The formation characteristics of nitrous acid (HONO) in a central Chinese city: A novel insight from various pollution types

Abstract

Nitrous acid (HONO) is an essential precursor for hydroxyl radical (·OH), exerting a significant impact on atmospheric oxidation chemistry. This study examined the contaminative properties and main sources of HONO across four distinct pollution categories, including a non-pollution period (Period I), a haze pollution period (Period II), an ozone pollution period (Period III) and a double high pollution period (Period IV). Nighttime HONO levels were predominantly driven by the heterogeneous transformation of NO₂ into HONO, with higher reaction rates (C_HONO) during Period IV (3.5 × 10⁻² h⁻¹) and Period II (3.3 × 10⁻² h⁻¹) compared to Period I and Period III (2.0 × 10⁻² h⁻¹). This indicates that elevated PM_2.5 and NO₂ concentrations enhance HONO production, while automobile exhaust and NO homogeneous reactions had limited influence on nighttime HONO formation. Daytime HONO budget analysis revealed that the unknown source emissions (P_unknown) were substantially higher during Period IV (1.21 ppb h⁻¹) than in Period II (0.77 ppb h⁻¹) and Period III (0.57 ppb h⁻¹). Specifically, P_unknown demonstrated a more pronounced positive correlation with relative humidity (RH) (R² = 0.66) and NH₃ (R² = 0.55) during Period IV. Peak P_unknown values coincided with warmer daytime periods, when NH₃ emissions and gas-phase HONO formation intensified, enhancing atmospheric oxidation capacity and exacerbating air pollution. The study enhanced the understanding on the pollution characteristics of HONO, which supported the development of targeted policies in the Central Plains Economic Region.