ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-6551-2010 Impacts of HONO sources on the photochemistry in Mexico City during the MCMA-2006/MILAGO Campaign Li G. 1 2 Lei W. 1 Zavala M. 1 Volkamer R. 3 4 Dusanter S. 5 Stevens P. 5 Molina L. T. 1 2 Molina Center for the Energy and the Environment, La Jolla, CA 92037, USA Massachusetts Institute of Technology, Cambridge, MA 02139, USA Department of Chemistry, University of Colorado at Boulder, Boulder, CO 80309, USA Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado at Boulder, Boulder,\newline CO 80309, USA Center for Research in Environmental Science, School of Public and Environmental Affairs, and Department of Chemistry, Indiana University, Bloomington, IN 47405, USA 16 07 2010 10 14 6551 6567 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/6551/2010/acp-10-6551-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/6551/2010/acp-10-6551-2010.pdf

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 NO<sub>2</sub> heterogeneous reaction with semivolatile organics, NO<sub>2</sub> reaction with freshly emitted soot, NO<sub>2</sub> 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<sub>x</sub> (OH+HO<sub>2</sub>) simulations during daytime and the partition of NO/NO<sub>2</sub> in the morning. The HONO sources also accelerate the accumulation of O<sub>3</sub> concentrations in the morning by about 2 h and subsequently result in a noticeable enhancement of O<sub>3</sub> 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 H<sub>2</sub>SO<sub>4</sub> from SO<sub>2</sub> reacting with OH.

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