## 3 Results and Discussion
### Influence of \(NO_{x}\) Acidity and RH on SOA (Experiments: Tol-7 to Tol-9)
#### 3.4.2 Influence of \(\rm NO_{x}\) Acidity and RH on SOA Light Absorption  
We also investigated the influence of SOA mass, acidity, \(\rm NO_{x}\), and RH on the optical properties of toluene SOA. Figure 13a-f shows the \(\rm B_{abs}\) and MAC of toluene SOA as functions of the \(\rm[NO_{x}]/[\Delta HC]\) ratio at three different RH values in the \(\rm H_{2}SO_{4}\) seeded  
Figure 12: **(a–c)**: SOA mass as a function of \(\rm NO_{x}\)\(\rm I\)/\(\rm[\Delta HC]\) and the influence of acidity on SOA mass at (**a**) 80% RH, (**b**) 65% RH, and (**c**) 20% RH. The dashed lines are included to guide the eye. experiments (Tol-7, Tol-8, and Tol-9). As in the non-seeded experiments, Babs increased with [NOx]/[AHC] in the H2 SO4 seeded experiments. However, the slope was slightly steeper in the seeded case and the absolute values of Babs were approximately 2 Mm\({}^{-1}\) higher at all RH values below 80% (See Figure 13a-c). At 80% RH, the Babs trends in the non-seeded (Tol-1) and H2 SO4 seeded (Tol-7) experiments were almost identical, indicating that acidity had no obvious effect on Babs under either the NP or the NR regime in this case. However, because SOA mass formation was  
Figure 13: (**a**–f): Effects of acidity, NOx and RH on the Babs and MAC of toluene SOA. (i) Babs as a function of [NOx]/[AHC] in H2 SO4 seeded and non-seeded experiments at (**a**) 80%, (**b**) 65%, and (**c**) 20% RH. (ii) MAC as a function of [NOx]/[AHC] in H2 SO4 seeded and non-seeded experiments at (**d**) 80%, (**e**) 65%, and (**f**) 20% RH. The solid and dotted lines in Figure 13 (**a**–**c**) are eye guides showing the trends in Babs for NP SOA in the H2 SO4 seeded and non-seeded cases, respectively; eye guides for the NR case are not shown. Conversely, in Figure 13 (**d**–**f**), the solid and dotted lines are eye guides for both the NP and NR SOA in H2 SO4 seeded and non-seeded experiments, respectively. The solid and dashed lines are included to guide the eye. reduced under acidic conditions at 80% RH (see Figure 12a) and the MAC is the quotient of [B\({}_{\text{abs}}\)] and the SOA mass, the MAC was higher in the seeded case than in the non-seeded case (see Figure 13d). The MAC of acid-seeded SOA in the 65% and 20% RH cases (experiments Tol-8 and Tol-9, respectively) was also higher than in the corresponding non-seeded cases, as shown in Figure 13e,f. This indicates that acidity plays an important role in SOA browning. The effect of RH on the light absorbing properties of toluene SOA formed with acid seeding is clearly shown in Figure 14a-c: under NP conditions, the MAC at all three RH values was almost identical to that in the corresponding non-seeded experiments, but the MAC in the H\({}_{2}\)SO\({}_{4}\)-seeded experiments was generally higher than that in the corresponding non-seeded experiments and increased as the RH decreased. The high MAC of acidic SOA at lower RH values is tentatively linked to the formation of organo-sulphate chromophores, which is suggested to be governed by two major parameters: (i) the pH of the SOA aqueous phase, and (ii) the ratio of [H\({}^{+}\)]free (the concentration of free protons in the aqueous phase) to the SOA mass. Under NP conditions, the highest acidity (i.e., the lowest pH) and the highest [H\({}^{+}\)]free / SOA mass ratio were both observed with seeding, as shown in Figures 13e and 14c, supporting our hypothesis. In addition, a modelling study conducted by [PERSON] et al. [53] suggested that organo-sulphate formation was maximized at low pH and low RH (<60%), i.e., when the aerosol was more concentrated. Conversely, under NR conditions the MAC of non-seeded SOA reached a minimum at an intermediate RH (65%), whereas the MAC of the H\({}_{2}\)SO\({}_{4}\) seeded SOA peaked at the same RH (65%) and was lowest at 80% RH (see Figure 14b). These contrasting trends for seeded and non-seeded SOA under NR conditions are interesting and suggest that NO\({}_{\text{x}}\) has a strong influence on light absorption at the intermediate RH (65%).