Abstract
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 HOx (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.
Highlights
Nitrous acid (HONO) plays an important role in the photochemistry of the troposphere, especially in the polluted urban atmosphere, because HONO is a significant photochemical precursor of the hydroxyl radical (OH), the key oxidant in the degradation of most air pollutants (e.g., Jenkin et al, 1988; Calvert et al, 1994; Aumont et al, 2003; Stemmler et al, 2006; Volkamer et al, 2007)
Two LP-Differential Optical Absorption Spectroscopy (DOAS) instruments were deployed at a mean height of 16 m and 70 m above ground at CENICA during MCMA-2003 (Volkamer et al, 2007)
If measurements are compared at a comparable mean height of the LPDOAS light path above ground, the mean HONO diurnal profiles during MCMA-2003 and the reference case agree within 10% at night
Summary
Nitrous acid (HONO) plays an important role in the photochemistry of the troposphere, especially in the polluted urban atmosphere, because HONO is a significant photochemical precursor of the hydroxyl radical (OH), the key oxidant in the degradation of most air pollutants (e.g., Jenkin et al, 1988; Calvert et al, 1994; Aumont et al, 2003; Stemmler et al, 2006; Volkamer et al, 2007). During the MCMA-2006 field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project in March 2006, an extensive data set was obtained, including highly time-resolved ambient gas phase species and aerosols (Molina et al, 2008; 2010), which provides a unique opportunity to investigate the impacts of HONO sources on the photochemistry in a polluted urban area. The objective of the present study is to examine the relative importance of different HONO sources and evaluate their contributions to the HOx/NOx/O3 chemistry and secondary aerosols using the WRF-CHEM model based on the measurements taken during MCMA-2006 field campaign.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
