Quantifying the Nitrogen Sources and Secondary Formation of Ambient HONO with a Stable Isotopic Method

Emission characteristics of indoor HONO from residential natural gas cooking stoves in a household in Kunming, China

Nitrous acid (HONO), a key precursor of hydroxyl radicals, significantly impacts outdoor and indoor air chemistry and poses health risks. Gas stove combustion is a major indoor HONO source, yet its emission factors (EFs) remain poorly quantified in China. This study quantified HONO emissions from natural gas (NG) stove combustion in a Kunming residential kitchen, yielding a HONO/NOx ratio of 5.8 ± 1.5 (1σ) % and a HONO/NG EF of 55.6 ± 13.8 mg m⁻³ . Estimated 2023 HONO emissions in Kunming are 17.9 ± 4.4 tons, with urban emission intensities comparable to traffic-related HONO emissions. Just a few minutes of cooking can elevate HONO concentrations to over 100 ppb and NOx concentrations to over 1000 ppb. Flame temperatures were 695.8 ± 22.6 °C (low flow) and 819.1 ± 7.2 °C (high flow), with no significant difference (p > 0.05) in HONO/NOx or HONO/NG ratios. Nitrogen in NOx and HONO likely originates from N₂ conversion through combustion radical chemistry. Three HONO reduction strategies-alkaline cleaning (30-60 % reduction), induction cookers (100 % reduction), and range hoods combining ventilation (>90 % reduction)-were evaluated, with benefits and limitations discussed. Uncertainties related to the reservoir effect, spatial heterogeneity, air exchange, and NO₂ heterogeneous reactions on HONO EFs were addressed. NO₂ heterogeneous reactions and air exchange contributed ∼20 % to peak HONO during combustion, with most HONO from primary emissions. These findings provide insights into indoor HONO emissions and highlight mitigation strategies to improve indoor air quality and public health.

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56-4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1-242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9-36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity.