The Differences in the Formation and Source Characteristics of Nitrous Acid (Hono)In a Central Chinese City: A Novel Insight from Various Pollution Types

Abstract

The compound nitrous acid (HONO) acts as an essential precursor for the hydroxyl radical (·OH), exerting a notable impact on atmospheric oxidation chemistry. This study analyzed the pollution characteristics of HONO and its main sources during severe air pollution periods at different pollution levels, 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). The elevation in nighttime HONO levels was mainly attributed to the transformation of NO2 into HONO through non-uniform reactions. The NO2 heterogeneous reaction efficiency CHONO exhibited higher values 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 indicated that as PM2.5 levels increased, the elevation in NO2 concentration levels enhanced the efficiency of NO2 conversion to HONO, while automobile exhaust emissions and NO homogeneous reactions did not contribute significantly to nighttime HONO formation. According to the daytime HONO budget analysis, the primary source of HONO was the homogeneous reaction of NO and ·OH, with a substantial contribution from unknown sources. Notably, the average value of Punknown during Period Ⅰ (3.69 ppb·h-1) was considerably higher than Period Ⅱ (0.77 ppb·h-1), Period Ⅲ (0.57 ppb·h-1), and Period Ⅳ (1.21 ppb·h-1). Specifically, Punknown demonstrated a more pronounced positive correlation with RH (R2=0.66) and NH3 (R2=0.55) during Period Ⅳ. The peak values of Punknown during pollution periods mainly occurred during the warmer time of the day, coinciding with NH3 readily entering the air. This accelerated the formation of HONO in the gas phase, enhancing atmospheric oxidation capacity and exacerbating air pollution.

Keywords: Nitrous acid, Haze pollution, Ozone pollution, Double high pollution, Unknown source analysis