## 4 Conclusions
There was no clear evidence that acid seeding or variation in the RH changed the chemical composition of the SOA in ways that greatly affected B\({}_{\rm scat}\), however; instead, the changes in B\({}_{\rm scat}\) and MSC in seeded SOA under NP and NR conditions are primarily attributed to increased surface growth driven by H\({}_{\rm 2}\)SO\({}_{\rm 4}\) seeds. Incorporating these findings into climate and air quality models will reduce uncertainties in the treatment of SOA and a deeper understanding of its role in climate change. **Supplementary Materials:** The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/atmos13071099/s1](https://www.mdpi.com/article/10.3390/atmos13071099/s1)]([https://www.mdpi.com/article/10.3390/atmos13071099/s1](https://www.mdpi.com/article/10.3390/atmos13071099/s1)). Table S1: The estimated total OH concentration exposure (OH exp) and equivalent atmospheric aging _(Hrs and Days)_ of SOA in CO:PAM. Table S2: NR Regime (NO\({}_{\rm x}\)/\(\Delta\)HC > 0.15) SOA properties and parameterization. Table S3: E-AIM model results and pH for 22.3 ug m\({}^{-}\) dried H\({}_{\rm 2}\)SO\({}_{\rm 4}\) injected in each experiment. Figure S1: Process flowdiagram of the experimental setup. Figure S2: SOA mass as a function of RH in the NP and NR SOA formation regimes. Figure S3: Mass enhancement factor (MEF) as a function of relative humidity for toluene-OH photo-oxidation SOA. A fit to data reported by [PERSON] et al. [33] is plotted alongside a fit to data originating from this work for toluene SOAs formed at different RH values and NOx concentrations. The MEF in this study was estimated based on the SOA mass at 20% RH with the assumption that the hygroscopic growth factor for SOA at 20% RH relative to that at around 0% RH is approximately 1.03, as reported by [PERSON] et al. [33]. Figure S4: B\({}_{\text{scat}}\) as a function of RH in the NP and NR SOA formation regimes. NP and NR results are plotted using empty blue and filled red circles, respectively. Figure S5: Aerosol liquid water (ALW) content as a function of RH in the NP and NR regimes of SOA formation. NP and NR results are plotted using empty blue and filled red circles, respectively. Figure S6: MSC as a function of RH in the NP and NR SOA formation regimes. NP and NR results are plotted using empty blue and filled red circles, respectively. Best fits for NP and NR are shown using solid blue and dotted red lines, respectively. Figure S7: Single Scattering Albedo (SSA) as a function of the NOx to reacted toluene ratio ([NOx]/[\(\Delta\)HC]) at 80% RH, 65% RH, 35% RH, and 20% RH. Empty and filled symbols show results for nitrogen poor (NP) and nitrogen rich (NR) SOA, respectively. The blue solid line is the trend line for NP SOA. Figure S8: SSA as a function of RH in the NP and NR SOA formation regimes. NP and NR results are plotted using empty blue and filled red circles, respectively. Best fit lines for NP and NR are solid blue and dotted red, respectively. Figure S9: (a,b): Effect of NHx\({}_{3}\) on the light scattering properties of SOA at 80% RH under low and high NOx conditions. (a) B\({}_{\text{scat}}\) as a function of the ([NHx]/[\(\Delta\)HC]) ratio; (b) MSC as a function of the ([NHx]/[\(\Delta\)HC]) ratio. Figure S10: (a-f): Effects of acidity and RH on SOA mass, ALW, and pH for NP and NR SOA: (a) NP SOA mass as a function of RH in the H2 SO4 seeded and non-seeded experiments; (b) NR SOA mass as a function of RH in the H2 SO4 seeded and non-seeded experiments; (c) NP and NR SOA mass as functions of the RH in the H2 SO4 seeded experiments; (d) ALW in NLW SOA as a function of RH in the H2 SO4 seeded and non-seeded experiments; (e) pH of the NP and NR SOA as functions of the RH in the H2 SO4 seeded experiments. Non-seeded experiments are Tol-1, Tol-2 and Tol-4, while H2 SO4 seeded experiments are Tol-7, Tol-8 and Tol-9. Figure S11: (a-f): Effects of acidity, NOx\({}_{\text{scat}}\), and RH on the B\({}_{\text{scat}}\) and MSC of toluene SOA.