## 2 Methods
### Analytical Method for PM\({}_{2.5}\) and Gaseous Compounds  
The mass concentration of PM\({}_{2.5}\) was determined using Teflon filter sampling and the beta-ray absorption method (5014i Beta Continuous Ambient Particulate Monitor, Thermo, Fisher Inc., Waltham, MA, USA). We conducted three-hour integrated sampling, employing Teflon filters with a Mid-Volume Air Sampler (MAS-16, APM Engineering Co, Gyeonggi-do, Republic of Korea) coupled with a cyclone (URG-2000-30 EC, URG Corp., Chapel Hill, NC, USA), which operated at a flow rate of 42 L/min. The beta-ray absorption method measured hourly mass concentrations at a flow rate of 16.7 L/min and was corrected using the difference in the weight of the Teflon filter before and after sampling. To collect PM\({}_{2.5}\) from the main duct on the roof, we adjusted the cross-sectional area of the inlet to achieve isokinetic sampling conditions. The Teflon filter was subsequently processed for extraction and analysis for anions and cations. Each Teflon filter was placed in a 20 mL vial, moistened with 3 mL of MeOH, followed by the addition of 16 mL of water. Following 2 h of sonication, the sample was filtered through a polyvinylidene fluoride (PVDF) syringe filter to prepare for IC analysis. Analyzed ionic components of PM\({}_{2.5}\) included NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\). For the cation analysis, a Metrohm Metrosep C4 250/4.0 column was utilized with a 5 mM HNO\({}_{3}\) eluent. We measured gaseous concentrations using the same duct sampling inlet method as for PM\({}_{2.5}\). The concentrations of NO-NO\({}_{2}\) (42 iQ NO-NO\({}_{2}\)-NO\({}_{\rm x}\) Analyzer, Thermo, Fisher Inc., Waltham, MA, USA), CO (48 iQ CO Analyzer, Thermo, Fisher Inc., Waltham, MA, USA), CO\({}_{2}\) (410 iQ CO\({}_{2}\) Analyzer, Thermo, Fisher Inc., Waltham, MA, USA), O\({}_{3}\) (49 iQ O\({}_{3}\) Analyzer, Thermo, Fisher Inc., Waltham, MA, USA), SO\({}_{2}\) (43 iQ SO\({}_{2}\) Analyzer, Thermo, Fisher Inc., Waltham, MA, USA), and NH\({}_{3}\) (LA331-EAA, ABB-Los Gatos Research, Zurich, Switzerland) were determined. All instruments were calibrated prior to measurements. Detailed methodologies and procedures on quality assurance/quality control (QA/QC) are documented in prior publications [30; 31; 32; 33; 34; 35]. Figure 1: Schematic diagram of a HONO collection system using an ultrasonic nozzle coupled with a spray chamber and ion chromatography. Configuration and Analytical Methods for HONO Collection Using Ultrasonic Nozzle Coupled with Spray Chamber and Ion Chromatography  
The configuration of the HONO collection system using an ultrasonic nozzle is illustrated in Figure 1. Air drawn in through the inlet and duct system was first purified by a Teflon filter, which was replaced daily. Subsequently, the air was channeled into the first spray chamber and then directed to an impinger. This impinger, containing a specific volume of an absorbing solution, was designed to minimize mist carryover to subsequent stages, operating in a similar role as the gas-liquid separator found in a stripping coil [13]. The spray chamber featured a diameter of 40 mm and a height of 200 mm, with the absorbing solution being sprayed from the top while air was introduced through a side branch. The duration of sampling using the spray chamber was set to 40 min. The impinger, equipped with branches on both sides, facilitated the transport and recirculation of absorbing solutions to the chamber for repeated spraying. Absorption solution circulation was achieved through a peristaltic pump connected to the impinger, maintaining a flow rate of approximately 13.06 mL/min. This recirculation aimed to reduce the method detection limits (MDLs) to below those of previous real-time systems [9; 10; 11; 12; 13]. The spray nozzle, constructed from stainless steel with a height of 140 mm and a diameter of 66 mm, atomized the injected fluid using a vibration frequency of 120 kHz.