The Linkage of Atmospheric Rivers and Air Quality over Northern California

This study investigated the characteristics of heavy precipitation atmospheric river (AR) events and their relationship with the air quality over Northern California. In early January 2017, we examined the air quality before, during, and after two AR storm events (January 8^th, 10^th) and focused on any significant enhancement in aerosol concentration, especially near Bodega Bay.

Although there was a narrow and abundant water vapor transport to the coastal area of California during both days, the dynamic features between the January 8^th and January 10^th AR storms were quite different. On January 8^th, there was a strong “T” shape storm with high winds (~10 -15 m/s) and a significant amount of water vapor penetrating inland, resulting in heavy rainfall over the north and south of the Bay Area, as well as the Sierra Nevada regions. For the January 10^th event, the winds were stronger (~20 m/s), especially near the border of California that lead to heavy rainfall. For both events, precipitation over the Sierra mountain side was predominant, while coastal precipitation was more significant for the January 8^th event. The difference in location and magnitude of precipitation on the January 8^th event was partly due to the contribution of the relatively weak zonal wind and the stronger development of the barrier jet.

Regarding the air quality associated with the AR storm period, all PM_2.5, NO₂, and CO showed increasing patterns 2 to 3 days prior to January 8^th. This was caused by the high pressure system that resided in the north of the Bay Area a few days prior to the January 8^th storm. As a result, the atmosphere was relatively stable which enhanced the subsidence. Subsequently, pollutants accumulated locally and were constrained from being diffusive. The warm temperature, that also occurred a few days before the January 8^th coastal storm, contributed to increasing the chemical reaction of the aerosols. For O₃, there was a slight increase before and after the January 8^th event, although the trend was not significant. For the January 10^th event, there was no significant increase in aerosol concentration before the storm, and this is believed to be attributed to the previous storm being so close in timing.

For this case study, we incorporated in hourly surface aerosol measurements, 449 MHz wind profiler data, and ERA-interim reanalysis data, which is the first step towards advancing our understanding of surface aerosol concentrations at various stages of AR events.