Anthropogenic factors, such as ignitions, climate, and land use changes, are the primary reason why the wildfire season today is three times longer than only twenty years ago, with fires now burning year-round in the continental United States1. The quantities of fuel that go up as smoke in the form of trace gases and aerosols affect public health2,3,4 and ecosystems5, but are highly uncertain6,7,8,9 due to a lack of measurement techniques to directly quantify wildfire emissions. Uncertain emissions contribute to errors in air quality models10,11 used to forecast smoke impacts on public health in affected communities, and to inform firefighting efforts. Here we show that destructive fires in northern California during October 2017 emitted 2040 ± 316 tonnes hr-1 carbon monoxide (CO), which is more than ten times larger than the emissions from all other anthropogenic CO pollution sources in the entire state of California combined. An assessment of satellite-based emission inventories reveals that the uncertainty in predicting pyrogenic CO emissions spans a factor of 83. Air quality impacts in the form of ozone and fine particulate matter range from insignificant to very severe (vary by two orders of magnitude), and scale with uncertain emissions caused primarily by the different ecosystem classification among emission inventories. This uncertainty is reduced to a factor of ~2 by our first CO mass flux measurements on the scale of actual wildfires. We derive the conversion factor of satellite fire radiative energy to CO emissions (α = 73 ± 11 to 149 ± 23 g CO MJ-1) for savannah and extratropical forest fuels of these fires, which is higher (+30 to +230%) than tabulated for these fuel types. Our results help to improve future model predictions, and suggest an additional organic soil contribution to emissions from wildfires in northern California.