Exploring the HONO source during the COVID-19 pandemic in a megacity in China

HONO concentrations were measured using an atmospheric HONO online detector (TiH300, Focused Photonics., China). An automatic weather station (QXZ1.0, Yigu Technologies, China) was used to observe temperature, relative humidity, wind direction, and wind speed. PM_2.5 was analyzed using a tapered element oscil-lating microbalance (Model 1400a, Thermo Electron, USA). Ozone (O₃) was monitored using UV absorption (TE, Model 49i, Waltham, USA), while carbon monoxide (CO) was quantified using a gas filter correlation/non-dispersive infrared analyzer (TE, Model 48i, Waltham,USA). Nitrogen oxides (NO-NO₂-NO_x) were measured using an ammonia analyzer (AQMS-650, Focused Photonics, China). Photolysis rates of HCHO, NO₂, H₂O₂, NO–3, and HONO were measured using an atmospheric photolysis rate analyzer (PFS-100, Focused Photonics, China). An ambient ion monitor (URG-9000D, Thermo Fisher Scientific, USA) was used to obtain the hourly mass concentrations of NO–3.