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Recent measurements in Houston show that HONO traffic emissions are 1.7%of NOx emissions, which is about twice the previously estimated value of0.8% based on tunnel measurements in 2001. The 0.8% value is widelyused to estimate mobile emissions of HONO for air quality modelingapplications. This study applies the newly estimated HONO / NOx ratio inthe WRF–SMOKE–CMAQ modeling system and estimates the impact of higher HONOtraffic emissions on its mixing ratios. Since applied emission inventoryresulted in overestimates of NOx mixing ratios and because HONOemissions and chemical formation depend on the magnitude of NOx, thus,before proceeding with HONO emission modifications emissions of NOxwere adjusted to reflect current emission trends. The modeled mixing ratiosof NOx were evaluated against measured data from a number of sites inthe Houston area. Overall, the NOx mean value dropped from 11.11 ppbvin the base case to 7.59 ppbv in the NOx-adjusted case becoming muchcloser to the observed mean of 7.76 ppbv. The index of agreement (IOA) isimproved in the reduced NOx case (0.71 vs. 0.75) and the absolute meanerror (AME) is lowered from 6.76 to 4.94. The modeled mixing ratios of HONOwere evaluated against the actual observed values attained at the MoodyTower in Houston. The model could not reproduce the morning HONO peaks whenthe low HONO / NOx ratio of 0.008 was used to estimate HONO emissions.Doubling HONO emissions from mobile sources resulted in higher mixingratios, and the mean value increased from 0.30 to 0.41 ppbv becoming closerto the observed mean concentrations of 0.69 but still low; AME was slightlyreduced from 0.46 to 0.43. IOA for simulation that used the 2001 emissionvalues is 0.63 while for simulation with higher HONO emission it increasedto 0.70. Increased HONO emissions from mobile sources resulted in a 14%increase in OH during morning time at the location of the Moody Tower and3% when averaged over an urban area. The increase calculated for daytime was7 and 1% for the Moody Tower and the urban area, respectively. Theimpact on ozone was found to be marginal. This study results shed light onthe underestimated HONO and OH in the morning from global/regional chemicaltransport models with the typical emission of 0.8% HONO emission out ofthe total NOx emissions

Impact of updated traffic emissions on HONO mixing ratios simulated for urban site in Houston, Texas