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

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 HOx (OH+HO2) simulations during daytime and the partition of NO/NO2 in the morning. The HONO sources also accelerate the accumulation of O-3 concentrations in the morning by about 2 h and subsequently result in a noticeable enhancement of O-3 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.