**3. Results**

Before investigating the production and sensitivity of nitrate aerosol, we checked the precursor gases of nitrate aerosol and oxidant concentrations that affect nitrate aerosol chemistry. Figure 3a indicates the meridionally averaged NOx concentration in the domain. The NOx concentration is highest at the street surface level, indicating that vehicular NOx emission is trapped by the canyon vortex in the street canyon. The NOx concentration reached 100 ppbv, ten times higher that outside the canyon, showing a steep gradient of NOx concentration in the street canyon. Note that concentrations in the three street canyons have slightly different values and dispersion patterns owing to different vortex patterns under non-infinite consecutive 3-D street canyons following the dispersion rates of TKE [26]. Figure 3b shows the spatial distribution of O3 concentration. The O3 concentration was lowest at the surface, showing a negative correlation with the NOx concentration. The O3 concentrations outside and inside the street canyon were 38 and 11 ppbv, respectively, suggesting NOx titration in the street canyon. This distribution of O3 and NOx fits the general dispersion pattern in the street canyon reported in previous studies [25,27]. Figure 3c displays the HNO3 concentration, which reached 3.8 ppbv in the street canyon. The HNO3 concentration in the street canyon was higher than that outside the street canyon, indicating the oxidation of HNO3 from vehicular NOx. However, the HNO3 concentration at the surface was the lowest, even though the NOx concentration was highest at the surface. The low level of HNO3 at the surface was caused by low O3 concentrations at the surface under VOC-limited conditions, suppressing the production of OH and HNO3. Figure 3d displays the NH3 concentrations; the daily averaged NH3 concentration reached 2.3 ppbv, with the highest value at the surface. The dispersion pattern of NH3 was similar to that of NOx, indicating the high impact of vehicular emissions.

**Figure 3.** Distributions of the daily average concentrations of (**a**) NOx, (**b**) HNO3, (**c**) O3, and (**d**) NH3 (ppbv) in the CNTL simulation.

Figure 4a shows the nitrate concentrations in the street canyon, which are higher than those outside the canyon, showing values of up to 11.4 μg m<sup>−</sup>3. The high concentration of nitrate aerosol is due to the high HNO3 concentration from vehicular NOx in the street canyon. However, the spatial pattern of nitrate aerosol in the street canyon differs from that of HNO3, because NH3 is a precursor gas of ammonium nitrate. The nitrate aerosol was highest at the surface following vehicular NH3. These high correlations between NH3 and nitrate aerosol indicate that ammonium nitrate forms under NH3-limited conditions in the street canyon. Figure 4b shows the ammonium concentration in the street canyon; the spatial distribution of ammonium aerosol is similar to that of nitrate, implying that most ammonium aerosols combine with nitrate aerosols in winter. The maximum concentration of ammonium aerosol was 4.7 μg m<sup>−</sup>3, and the spatial patterns of ammonium also indicate a low concentration of ammonium sulfate in winter. Note that the vehicular emission rate of SO2 was very low, implying that ammonium sulfate inside the street canyon might also be low [55]. The sum of the ammonium and nitrate concentrations was 16.1 μg m<sup>−</sup>3, higher than the air quality guidelines set by the World Health Organization (WHO, 10 μg m<sup>−</sup>3) and 46% of the WHO Interim Target-1 (35 μg m<sup>−</sup>3), indicating the hazardous effect of ammonium nitrate aerosols on pedestrians [56]. Considering that nitrate chemistry is highly nonlinear, this cannot be resolved and may lead to uncertainty in regional models due to their coarser resolution.

**Figure 4.** Distribution of the daily average concentrations of (**a**) nitrate and (**b**) ammonium (μg m<sup>−</sup>3) in the CNTL simulation.

We investigated the sensitivity of nitrate aerosol production to NOx emission. Figure 5a shows the average nitrate concentration in the street canyon (i.e., below 20 m) by following vehicular NOx emission changes to investigate the sensitivity of nitrate aerosol production to the vehicular NOx emission rate. Surprisingly, the nitrate concentration did not show a clear relationship with the NOx emission rate. Nevertheless, the change in nitrate concentrations was at most 2% compared to the standard simulation, and the average nitrate concentration was highest in the CNTL and lowest in the NOx × 0.25 simulations, indicating the nonlinearity of the nitrate aerosol production to the NOx emission rate. These results contradict the conventional belief that high NOx emissions from vehicles can cause nitrate aerosol air quality problems.

Figure 5b,d display the average HNO3, O3, and NO2 concentrations, respectively, in the street canyon according to the sensitivity simulations. The HNO3 concentration in the street canyon shows

changes similar to those of nitrate aerosol, indicating that the changes in nitrate aerosol in the sensitivity simulations are closely related to the changes in HNO3 (Figure 5b). The O3 concentration decreases as the NOx emissions increase because of NOx titration (Figure 5d). Note that O3 formation falls under a VOC-limited regime in the street canyon due to vehicular emissions. A low concentration of O3 prevents the conversion of NO2 to HNO3 through either photochemical production during the daytime due to inhibited OH production and heterogeneous nitrate production at night. The NO2 (the precursor gas of HNO3), concentration in the street canyon is proportional to the NOx emissions because it affects the direct NO2 emissions from vehicles and the reduction in photodissociation of NO2 under low O3 concentrations (Figure 5c). High NO2 creates suitable conditions for HNO3 formation, compensating for the effect of decreased O3 on HNO3. Thus, HNO3 and nitrate aerosols have no clear relationship with NOx emissions and only undergo small changes. These results imply that NOx emission controls cannot improve PM2.5 levels in urban street conditions.

**Figure 5.** Average (**a**) nitrate, (**b**) HNO3, (**c**) NO2, and (**d**) O3 concentrations in the street canyon (i.e., below 20 m) following a change in the emission rate of vehicular NOx.

In addition, we estimated the sensitivity of nitrate aerosol production to VOC emissions. Figure 6a shows the average concentration of nitrate aerosol in the street canyon following vehicular VOC emission changes; nitrate concentrations are proportional to VOC emissions. The average nitrate concentration in the VOC × 0.25 simulation was 8% lower than that of the CNTL simulation, indicating the higher sensitivity of VOC emissions to nitrate aerosols compared with that of NOx emissions. The nitrate aerosols in VOC × 0 only showed a 12% difference with those of the CNTL simulation, which implies the large impact of the boundary condition on nitrate formation. The changes in HNO3 concentration follow the changes in nitrate concentration, implying that the former is caused by the latter (Figure 6b). Reducing VOC emissions drives a decrease in both NO2 and O3 concentrations, creating unfavorable conditions for HNO3 production, in contrast to the effect of NOx emissions (Figure 6c,d). These results are consistent with a previous box modeling study, which suggests that increases in VOC emissions induce nitrate production via O3 increases under a VOC-limited regime

for O3 production [57]. Considering that megacities are generally under a VOC-limited regime [58,59], VOC emission control can improve both O3 control and PM2.5 control in urban street canyons.

**Figure 6.** Average (**a**) nitrate, (**b**) HNO3, (**c**) NO2, and (**d**) O3 concentrations in the street canyon (i.e., below 20 m) following changes in the emission rate of vehicular VOC.

Finally, we investigated the sensitivity of NH3 emissions to nitrate production. Figure 7a shows the average concentration of nitrate aerosols in the street canyon following vehicular NH3 emission changes. The concentration of nitrate aerosol was considerably influenced by the NH3 emission changes; the nitrate concentration in the NH3 × 4 simulation was 42% higher than that in the standard simulation, and the nitrate concentration in NH3 × 0.25 was 85% of that in the standard simulation. These results indicate that the sensitivity of NH3 emissions to nitration is much higher than that of NOx and slightly higher than that of the VOC emissions. The HNO3 concentrations are inversely proportional to NH3 emissions, indicating that higher NH3 emissions induce a higher conversion rate of HNO3 to nitrate aerosol (Figure 7b). These results sugges<sup>t</sup> that the production of ammonium nitrate is reduced by the low concentration of NH3 under an NH3-limited regime for nitrate production. Studies based on both modeling and observed campaigns have reported that nitrate formation occurs under an NH3-limited regime in East Asian megacities, including SMR [21,60]. The nitrate aerosols in the NH3 × 0 simulation were 19% lower than those with the CNTL simulation, indicating that ammonium and NH3 concentrations from the boundary also have an important role in nitrate formation in the urban street canyon. These results indicate that the control of NH3 emissions might be the most effective way to degrade PM2.5 problems where vehicular emissions are dominant in winter. The regulation of vehicle emissions is mostly focused on the control of NOx emissions; considering the present findings, we should instead focus on controlling VOC and NH3.

Though we used the coupled CFD–chemistry model to investigate the sensitivity of nitrate aerosols from vehicular emissions under complex geometry, our simulation still has limitations. Sea-salt aerosol significantly impacts the formation of nitrate aerosols via heterogeneous reactions when interacting with trace gases on the surface of sea-salt aerosol [61]. This process drives the efficient production of nitrates under an NH3-limited environment. This model does not account for the effect of sea-salt aerosol on nitrate aerosol production. Therefore, the simulation might underestimate the nitrate formation of heterogeneous chemistry. Moreover, we only considered the effect of vehicular emissions; NH3 and VOCs emissions from heating or biogenic emissions might affect the sensitivity of nitrate formation in the street canyon. The absence of these emissions in the model domain might create uncertainty in the nitrate aerosol calculation in this simulation.

**Figure 7.** Average (**a**) nitrate and (**b**) HNO3 concentrations in the street canyon (i.e., below 20 m) following a change in the emission rate of vehicular NH3.

#### **4. Model Sensitivity to Geometry and Speciation of VOC Emissions**

We examined the sensitivity of the model to the canyon geometry by conducting sensitivity model simulations in which we changed the street canyon aspect ratios (the ratio of building height to street width) of the street canyon to 0.5 and 2.0. The conditions of the sensitivity simulations were identical to those of the CTNL simulation except that the height of the buildings, 10 m and 40 m, respectively, indicating canyon aspect ratios of 0.5 and 2.0 (Figure 8). We named the sensitivity simulations for different aspect ratios "species name" × "multiplying factor" \_A "aspect ratio" (e.g., CNTL\_A2.0 and NOx × 2\_A0.5). Figures 9 and 10 indicate the meridionally averaged NOx, O3, HNO3, and nitrate aerosol concentrations in the CNTL\_A0.5 and CNTL\_A 2.0 simulations. The nitrate aerosol and their precursors showed similar distributions as those of the CNTL simulations despite the difference in their aspect ratios. The NOx concentration was highest at the surface and is an order of magnitude higher than that outside the canyon, indicating the trapping of vehicular emissions due to the strong canyon vortex in the street canyon (Figures 9a and 10a). The spatial distribution of NOx indicates that concentrated vehicular emissions drive the NOx titration of the O3 concentration at the surface under a low VOC emissions condition in both cases (Figures 9b and 10b). The HNO3 concentrations also show similar distribution to those of the CTNL simulation, indicating the suppressing of the production of OH and HNO3 (Figures 9c and 10c). Figures 9d and 10d show the nitrate concentrations in the street canyon for different canyon aspect ratios. The averaged nitrate concentrations in the street canyon (<10 m and <40 m, respectively) show 5.7 and 6.2 μg m<sup>−</sup><sup>3</sup> in the CNTL\_A0.5 and CNTL\_A 2.0 simulations, respectively, which are 9%, and 19% higher than those of the CTNL simulation. These differences are mainly due to the complex canyon vortex driving for the building geometry [27]. Despite the dispersion patterns differing according to canyon geometry, the mechanism of nitrate aerosol formation was consistent with that of CNTL, suggesting high nitrate formation from vehicular emissions in the street canyon.

**Figure 8.** Schematic diagrams of the coupled sensitivity simulation domain for the canyon aspect ratios (**a**) 0.5 and (**b**) 2.0.

**Figure 9.** Distribution of the daily average concentrations of (**a**) NOx, (**b**) HNO3, (**c**) O3 (ppbv), and (**d**) nitrate aerosol (μg m<sup>−</sup>3) in the CNTL\_A0.5 simulation.

**Figure 10.** Distribution of the daily average concentrations of (**a**) NOx, (**b**) HNO3, (**c**) O3 (ppbv), and (**d**) nitrate aerosol (μg m<sup>−</sup>3) in the CNTL\_A2.0 simulation.

We tested the NOx, VOC, and NH3 emission sensitivity to nitration formation for different canyon aspect ratios (0.5 and 2.0). Figures 11 and 12 show the averaged nitrate concentration in the street canyons (<10 m and <40 m, respectively) by following the vehicular NOx, VOC, and NH3 emission changes for the different canyon aspect ratios of 0.5 and 2.0, respectively. The sensitivity of the nitrate formation following NOx, VOC, and NH3 emission changes is also similar to those for the canyon aspect ratio of unity. The nitrate concentration changes show no clear relationship with the NOx emission rate in either case, which is related to the nitrate precursor changes, particularly in the daytime as we mentioned (not shown). The maximum concentrations occurred in NOx × 0.5\_A0.5 and NOx × 0.5\_A2.0, which differed slightly with the simulations for the canyon aspect ratio of unity. Nevertheless, the difference in the nitrate concentrations between the simulations was only 2%. The sensitivity of nitrate formation following VOC and NH3 emissions also follows consistent results with those for the aspect ratio of unity. Enhanced (reduced) VOC emissions drive an increase (decrease) in the nitrate aerosol concentration affecting the O3 concentration under a VOC limited regime for O3 production. The nitrate concentration in the street canyon is proportional to the vehicular NH3 emission, indicating that NH3 limits the condition of nitrate formation in both cases. These results

indicate that the sensitivity of nitrate formation following emission changes in the street canyon is consistent, regardless of the building aspect ratio, due to concentrated emissions from vehicles and the canyon vortex.

**Figure 11.** The average nitrate concentration in the street canyon following changes in the emission rate of vehicular (**a**) NOx, (**b**) VOC, and (**c**) NH3 for the 0.5 canyon aspect ratio; units are μg m<sup>−</sup>3.

**Figure 12.** The same as Figure 11 but for the 2.0 canyon aspect ratio; units are μg m<sup>−</sup>3.

The VOC speciation of vehicular emissions might change the model sensitivity on nitrate formation by affecting the ozone and OH production, considering all mechanisms are explained under a VOC limited regime for O3 production. Therefore, we checked the sensitivity of VOC speciation of emissions on nitrate aerosol formation using the different VOC chemical speciation used by Kim et al. (2006) [62]. Table 5 summarizes the emission rates and the ratio of speciated VOC with the method of Kim et al. (2006). Figure 13 indicates the averaged nitrate concentration in the street canyon (i.e., below 20 m) by following vehicular NOx, VOC, and NH3 emission changes with the VOC speciation of Kim et al. (2006). Similar to other sensitivity simulations, NOx emission changes did not affect the nitrate formation (due to the conflicting effects of NO2 and O3) even though we changed the VOC speciation (Figure 13a). The sensitivity of VOC concentration to nitrate formation also shows a similar relationship to that with EMEP/EEA speciation. However, the sensitivity was slightly lower than the nitrate concentration in CNTL, showing only a 6% difference between VOC × 0.25 and CNTL (Figure 13b). These results show that the reduction of vehicular VOC emission is a more effective way to regulate nitrate problems in urban streets than NOx emissions under different VOC speciations.

**Table 5.** Emission rates per vehicle and **r**atios of speciated VOC obtained by following the method developed by Kim et al. (2006).



**Table 5.** *Cont.*

**Figure 13.** Average nitrate concentration in the street canyon following changes in the emission rate of vehicular (**a**) NOx and (**b**) VOC with the speciation method of VOC emission used by Kim et al. (2006); units are μg m<sup>−</sup>3.
