## 3 Results and Discussion
### Influence of NH\({}_{3}\) and NO\({}_{x}\) on SOA at High RH (Experiments: Tol-5 and Tol-6)
#### 3.3.2 Influence of NH\({}_{3}\), NOx on SOA Light Absorption at 80% RH  
Two results shown in Figures 9 and 10 are particularly noteworthy: (i) the B\({}_{\rm abs}\) and MAC in the low NOx (Tol-5) also showed similar behaviour as SOA mass (see Figure 8) in response to increasing [NH\({}_{3}\)]/[\(\Delta\)HC] but peaked at [NH\({}_{3}\)]/[\(\Delta\)HC] \(\sim\)0.11, however, (ii) the B\({}_{\rm abs}\) and MAC in high NOx (Tol-6) were much higher than low NOx experiment (Tol-5 ) and SOA mass in the high NOx case (see Figure 8). The similar behaviors of B\({}_{\rm abs}\) and MAC as functions of [NH\({}_{3}\)]/[\(\Delta\)HC] suggest that N-H-containing organic chromophores (e.g., cyclic amines such as azo compounds) with similar light absorption profiles were formed in the low NOx regime [48; 49]. In the high NOx regime, SOA formation via the NOx -water mediated chemistry appears to dominate because the B\({}_{\rm abs}\) and MAC in this case behaved similarly to those of the NR SOA formed in experiments Tol-1 to Tol-4. That is to say, B\({}_{\rm abs}\) and MAC were independent of [NH\({}_{3}\)]/[\(\Delta\)HC] in the high NOx regime and almost constant (B\({}_{\rm abs}\)\(\sim\)40 Mm\({}^{-1}\), MAC\(\sim\)0.09 m\({}^{2}\) g\({}^{-1}\)). Figure 10: MAC as a function of the ([NH\({}_{3}\)]/[\(\Delta\)HC]) ratio at 80% RH under low and high NOx scenarios. Blue and red dotted lines are the eye guiding lines for low and high NOx scenarios, respectively. The dashed lines are included to guide the eye. Figure 9: B\({}_{\rm abs}\) as a function of the ([NH\({}_{3}\)]/[\(\Delta\)HC]) ratio at 80% RH under low and high NOx scenarios. Blue and red dotted lines are the eye guiding lines for low and high NOx scenarios, respectively. The dashed lines are included to guide the eye. 3.3.3. Influence of NH\({}_{3}\) and NO\({}_{x}\) on SOA Light Scattering at 80% RH  
Interestingly, the scattering coefficient (B\({}_{\rm scat}\)) and potential (MSC) were both high and did not respond to increases in [NH\({}_{3}\)]/[\(\Delta\)HC] in the same way as B\({}_{\rm abs}\) and the SOA mass under either high or low NO\({}_{x}\) regimes (see Figure S9a,b in the Supplementary Materials). B\({}_{\rm scat}\) and MSC initially increased with [NH\({}_{3}\)]/[\(\Delta\)HC] in both high and low NO\({}_{x}\) regimes but then remained almost constant when the [NH\({}_{3}\)]/[\(\Delta\)HC] ratio rose above 0.11. In other words, although the SOA mass decreased as the [NH\({}_{3}\)]/[\(\Delta\)HC] ratio increased above 0.13, B\({}_{\rm scat}\) and MSC did not change significantly. These outcomes can potentially be attributed to the presence of highly scattering coatings of condensed ammonium nitrate on SOA in both NO\({}_{x}\) regimes. The MSC for SOA formed under the low NO\({}_{x}\) regime was significantly higher than in the high NO\({}_{x}\) regime. However, it is noteworthy that in both regimes the MSC was significantly higher than in any previous experiment that is Tol-1 to Tol-4. B\({}_{\rm scat}\) was also lower in the high NO\({}_{x}\) regime. This is probably because the effects of high NO\({}_{x}\) concentrations, which favour the formation of comparatively volatile organo-nitrates, outweigh those of organo-amines from NH\({}_{3}\) reaction during SOA formation. These findings show that NH\({}_{4}\)NO\({}_{3}\) strongly affects the formation and properties of SOA under the NP regime, and that the presence of NH\({}_{3}\) under high NO\({}_{x}\) conditions promotes the formation of more light-absorbing SOA. Previous studies have similarly concluded that NH\({}_{3}\) increases the O\(\circ\)C and N\(\circ\)C ratios of compounds formed by toluene-OH water-mediated chemistry and promotes the formation of SOA compounds containing carbonyl and carboxylic acid functional groups under low NO\({}_{x}\) conditions [48; 49]. Figure 11 summarizes the complex influences of NH\({}_{3}\) on the light absorption and scattering characteristics (i.e., MAC, MSC, B\({}_{\rm abs}\), and B\({}_{\rm scat}\)) of toluene SOA under the NP and NR regimes.