Evidence for Strong HONO Emission from Fertilized Agricultural Fields and its Remarkable Impact on Regional O3 Pollution in the Summer North China Plain

Summertime HONO concentrations were synchronously measured at two (agricultural and nonagricultural) sites in the North China Plain (NCP). Daytime HONO (1.4 +/- 0.6 ppbv) and HONO/NO2 ((12 +/- 8)%) over the agricultural field after fertilization were found to be remarkably higher than those before fertilization, implying strong HONO emission from the fertilized fields. Synchronous enhancements of HONO and O-3 after fertilization at both sites suggested that the emitted HONO accelerated the local and the regional O-3 pollution. HONO budget analysis further revealed that its emission was significantly enhanced after fertilization. Soil HONO emission flux and its uncertainty were estimated and discussed. The estimated emission flux exhibited a distinct diurnal variation with a noontime maximum. The net OH production rate from HONO photolysis greatly exceeded that from O-3 photolysis over the agricultural field, and their maximum ratio of 4.7 was obtained after fertilization. We provide field evidence that fertilized fields in the NCP act as a strong HONO source, which accelerates daytime photochemistry, leading to an increase of regional photo-oxidants such as O-3. Considering the severe O-3 pollution in the summer NCP and that the large area of the agricultural field is regularly treated with high fertilization amount in this region, HONO emission should be taken into account in the regional air quality deterioration.

Automated summary

In summertime, fertilized fields in the North China Plain act as a strong source of HONO, accelerating daytime photochemistry and increasing regional pollutants like O-3. This emphasizes the need to consider HONO emissions in addressing regional air quality deterioration, especially considering the severe O-3 pollution in the region.