At Trinidad, California we observed elevated CO2 concentrations and concomitant lowered O2 levels coincident with forest fires 70 km distant (from 10/8/99 to 10/21/99). The precision of our O2 data, ¿1 &mgr;mol O2/mol dry air, revealed the reduction of atmospheric oxygen resulting from the combustion of biomass, and the stoichiometric ratios (-O2/CO2) of the wildfire emissions. Estimates of daily -O2/CO2 ratios were obtained by regression of CO2 against corresponding O2 data (R2 0.86 to 0.96). Daily -O2/CO2 ratios changed from 1.15 to 1.41 on a particularly smoky day that coincided with elevated levels of CH4 and increased CH4/CO2 ratios. The change to a higher ratio during smoky conditions illustrates the association between changing emissions and -O2/CO2 ratios, possibly due to changing wildfire dynamics. ¿ 2001 American Geophysical Union