Modeling of daytime HONO vertical gradients during SHARP 2009

Abstract

Nitrous acid (HONO) acts as a major precursor of the hydroxyl radical (OH) in the urban atmospheric boundary layer in the morning and throughout the day. Despite its importance, HONO formation mechanisms are not yet completely understood. It is generally accepted that conversion of NO₂ on surfaces in the presence of water is responsible for the formation of HONO in the nocturnal boundary layer, although the type of surface on which the mechanism occurs is still under debate. Recent observations of higher than expected daytime HONO concentrations in both urban and rural areas indicate the presence of unknown daytime HONO source(s). Various formation pathways in the gas phase, and on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO. However, it is unclear which mechanism dominates and, in the cases of heterogeneous mechanisms, on which surfaces they occur. <br><br> Vertical concentration profiles of HONO and its precursors can help in identifying the dominant HONO formation pathways. In this study, daytime HONO and NO₂ vertical profiles, measured in three different height intervals (20–70, 70–130, and 130–300 m) in Houston, TX, during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP) are analyzed using a one-dimensional (1-D) chemistry and transport model. Model results with various HONO formation pathways suggested in the literature are compared to the the daytime HONO and HONO/NO₂ ratios observed during SHARP. The best agreement of HONO and HONO/NO₂ ratios between model and observations is achieved by including both a photolytic source of HONO at the ground and on the aerosol. Model sensitivity studies show that the observed diurnal variations of the HONO/NO₂ ratio are not reproduced by the model if there is only a photolytic HONO source on aerosol or in the gas phase from NO₂^* + H₂O. Further analysis of the formation and loss pathways of HONO shows a vertical dependence of HONO chemistry during the day. Photolytic HONO formation at the ground is the major formation pathway in the lowest 20 m, while a combination of gas-phase, photolytic formation on aerosol, and vertical transport is responsible for daytime HONO between 200–300 m a.g.l. HONO removal is dominated by vertical transport below 20 m and photolysis between 200–300 m a.g.l

Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010

“…These HONO (g) formation mechanisms are consistent with field observations that suggest a relationship between the strength of the daytime HONO (g) source and the product of NO 2 and surface irradiance [Li et al, 2012;Ren et al, 2011;Wong et al, 2012]. These field observations also suggest that the NO 2 heterogeneous reactions described above can only account for a fraction of the required HONO (g) production [Sörgel et al, 2011b;Wong and Stutz, 2010;Wong et al, 2011;Wong et al, 2012].…”

“…In fact, some points where the AIM-IC measures elevated mixing ratio equivalents of NO 2 À (p) are also periods where it measures more HONO (g) than the SC-AP instrument. This is expected if the ground surface is dominating conversion of NO 2 to HONO (g) at night, as vertical gradients have been observed to develop at the surface, and HONO (g) mixing ratios decrease with altitude VandenBoer et al, 2013;Villena et al, 2011;Vogel et al, 2003;Wong et al, 2011;Wong et al, 2012;Zhang et al, 2009]. To the extent that the AIM-IC and SC-AP were measuring air containing comparable levels of HONO (g) , Figure 1b further suggests that the PM 2.5 particle nitrite measurements from the AIM-IC are not the result of HONO (g) breakthrough from the gas channel.…”

“…In other field observations, the photolysis of NO 3 À [Handley et al, 2007;Schuttlefield et al, 2008;Zhou et al, 2003;Zhou et al, 2011], photocatalytic conversion of NO 2 on humic and organic surfaces , or a link between measured NO 2 and surface irradiance [Wong et al, 2012] are suggested as likely pathways to HONO (g) daytime production. At most, 30% of the daytime HONO (g) source has been accounted for at the measurement height, dominated by the conversion of NO 2 on photoexcited humic acids [Sörgel et al, 2011b].…”

Nitrous acid formation in a snow-free wintertime polluted rural area

“…RCAT8.2 is a highly resolved one-dimensional chemistry and transport model that has been used to investigate the impact of nocturnal processes and daytime chemistry of HONO and NO 3 (e.g., Wong and Stutz, 2010;Wong et al, 2013;Tsai et al, 2014). It is based on the gas-phase Regional Atmospheric Chemistry Mechanism (RACM), which contains 77 species, 237 reactions, and aggregates atmospheric VOCs into 23 classes: four alkanes, four alkenes, three biogenic, three aromatics, and nine carbonyls (Stockwell et al, 1997).…”

Impact of updated traffic emissions on HONO mixing ratios simulated for urban site in Houston, Texas

“…Emissions of HONO from traffic were estimated by Kirchstetter et al (1996) and Kurtenbach et al (2001), who performed tunnel studies and reported exhaust emission ratio of HONO to NO x in a range of 0.003-0.008. The value of 0.008 is used in the Community Multiscale Air Quality (CMAQ) model to calculate HONO emissions from mobile sources (Foley et al, 2010) as well as in other models, for example, in a box model employed to study HONO sources in Houston (Wong et al, 2013). The relative contribution of HONO emissions from traffic to other sources when using the HONO to NO x ratio of 0.008 is about 9 % based on simulations for eastern US (Sarwar et al, 2008).…”