## 3 Results and Discussion
### Influence of NO\({}_{x}\) and RH on SOA (Experiments: Tol-1 to Tol-4)
#### 3.2.3 Influence of NOx and RH on Light Scattering  
The SOA light scattering coefficient (Bscat) decreased non-linearly in response to linear increases in the NOx concentration at a constant RH. This decrease correlated non-linearly with the [NOx]/[LHC] ratio at RH values between 80% and 20%, as shown in Figure 6. Interestingly, the non-linearity in these curves again occurred at an [NOx]/[LHC] value of around 0.15. The total scattering (Bscat) of NP SOA did not vary greatly with [NOx]/[LHC] at constant RH. However, the Bscat for NR SOA decreased with increasing [NOx]/[LHC]. This decreasing trend in Bscat values mirrored that seen for the SOA mass (see Figure 1a-d) and can be attributed to the reduction in the mass (see the slopes of the parameterization of NR SOA against [NOx]/[LHC] in Table S2 of the Supplementary Materials) and surface area (data not shown) of the SOA at higher RH values. Figure 6: Bscat as a function of the NOx to reacted toluene ratio ([NOx]/[LHC]) at 80% RH, 65% RH, 35% RH and 20% RH. Empty and filled symbols show results for NP and NR SOA, respectively. The dashed lines are included to guide the eye. In both the NP and NR regimes, \(\rm B_{scat}\) increased non-linearly with RH (see Figure S4 of the Supplementary Materials), with the non-linearity becoming apparent above 65% RH; the \(\rm B_{scat}\) at 80% RH was substantially higher than at lower RH values. This can be attributed to the high ALW content of the SOA at 80% RH (see Figure S5 in Supplementary Materials). As discussed previously, because the SOA hygroscopicity was similar in the NP and NR regimes, the ALW showed similar responses to increasing RH in both cases (see Figure S5). As shown in Figure 7, the MSC (mass scattering cross-section) of the SOA varied non-linearly with increasing [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] and RH. In the NP regime, MSC increased slightly with increasing [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] for all RH values bar the lowest (20% RH), in which case the MSC decreased slightly as [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] increased. In the NR regime, MSC increased moderately with [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] at 80% and 65% RH but decreased as [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] increased at 20% RH and did not vary appreciably with increasing [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] at 35% RH. These results clearly reveal a switching point in the relationship between MSC and [NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C] at around 35% RH. Plotting the MSC as a function of the RH (see Figure S6 of the Supplementary Materials) revealed a positive linear relationship between these two quantities in both the NP and NR regimes, so the scattering potential of the SOA was greatest at high RH. This can be attributed to the greater adsorption of water onto the SOA at higher RH values. Interestingly, however, the slope of the plot was steeper in the NR case (see Figure S6 in Supplementary Materials). As a result, the NR MSC was lower than the NP MSC when the RH was 35% or less, but the opposite was true at higher RH values. The light scattering potential of the NR SOA was thus more sensitive to the RH than that of the NP SOA. This outcome can be attributed to the ALW content of the SOA, which is higher in the NR case at high RH (>35%) and in the NP case at low RH (35% or less). In other words, the NR SOA absorbed more water and became more strongly scattering at high RH, while the reverse  
Figure 7: MSC as a function of the \(\rm NO_{x}\) to reacted toluene ratio ([NO\({}_{\rm x}\)]/[\(\rm\Delta H\)C]) at 80% RH, 65% RH, 35% RH and 20% RH. Empty and filled symbols show results for NP and NR SOA, respectively. The solid and dashed lines are included to guide the eye. was true at low RH. This may indicate that the molecular composition of the SOA (i.e., its product distribution) changes significantly between 65% RH and 35% RH. As expected, the single scattering albedo (SSA) of both NP and NR SOA was significantly affected by both NOx and water vapour.