Impacts of HONO sources on the photochemistry in Mexico City during the MCMA-2006/MILAGO Campaign

Abstract. The contribution of HONO sources to the photochemistry in Mexico City is investigated during the MCMA-2006/MILAGO Campaign using the WRF-CHEM model. Besides the homogeneous reaction of NO with OH, four additional HONO sources are considered in the WRF-CHEM model: secondary HONO formation from NO₂ heterogeneous reaction with semivolatile organics, NO₂ reaction with freshly emitted soot, NO₂ heterogeneous reaction on aerosol and ground surfaces. The WRF-CHEM model with the five HONO sources performs reasonably well in tracking the observed diurnal variation of HONO concentrations. The HONO sources included are found to significantly improve the HO_x (OH+HO₂) simulations during daytime and the partition of NO/NO₂ in the morning. The HONO sources also accelerate the accumulation of O₃ concentrations in the morning by about 2 h and subsequently result in a noticeable enhancement of O₃ concentrations over the course of the day with a midday average of about 6 ppb. Furthermore, these HONO sources play a very important role in the formation of secondary aerosols in the morning. They substantially enhance the secondary organic aerosol concentrations by a factor of 2 on average in the morning, although they contribute less during the rest of the day. The simulated particle-phase nitrate and ammonium are also substantially enhanced in the morning when the four HONO sources are included, in good agreement with the measurements. The impact of the HONO sources on the sulfate aerosols is negligible because of the inefficient conversion of H₂SO₄ from SO₂ reacting with OH.