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

Abstract. Recent measurements in Houston show that HONO traffic emissions are 1.7% of NO_x emissions, which is about twice the previously estimated value of 0.8% based on tunnel measurements in 2001. The 0.8% value is widely used to estimate mobile emissions of HONO for air quality modeling applications. This study applies the newly estimated HONO / NO_x ratio in the WRF–SMOKE–CMAQ modeling system and estimates the impact of higher HONO traffic emissions on its mixing ratios. Since applied emission inventory resulted in overestimates of NO_x mixing ratios and because HONO emissions and chemical formation depend on the magnitude of NO_x, thus, before proceeding with HONO emission modifications emissions of NO_x were adjusted to reflect current emission trends. The modeled mixing ratios of NO_x were evaluated against measured data from a number of sites in the Houston area. Overall, the NO_x mean value dropped from 11.11 ppbv in the base case to 7.59 ppbv in the NO_x-adjusted case becoming much closer to the observed mean of 7.76 ppbv. The index of agreement (IOA) is improved in the reduced NO_x case (0.71 vs. 0.75) and the absolute mean error (AME) is lowered from 6.76 to 4.94. The modeled mixing ratios of HONO were evaluated against the actual observed values attained at the Moody Tower in Houston. The model could not reproduce the morning HONO peaks when the low HONO / NO_x ratio of 0.008 was used to estimate HONO emissions. Doubling HONO emissions from mobile sources resulted in higher mixing ratios, and the mean value increased from 0.30 to 0.41 ppbv becoming closer to the observed mean concentrations of 0.69 but still low; AME was slightly reduced from 0.46 to 0.43. IOA for simulation that used the 2001 emission values is 0.63 while for simulation with higher HONO emission it increased to 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 and 3% when averaged over an urban area. The increase calculated for daytime was 7 and 1% for the Moody Tower and the urban area, respectively. The impact on ozone was found to be marginal. This study results shed light on the underestimated HONO and OH in the morning from global/regional chemical transport models with the typical emission of 0.8% HONO emission out of the total NO_x emissions.