## 2 Materials and Methods  
### Experiments  
#### 2.1.1 Experimental Setup  
The experiment was designed to generate SOA with tight control over experimental parameters and to measure and record changes in its formation, transformation, and properties under diverse experimental conditions. As shown in Figure S1 of the Supplementary Information (henceforth referred to as \"Supplementary Materials\"), the experimental set-up consisted of three serial zones: (i) the reactants input control zone, (ii) the reaction zone, and (iii) the instrumentation and measurement zone. The reactants input control zone regulated the flows and concentrations of gases and aerosols in the reaction zone. In the reaction zone, photo-induced reactions leading to SOA formation and transformation were allowed to occur subject to tightly controlled time constraints. In the instrumentation and measurement zone, various parameters, concentrations, and properties of the gas and aerosol mixture at the outflow of the reaction zone were measured and recorded. The Gothenburg Potential Aerosol Mass chamber (Go:PAM) was used as the photo-oxidation reaction zone [35]. This apparatus is depicted schematically in Figure S1 OH radicals are produced in Go:PAM by photolyzing O\({}_{3}\) in the presence of water vapour. Total flows through the apparatus were measured, revealing that the reactants had a median residence time of 100 s and SOA was formed in a laminar flow. The main inputs in the reactant input control zone were toluene (VOC), water vapour, and O\({}_{3}\). NO\({}_{\rm x}\), NH\({}_{3}\), and H\({}_{2}\)SO\({}_{4}\) were also added in some experiments as described below. To the check _the accelerated aging in lab environment with 100 s median residence time_, the Toluene & O\({}_{3}\) was taken in higher quantities than atmospherically-relevant conditions. Most of the experiments used ramping NOx\({}_{\rm x}\) concentrations but some used an NH\({}_{3}\) ramp. If a given input was not used in an experiment, the pipe connecting that input to the Go:PAM chamber was blocked to ensure that only the desired reactants were admitted. VOC-free air was generated with a Zero-Air generator. O\({}_{3}\) was generated by photolyzing VOC-free air with UVP Pen-Ray@mercury lamps (185 nm) and allowed to flow into the Go:PAM chamber. The desired RH in the Go:PAM chamber was set using an RH generator consisting of a water bubbler maintained at a user-specified temperature; VOC-free air was used to carry the required amount of moisture into the Go:PAM chamber. Similarly, to establish the desired VOC concentration in the Go:PAM chamber, VOC-free air was passed through a VOC container (which was heated to a user-specified temperature below the VOC's boiling point using a water bath) to carry the VOC into Go:PAM chamber. H\({}_{2}\)SO\({}_{4}\) seeds were generated from an H\({}_{2}\)SO\({}_{4}\) solution using the TSI Atomizer flow with VOC-free air; the flow was controlled with a needle valve that allowed a majority of the H\({}_{2}\)SO\({}_{4}\) to be sent to the exhaust with a small seed fraction being dried with a silicon drier and sent to the Go:PAM chamber. The required flow through the FIGAERO-HR-ToF-CIMS instrument during gas phase sampling differed from that for particle phase sampling. Therefore, VOC-free Air was used to dilute the FIGAERO-HR-ToF-CIMS flow to enable control over the flow through the instrument. In the instrumentation and measurement zone, a SMIPS (CPC 3775; EC3080 TSI Inc. Shoreview, MN, USA) was used to measure the particle number concentration and volume concentration of SOA in the sample outflow. The absorption and scattering properties of the SOA were measured with a 3-wavelength photoacoustic soot spectrometer (PASS-3) after passage through a charcoal filter to remove residual VOCs and NO\({}_{\rm x}\). Both instruments were described previously by [PERSON], et al.