**5. Conclusions**

Nitrate contributions to PM2.5 mass have increased in polluted urban areas, with an increasing number of severe haze events in East Asia. This study investigates the favorable conditions for the production of nitrate aerosols in urban streets using a coupled CFD–chemistry model. It was found that the nitrate concentrations in street canyons are higher than those outside the canyons due to the high HNO3 concentrations from vehicular NOx in these canyons. However, the spatial pattern of nitrate aerosols in street canyons differs from that of HNO3 due to NH3, thus indicating that ammonium nitrate formation occurs under NH3-limited conditions in street canyons.

Sensitivity simulations indicate that nitrate concentration does not show a clear relationship with the NOx emission rate, with nitrate changes of only 2% across among 16 time differences in NOx emissions. The HNO3 concentration in street canyons changes in a similar manner to that of nitrate aerosols, indicating that the changes in nitrate aerosols in the sensitivity simulations are closely related to HNO3 changes. An increase in the NOx emissions induces a decrease in O3 and an increase in NO2 under a VOC-limited regime for O3 production. These changes in O3 and NO2 have a conflicting effect on the HNO3 production in urban streets. Therefore, HNO3 and nitrate aerosols have no linear relationship with NOx emissions and only undergo small changes. The sensitivity simulations were conducted by varying the vehicular VOC emissions to investigate their effect on nitrate production. The results show that nitrate concentrations are proportional to VOC emissions. Nitrate was decreased by 9% in the VOC × 0.25 simulation, indicating a relatively high sensitivity compared to that of NOx. Decreased VOC emissions drive a decrease in both NO2 and O3 concentrations, creating unfavorable conditions for HNO3 production, unlike the effect of changes in NOx emissions. The nitrate aerosol concentration is considerably influenced by NH3 emissions, which show a higher sensitivity to nitrate production than do NOx and VOC emissions in urban streets. The nitrate concentration is proportional to NH3 emissions with the additional production of ammonium nitrate under an NH3-limited regime

for nitrate production. This research implies that, where vehicular emissions are dominant in winter, the control of vehicular VOC and NH3 emissions might be a more effective way to degrade PM2.5 problems than controlling NOx.

We checked the model sensitivity by changing the model's building geometry and VOC speciation. The sensitivity of nitrate formation by following emissions changes acts in a similar direction as CNTL simulation despite changing the building geometry and speciation of VOC emissions. The sensitivity simulations revealed that our results about the sensitivity of nitrate production to emission changes are robust.

**Funding:** This work was funded by the 2017 Research Fund of Myongji University.

**Acknowledgments:** This work was supported by the 2017 Research Fund of Myongji University.

**Conflicts of Interest:** The authors declare no conflict of interest.
