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

Abstract:

The soot redox reaction with an uptake coefficient (γ) of 10^-4 at night and the soot heterogeneous photochemistry with the γ formula of J_NO2/10 in the daytime, where J_NO2 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, HO_x, 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, HO_x, 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 ~ three times those in the daytime.