. Emissions, Transport, and Chemistry of Smoke from the October 2017 Northern California Fires

Abstract
During October 2017, the fires in Northern California (N. CA) counties of Sonoma, Napa, Solano, and Mendocino killed 44 people, destroyed more than 10,000 structures, and resulted in reported economic losses of 13.2 billion. The Oct. 2017 N. CA fires were the second most destructive fires in CA history and resulted in the highest particulate matter (PM2.5) levels recorded in the Bay Area since 1999. Human caused fires are the primary reason that the wildfire season is now three times longer than 20 years ago, with fires burning year-round in the continental US today. The quantities of fuel that go up in smoke affect public health and ecosystems, but the emitted amounts of trace gases and aerosols remain poorly understood. Here we demonstrate airborne absorption measurements of carbon monoxide (CO) and ammonia (NH3) by deploying the University of Colorado Solar Occultation Flux (CU SOF) instrument downwind of the Oct. 2017 N. CA fires. The CU SOF sampling enabled the first emission flux estimates on the scale of actual wildfires based on absorption measurements, which reduce the uncertainty in the Oct. 2017 N. CA fire emissions from four orders of magnitude among satellite-based emission inventories to about a factor of two. The Oct. 10, 2017 CO emissions estimated from the CU SOF for the N. CA fires are two to seven times larger than all other anthropogenic CO pollution sources in the entire state of California combined. Air quality forecasts using regional chemical models help mitigate public health concerns in affected communities and inform firefighting efforts, yet many models fail to accurately predict ozone (O3) and PM2.5 levels during fire events. Model simulations of the Oct. 2017 N. CA fires are found to best reproduce observations when the fire diurnal cycle, size, and spatial variation indicated by satellite retrievals of Fire Radiative Power are represented in the model. The model simulations are compared with satellite retrievals of aerosol optical depth, and show that O3 and secondary aerosol mass were produced from the Oct. 2017 N. CA fire emissions.