## 3 Results and Discussion
### Influence of NO\({}_{x}\) and RH on SOA (Experiments: Tol-1 to Tol-4)
#### 3.2.2 Influence of NO\({}_{\rm x}\) and RH on Light Absorption  
As outlined previously (see Section 3.2.1 and Figure 2), NO\({}_{\rm x}\) promoted the formation of BrC-rich SOA. However, unlike the SOA mass, B\({}_{\rm abs}\) increased steadily with [NO\({}_{\rm x}\)]/[\(\Delta\)HC] in the NP regime at the higher RH. Both the absolute values of B\({}_{\rm abs}\) and the slope of the plot of B\({}_{\rm abs}\) against RH became higher as the NO\({}_{\rm x}\) increased from NP to NR regimes. The trend towards higher B\({}_{\rm abs}\) values at higher RH is consistent with the trends observed for mass (see Figure 1). These results indicate that NO\({}_{\rm x}\)- and water-mediated toluene-OH chemistry promote both SOA mass formation and BrC formation at high RH values (e.g., 80%) due to the synergistic effects of water vapour and high OH concentrations on water-mediated OH chemistry. These effects can also be seen in Figure 3, which shows the variation of B\({}_{\rm abs}\) with RH under NR and NP conditions. Under NP conditions, B\({}_{\rm abs}\) increases linearly with RH, but under NR conditions the increase becomes steeper and non-linear, as shown in Figure 3. These results suggest that the effect of RH on B\({}_{\rm abs}\) (and thus BrC formation) is primarily due to water-mediated OH chemistry, and that the effect of NO\({}_{\rm x}\) on the formation of BrC compounds (i.e., chromophores) is modest. In addition, the sudden non-linear increase in B\({}_{\rm abs}\) at high RH (80%) under the NR regimes suggests that water-mediated NO\({}_{\rm x}\) chemistry plays a significant role in the formation of strongly light-absorbing compounds under these conditions without greatly increasing the total SOA mass; the SOA masses under the NP and NR regimes were within 10-20% of each other at the same RH (see Figure S2 in Supplementary Materials). Figure 3: B\({}_{\rm abs}\) as a function of RH in the NP and NR SOA formation regimes. NP (empty) and NR (filled) symbols are blue and red coloured, respectively. The solid and dashed lines are included to guide the eye. The mass absorption cross section (MAC) of the SOA was estimated by normalizing B\({}_{\text{abs}}\) against the SOA mass (see Table S2 in the Supplementary Materials and Table 3). Figure 4 clearly shows that like the B\({}_{\text{abs}}\), MAC of the SOA increased with [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] under all RH conditions, with a clear \"switching point\" at [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] = 0.15. The clear increase in MAC with [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] in the NP regime shows that NO\({}_{\text{x}}\) played a significant role in chromophore formation chemistry even when its concentration was low and that more strongly light-absorbing chromophores were formed as [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] increased. In other words, the light absorption potential of the formed SOA was highly sensitive to the [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] ratio under both NP and NR regimes. Interestingly, the increase in MAC due to the influence of NO\({}_{\text{x}}\) under NP conditions was independent of RH. At 80% RH, the increase in MAC with increasing [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] was slightly steeper under NR conditions than under NP conditions. However, the difference between NP and NR conditions was less clear at lower RH values (e.g., 35% RH). Figure 5 shows how the influence of RH on the MAC clearly differed between the NP and NR regimes. This outcome is not easily explained and sugg  
Figure 4: MAC (mass absorption cross-section) as a function of the [NO\({}_{\text{x}}\)]/[\(\Delta\)HC] ratio at 80%, 65%, 35% and 20% RH. Empty and filled symbols show results for NP and NR SOA, respectively. The dashed lines are included to guide the eye. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Avg. SOA**} & \multicolumn{3}{c}{**Avg. ALW ***} & \multicolumn{3}{c}{**Avg. SOA**} & \multicolumn{3}{c}{**Avg. SOA**} & \multicolumn{3}{c}{**Avg.