## 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
**} & \multicolumn{3}{c}{**Dry SOA**} & \multicolumn{3}{c}{**ALW+SOA**} \\
**Exp. ** & **RH** & \multicolumn{3}{c}{**Mass**} & \multicolumn{3}{c}{**Avg. **} & \multicolumn{3}{c}{**Avg. ALW ***} & \multicolumn{3}{c}{**Mass * (\(dry\))**} & \multicolumn{3}{c}{**B\({}_{\text{abs}\text{405}}\)**} & \multicolumn{3}{c}{**MAC\({}_{\text{405}}\)**} & \multicolumn{3}{c}{**MAC\({}_{\text{405}}\)**} \\
**ID** & **(\%)** & **NR** & **NP** & **NR** & **NP** & **NR** & **NP** & **NR** & **NP** & **NR** & **NP** & **NR** & **NP** & **NR** \\ \hline Tol-1 & 80 & 545 & 508 & 0.094 & 0.288 & 167 & 155 & 378 & 353 & 10.3 & 28.0 & 0.027 & 0.  
or more competing processes, notably (i) hygroscopic growth of the formed SOA, which increases the SOA mass through the incorporation of aerosol liquid water (ALW); and (ii) the water-mediated chemistry of the toluene-NO\({}_{\rm x}\)-OH system and its effects on the composition and hygroscopicity of the formed SOA. At a given RH, the B\({}_{\rm abs}\) value of the SOA reflects only the number of chromophores formed as a result of toluene-NO\({}_{\rm x}\)-OH-water-mediated chemistry. The increase in B\({}_{\rm abs}\) with [NO\({}_{\rm x}\)]/[AHC] indicates that NO\({}_{\rm x}\)-water chemistry has an important effect on MAC at high RH levels. At the same time, hygroscopic growth at high RH enhances the contribution of ALW to the SOA mass measured by the SMPS, which is calculated using the SOA density functions of [PERSON] et al. [20]. This in turn leads to underestimation of the MAC at high RH. These two processes may have opposing effects on the MAC because hygroscopic growth increases SOA mass, while toluene-NO\({}_{\rm x}\)-OH-water-mediated chemistry promotes BrC formation. At high RH, hygroscopic growth reduces MAC, while VOC-NO\({}_{\rm x}\)-OH-H\({}_{2}\)O-mediated chemistry increases B\({}_{\rm abs}\). The trends in SOA formation observed in experiments Tol-1 to Tol-4 at RH values of 80% to 20% under NP and NR regimes were similar to those reported by [PERSON] et al. [33]. However, the mass enhancement factor (MEF)determined at the lowest RH examined in our experiments (20%; see Figure S3 of the Supplementary Materials) was significantly higher than that reported by _[PERSON] et al._ [33], which is notable because these authors studied a very hygroscopic toluene SOA formed at almost 1% RH and examined its absorption of water at higher RH values. Therefore, hygroscopic growth alone cannot explain the high MEF observed in this work. Instead, the MEF trends observed here can be attributed to several factors that were discussed above, namely: (i) the increase in the SOA mass due to increased concentrations of both OH radicals and reacted toluene [AHC] at higher RH values; (ii) the greater hygroscopicity of SOA formed at higher RH values and more  
Figure 5: MAC as a function of RH in the NP and NR SOA formation regimes. Results for the NP and NR regimes are plotted using empty blue and filled red circles, respectively. Solid and dashed lines are included to guide the eye. extensive heterogeneous oxidation of SOA due to aging; and (iii) the influence of NOx chemistry on the gas-aerosol partitioning and hygroscopicity of the SOA. In addition, [PERSON] et al. [34] showed that increasing OH exposure during SOA formation caused a sharp increase in ALW at constant RH in the low NOx regime. However, when normalized against the dry SOA mass, the ALW did not increase significantly as the OH concentration increased, indicating that the hygroscopicity of toluene SOA (i.e., its capacity to absorb water) at a given RH was independent of the OH concentration and was thus unaffected by OH-water mediated chemistry. This in turn implies that the large differences between the MEF values obtained in this work and those reported by [PERSON] et al. [33] can be attributed to the water-mediated toluene-OH chemistry of NP SOA.