## 3 Results and Discussion
### Influence of \(NO_{x}\) Acidity and RH on SOA (Experiments: Tol-7 to Tol-9)  
#### 3.4.1 Influence of \(NO_{x}\) Acidity and RH on SOA Mass  
As shown in Figure 12a-c, SOA formation in the H\({}_{2}\)SO\({}_{4}\)-seeded experiments with NO\({}_{x}\) ramps (see Tables 1 and 2; experiments Tol-7 to Tol-9) clearly depended on both NOx and RH. As in the case without H\({}_{2}\)SO\({}_{4}\) seeding (experiments Tol-1 to Tol-4; see Figure 1a-d, the dependence of the SOA mass on [NO\({}_{x}\)]/[\(\Delta\)HC] differed markedly between NP and NR conditions. However, the response of the SOA mass to increases in the [NO\({}_{x}\)]/[\(\Delta\)HC] ratio also differed in some respects from that seen under non-seeded conditions (see Figure 1a-d). The similarities are due to the fact that SOA is formed by toluene-NO\({}_{x}\) chemistry in both cases, while the differences can be attributed to the impact of acidity-mediated heterogeneous chemistry and NOx on SOA formation [50; 51; 52]. In the acid-seeded experiment with 80% RH (Tol-7), the SOA mass increased with the [NO\({}_{x}\)]/[\(\Delta\)HC] ratio under NP conditions but then became independent of [NO\({}_{x}\)]/[\(\Delta\)HC] under NR conditions (see Figure 12a). Conversely, in the H\({}_{2}\)SO\({}_{4}\) seeded experiments at 65% and 20% RH (Tol-8 and Tol-9), the SOA mass decreased significantly as [NO\({}_{x}\)]/[\(\Delta\)HC] increased under NR conditions but did not vary greatly with [NO\({}_{x}\)]/[\(\Delta\)HC] under NP conditions. Under both NP and NR conditions with acid seeding, the SOA mass at 80% RH was lower than in the corresponding non-seeded experiments whereas the opposite was true at 20% RH (see Figure S10a,b in the Supplementary Materials). At 65% RH, the SOA masses in the H\({}_{2}\)SO\({}_{4}\)-seeded and non-seeded experiments were comparable. To better understand these outcomes, the influence of ALW and pH on SOA mass formation was investigated (see Figure S10c-f and Table S3 in the Supplementary Materials). This suggested that ALW (see Figure S10c,d)) increased the availability of free acid, i.e., [H\({}^{+}\)]free (see Figure S10e) formed by H\({}_{2}\)SO\({}_{4}\) dissociation at all RH levels (see Table S3 in Supplementary Materials). Therefore, the pH of the SOA was lowest at the lowest RH (20%) (see Figure S10f in Supplementary Materials). Moreover, whereas both ALW and the SOA mass increased with RH, the acidity of the SOA decreased (i.e., its pH increased; see Figure S10a-e in Supplementary Materials). Accordingly, the SOA mass increase due to acidity was most pronounced (30% in the NR case and 21% in the NP case) at the lowest pH at 20% RH. However, at 65% RH, the pH and SOA mass were both higher and acidity had no appreciable effect on the SOA mass, while at the highest pH (80% RH), acidity reduced the SOA mass significantly when compared to the non-seeded experiments-by 35% and 21% under NP and NR conditions, respectively. These results show that the effects of acidity on the formation and properties of SOA are complex. The observation that acid seeding and high RH (80%) caused NOx to increase SOA formation in the NR regime but not in the NP regime (in the Tol-7 experiment) is unprecedented and contrary to the observed SOA formation behaviour in the absence of H\({}_{2}\)SO\({}_{4}\) seeding (i.e., in the Tol-1 experiment). [PERSON] et al. [51] reported that SOA mass was increased by acidic seeding when SOA loadings were low, but that this increase was not observed at higher SOA loadings. Additionally, [PERSON] and [PERSON] [52] reported that the mass of toluene SOA formed under low and intermediate NOx conditions (which both fall within the NP regime as defined in this work) with acidic sulphate seeding was considerably higher (by \(\sim\)15-21%) at low RH when compared to the case with neutral seeding, but was reduced at high RH (by \(\sim\)38-48%). No significant particle acidity effect was observed in the high NOx experiments in their study [52].