Uncertainty in the uptake coefficient for HONO formation on soot and its impacts on concentrations of major chemical components in the Beijing-Tianjin-Hebei region

- Simulations support the soot heterogeneous photochemistry mechanism in the daytime.
- Simulations support the soot redox mechanism at night.
- The order of magnitude of the uptake coefficient for HONO formation on soot is likely 10−4.
- The surface areas of major aerosol components at night are ∼3 times those in the daytime.
- The nocturnal soot surface area shows a sharp decrease from near the ground to ∼200 m.

The soot redox reaction with an uptake coefficient (γ) of 10−4 at night and the soot heterogeneous photochemistry with the γ formula of JNO2/10 in the daytime, where JNO2 denotes the NO2 photolysis frequency, were inserted into the WRF-Chem model to evaluate the effects of γ changes of several orders of magnitude on the concentrations of HONO, HOx, O3, NO3−, and NH4+ in the Beijing-Tianjin-Hebei (BTH) region. Results demonstrated that the simulations in the BTH region support the redox mechanism at night and the heterogeneous photochemistry mechanism in the daytime for HONO formation, and suggested that the order of magnitude of γ is likely to be 10−4. Minor increases in HONO, HOx, O3, NO3−, and NH4+ were produced when γ was decreased to ≤10−5, whereas the concentrations of HONO and NO3− were substantially overestimated in most cases when γ was increased to 10−3. The soot surface area per unit volume of air showed a sharp decrease from 3 × 103 μm2 cm−3 near the ground to 6 × 102 μm2 cm−3 at ∼200 m at night. The surface area fractions of soot, nitrate, and sulfate showed minor variations between day and night, although the surface areas of the major aerosol components at night were approximately three times those in the daytime.