Fine particulate matter (PM2.5) in the air, defined as bits of debris and aerosols with diameters less than 2.5 micrometers, has negative impacts on human health, including contributing to cardiovascular and respiratory illnesses. This material is emitted into the skies from a variety of sources, such as combustion in automobile engines, coal-fired power plants, and wildfires. Once in the atmosphere, how PM2.5 particles circulate depends greatly on local winds.
In Southern California, for example, where wildfires occur most often at the end of the dry season, Santa Ana winds can shift where fine particulate matter winds up. These northeasterly winds occur between September and May and result from dry air that warms over southwest facing coastal topography before flowing down from the mountains and offshore. In fall, when conditions are driest, the Santa Ana winds can be a major exacerbator of wildfires.
In a new study, Aguilera et al. compare local data on PM2.5 concentrations against records of Santa Ana winds in Southern California. Their combined data set spans from 1999 to 2012 and reveals that the influence of the Santa Ana winds on PM2.5 concentrations depends on if and where fires are burning.
In the absence of upwind wildfires, strong Santa Anas reduced PM2.5 air pollution over Southern California by sweeping PM2.5 out to sea. However, when wildfires are burning, the winds had the opposite effect, stoking the fires inland and transporting ash and other particulate debris from them to coastal cities.
In general, the Santa Ana winds decreased PM2.5 particulates most in inland ZIP codes during years with fewer fires, whereas the biggest increases in PM2.5 were observed in coastal ZIP codes in years with widespread wildfires.
Projections of the dynamics of Santa Ana winds as well as of the changing precipitation regime in Southern California (i.e., with the wet season starting later) suggest that there could be more chances for consecutive Santa Ana wind days fanning wildfires and burning larger areas, which in turn would increase human exposure to fine particulates from wildfire smoke. (GeoHealth, https://doi.org/10.1029/2019GH000225, 2020)
—David Shultz, Science Writer