Wildfires are important sources of ozone by emitting large amounts of NOx and NMVOC, main ozone precursors, at both global and regional scales.Their influences on ozone in the U.S.Intermountain West have recently received much interest because surface ozone concentrations over that region showed an increasing trend in the past two decades likely due to increasing wildfire emissions in a warming climate.Here we use the Lagrangian particle dispersion model (FLEXPART) as well as the GEOS-Chem chemical transport model to study the relationship between wildfires and ozone concentration in the US Intermountain West in the past 22 years (1989-2010) at both daily and inter-annual timescales.We combined the resident time estimates from the FLEXPART 5-day backward trajectories and a high-resolution fire inventory to define a fire index describing the impact of wildfires on ozone concentration at a particular site for each day of the summers 1989-2010.Over 26000 simulations of FLEXPART back-trajectories were conducted for the whole time period and for the 13 CASTNet surface monitoring sites.High ozone episodes were found associated with high fire index and temperature.We built multiple linear regression (MLR) models of daily ozone concentrations using fire index and other meteorological variables (temperature, wind speed, and relative humidity) for each year at each site.The regression models explained 42% of the ozone variations (ranging from 23% to 68% for each site), and showed that ozone productions of biomass burning are of high variability (ranging from 0.1 ppbv to 18.7 ppbv).We further conducted the GEOS-Chem nested simulation for summer 2006-2008.Wildfires ozone influences were calculated as the difference between the standard simulations and a sensitivity simulation with wildfire emissions turned off.We showed wildfire ozone influences estimated from the two approaches were highly correlated, indicating consistency between the Lagrangian and Eulerian models.