Effect of vertical parameterization of a missing daytime source of HONO on concentrations of HONO, O3 and secondary organic aerosols in eastern China

Unexpectedly high daytime concentrations of nitrous acid (HONO) measured by field observations cannot be explained by theoretical calculations, implying that there may be a missing source of HONO in the daytime (P_missing). The value of P_missing near the ground (P_Gmissing) is different from that measured higher in the atmosphere (P_Hmissing) according to previous field studies, but the contribution of the vertical P_missing profile in the atmospheric boundary layer (ABL) to air quality remains unknown. We derived a new formula P_Gmissing = 0.180 × J(NO₂) [ppb s^-1] based on field measurements near the ground, where J(NO₂) is the photolysis frequency of NO₂, and used the value of P_Hmissing inferred from Zeppelin measurements in the troposphere to parameterize P_missing in the ABL. This parameterization was incorporated into the Weather Research and Forecasting model with Chemistry (WRF-Chem) to quantify the vertical effects of P_missing on the concentrations of HONO, O₃ and secondary organic aerosols (SOAs) in eastern China. Our results showed that P_Gmissing and P_Hmissing together further narrowed the gap between the simulations and observations, leading to a daytime increase in HONO concentrations of about 160 ppt near the ground compared with P_Gmissing only, an increase in the daytime concentrations of O₃ of 8–37 ppb within the ABL in almost all of the studied domain in summer (1–19 ppb in winter and 4–21 ppb in autumn) and the largest hourly increase in the concentration of SOAs of 22.5 (18.6) μg m^-3 in winter (summer). The results indicated that HONO sources near the ground have a limited effect on the HONO concentrations in the upper ABL even in summer in the presence of strong convective activities, while the HONO increase in the upper ABL can affect the concentration of HONO near the ground. When P_Gmissing was inserted into each model layer in the ABL, the concentrations of HONO higher in the atmosphere were substantially overestimated, suggesting that observations of the vertical distribution of HONO in the ABL are required in polluted areas.