing PM2.5 in the Basin.

To assess the potential impact of the additional ship emissions on air quality in the Basin, we simulated atmospheric transport of ship emissions from a grid of 1302 ship emission locations over the ocean areas, spanning from the Los Angeles and Orange County coastline to over 200 miles away from the coast. At each location, 48-hour HYSPLIT forward trajectories starting every two hours on each day of 2021 were calculated. We calculated the fraction of trajectories reaching the Basin from each location to assess the potential of the location to affect PM2.5 concentrations in the Basin. Time- and location-dependent planetary boundary layer heights were used to remove trajectory points above the Basin's planetary boundary layer. We used NAM-12 km forecast as the meteorological input, and the spatial- and time-dependent plume rise heights of the simulated ship emissions were estimated using EPA's AERMOD. In addition, we ran sensitivity tests for many other factors that could affect the results, including the HYSPLIT vertical motion mode, the time resolution of the model run, and the input meteorological field.

Results show that the plume transport to the Basin from locations 50-150 miles offshore is rare, which indicates that the new queuing process adopted by the Marine Exchange will help reduce the transport of PM2.5 and precursors emitted by ships to the Basin. In addition, the impact from emissions located 25-50 miles offshore is much lower in winter than in summer. Finally, the Riverside/San Bernardino metropolitan area is mainly affected by emissions south of the Palos Verdes peninsula, while Los Angeles County is primarily influenced by emissions north of the Palos Verdes peninsula.