## 4 Conclusions  
We observed that the pattern of SOA mass formation at [NO\({}_{\rm x}\)]/[\(\Delta\)HC] molar ratios below 0.15 differed markedly from that seen at higher [NO\({}_{\rm x}\)]/[\(\Delta\)HC] ratios. We therefore distinguish between nitrogen-poor (NP) SOA formed under conditions with low initial NO\({}_{\rm x}\) concentrations and nitrogen-rich (NR) SOA, which has a higher content of compounds such as organo-nitrates. We suggest that this distinction is valuable for understanding trends in the formation and properties of SOA formation in the presence of varying concentrations of NO\({}_{\rm x}\). The SOA mass generally decreased as the [NO\({}_{\rm x}\)]/[\(\Delta\)HC] ratio increased, especially under the NR regime of our proposed framework. In addition, NO\({}_{\rm x}\) promoted SOA browning under both regimes. Under the NP regime, MAC increased with [NO\({}_{\rm x}\)]/[\(\Delta\)HC] independently of the RH. However, for NR SOA, the MAC depended significantly on both [NO\({}_{\rm x}\)]/[\(\Delta\)HC] and the RH. The RH also had a complex effect on SOA browning (measured in terms of the MAC) under NR conditions but did not promote browning under NP conditions. The mass scattering cross-section (MSC, defined as [B\({}_{\rm scat}\)]/[SOA Mass]) of NR SOA also depended on the RH (and by extension, on OH exposure): it increased with [NO\({}_{\rm x}\)]/[\(\Delta\)HC] at RH values above 65%but decreased as [NO\({}_{\rm x}\)]/[\(\Delta\)HC] increased at RH values below 35%. Both B\({}_{\rm abs}\) and MAC increased with the [NH\({}_{\rm 3}\)]/[\(\Delta\)HC] ratio, suggesting that NH\({}_{\rm 3}\) promotes the formation of N-H-containing organic chromophores such as cyclic amines and azo compounds with similar peak B\({}_{\rm abs}\) values under low NO\({}_{\rm x}\) conditions. In addition, the formation of ammonium nitrate was observed in toluene-NH\({}_{\rm 3}\)-OH systems at low NO\({}_{\rm x}\) concentrations. NH\({}_{\rm 3}\) had no strong effect on SOA mass formation under high NO\({}_{\rm x}\) conditions but when the NO\({}_{\rm x}\) concentration was low, NH\({}_{\rm 3}\) significantly increased both the mass of SOA that formed and its light absorbing and scattering capacity. In addition, under high NO\({}_{\rm x}\) conditions, the presence of NH\({}_{\rm 3}\) increased the MAC and MSC when compared to the case without NH\({}_{\rm 3}\). Acidity strongly promoted SOA mass formation. The SOA mass enhancement due to acidity was highest (30% and 21% in the NR and NP SOA, respectively) at the lowest RH (20%); under higher pH conditions with 65% RH, acidity had no appreciable effect on SOA mass formation. Moreover, at 80%, when the pH was highest, H\({}_{\rm 2}\)SO\({}_{\rm 4}\) seeding _reduced_ SOA mass formation by 35% and 21% in the NP and NR regimes, respectively. These results show that acidity has complex effects on SOA mass formation that depend on both the NO\({}_{\rm x}\) concentration and the RH. The MAC was significantly enhanced in both NP and NR regimes in the H\({}_{\rm 2}\)SO\({}_{\rm 4}\) seeded SOA, indicating that acidity plays an important role in SOA browning. In addition, the MACs of the H\({}_{\rm 2}\)SO\({}_{\rm 4}\)-seeded SOA were higher at intermediate and low RH than at high RH (80%). This finding was rationalized by suggesting that the MAC enhancement of acidic SOA is linked to the formation of organosulphate chromophores and governed by two major parameters: the pH of the SOA aqueous phase and the ratio of the free hydrogen ion concentration in the aqueous phase ([H\({}^{\rm 4}\)]\({}_{\rm free}\)) to the SOA mass. Acid seeding also increased the B\({}_{\rm scat}\) of the SOA, which can be attributed to increases in the surface area and scattering potential of the SOA caused by the sulfuric acid seeds.