Abstract
Abstract. Recent measurements in Houston show that HONO traffic emissions are 1.7% of NOx 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 / NOx 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 NOx mixing ratios and because HONO emissions and chemical formation depend on the magnitude of NOx, thus, before proceeding with HONO emission modifications emissions of NOx were adjusted to reflect current emission trends. The modeled mixing ratios of NOx were evaluated against measured data from a number of sites in the Houston area. Overall, the NOx mean value dropped from 11.11 ppbv in the base case to 7.59 ppbv in the NOx-adjusted case becoming much closer to the observed mean of 7.76 ppbv. The index of agreement (IOA) is improved in the reduced NOx 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 / NOx 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 NOx emissions.
Highlights
Photolysis of nitrous acid (HONO) is an important source of hydroxyl radical (OH)
The measured mean value from all sites is 7.76 ppbv; the simulated mean value dropped from 11.11 ppbv in the base case to 7.59 ppbv in the reduced NOx case becoming closer to the observed mean
The Weather Research and Forecasting (WRF)–Sparse Matrix Operator Kernel Emissions (SMOKE)–Community Multiscale Air Quality (CMAQ) modeling system was used for evaluation and adjustment of NOx emissions
Summary
Photolysis of nitrous acid (HONO) is an important source of hydroxyl radical (OH). Main sources of OH are photolysis of ozone, formaldehyde, alkenes, and nitrous acid (Elshorbany et al, 2009; Mao et al, 2010; Kim et al, 2014). Photolysis of ozone and formaldehyde are the most important sources of OH during midday and afternoon hours; the highest contribution to radical production during early morning hours comes from photolysis of HONO (e.g., Perner and Platt, 1979; Harris et al, 1982; Czader et al, 2012, 2013). Among the most known chemical sources of HONO is the gas-phase formation from the reaction between OH and nitric oxide (NO) (Pagsberg et al, 1997) and the heterogeneous formation on surfaces from the hydrolysis of nitrogen dioxide (NO2) (Kleffmann et al, 1998; Finlayson-Pitts et al, 2003).
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