## 3 Results and Discussion
### Influence of NO\({}_{x}\) and RH on SOA (Experiments: Tol-1 to Tol-4)  
#### 3.2.1 Influence of NO\({}_{\lambda}\) and RH on SOA Mass Formation  
As shown in Figure 1a-d, the SOA mass generally declined non-linearly as [NO\({}_{\lambda}\)]/[\(\Delta\)HC] increased in experiments Tol-1 to Tol-4; it was largely independent of [NO\({}_{\lambda}\)]/[\(\Delta\)HC] under NP conditions ([NO\({}_{\lambda}\)]/[\(\Delta\)HC] \(\leq\) 0.15) but decreased with increasing [NO\({}_{\lambda}\)]/[\(\Delta\)HC] under
Conversely, the light absorption of the SOA (measured in terms of the aerosol absorption coefficient at 405 nm, i.e., \(\mathrm{B_{abs,405}}\)) generally increased with [NO\({}_{\mathrm{x}}\)]/[\(\Delta\)HC], irrespective of NP or NR regimes. However, the increase was again non-linear; in experiments Tol-1-Tol-4, \(\mathrm{B_{abs}}\) increased moderately with increasing [NO\({}_{\mathrm{x}}\)]/[\(\Delta\)HC] under NP conditions but the increase became steeper under NR conditions, as shown in Figure 2. These trends confirm the formation of some prominent SOA chromophores due to the influence of NO\({}_{\mathrm{x}}\) on toluene-OH chemistry in the presence of water.  
Figure 1: **(a–d)**: SOA mass formed as a function of [NO\({}_{\mathrm{x}}\)]/[\(\Delta\)HC] at **(a)** 80% RH, **(b)** 65% RH, **(c)** 35% RH, and **(d)** 20% RH. Empty and filled symbols show results for NP and NR SOA, respectively. Dashed lines are included to guide the eye.  
Figure 2: The aerosol absorption coefficient \(\mathrm{B_{abs}}\) as a function of [NO\({}_{\mathrm{x}}\)]/[\(\Delta\)HC] at 80% RH, 65% RH, 35% RH and 20% RH. Empty and filled symbols show results for NP and NR SOA, respectively. Dashed lines are included to guide the eye.  
The results shown in Figures 1 and 2 indicate that [NO\({}_{\rm x}\)]/[\(\Delta\)HC] has two opposing effects on SOA formation; higher [NO\({}_{\rm x}\)]/[\(\Delta\)HC] levels promote the formation of light absorbing chromophores (browning molecules) but reduce that of molecules contributing to SOA mass.  
The effect of RH on the SOA mass was also non-linear. Under both NP and NR conditions, the SOA mass increased with the RH (see Figure S2 in Supplementary Materials) but the SOA mass was always higher under NP conditions than NR conditions, all else being equal. The increase in the SOA mass with the RH can be attributed to several factors including the amount of toluene reacted, the initial OH concentration, and the hygroscopic growth of SOA. As the RH increases, so too does the OH concentration in the reaction zone of the GO:PAM flow-tube, leading to higher toluene consumption and thus greater SOA mass formation. Water-mediated OH chemistry may also have contributed to SOA mass formation at the highest tested RH (80%). The effect of NO\({}_{\rm x}\) on the SOA mass was comparatively small (the SOA masses at the highest and lowest NO\({}_{\rm x}\) concentrations differed by \(<\)20%) and may be linked to the formation of nitrogen-rich SOA via RO\({}_{\rm 2}\)+NO reactions. These processes influenced the composition of the product compounds, for example by increasing the abundance of organo-nitrates (RONO\({}_{\rm 2}\)) at high NO\({}_{\rm x}\) concentrations. They also influenced the distribution of common products such as carbonyls (R=O) [45]. Because the SOA mass depends on the distribution of product vapor pressures, it would be expected to be influenced by the [NO\({}_{\rm x}\)]/[\(\Delta\)HC] ratio, especially in the NR regime.