Detailed budget analysis of HONO in central London reveals a missing daytime source

Abstract

Abstract. Measurements of HONO were carried out at an urban background site near central London as part of the Clean air for London (ClearfLo) project in summer 2012. Data were collected from 22 July to 18 August 2014, with peak values of up to 1.8 ppbV at night and non-zero values of between 0.2 and 0.6 ppbV seen during the day. A wide range of other gas phase, aerosol, radiation, and meteorological measurements were made concurrently at the same site, allowing a detailed analysis of the chemistry to be carried out. The peak HONO / NO x ratio of 0.04 is seen at ∼ 02:00 UTC, with the presence of a second, daytime, peak in HONO / NO x of similar magnitude to the night-time peak, suggesting a significant secondary daytime HONO source. A photostationary state calculation of HONO involving formation from the reaction of OH and NO and loss from photolysis, reaction with OH, and dry deposition shows a significant underestimation during the day, with calculated values being close to 0, compared to the measurement average of 0.4 ppbV at midday. The addition of further HONO sources from the literature, including dark conversion of NO 2 on surfaces, direct emission, photolysis of ortho-substituted nitrophenols, the postulated formation from the reaction of HO 2 × H 2 O with NO 2 , photolysis of adsorbed HNO 3 on ground and aerosols, and HONO produced by photosensitized conversion of NO 2 on the surface increases the daytime modelled HONO to 0.1 ppbV, still leaving a significant missing daytime source. The missing HONO is plotted against a series of parameters including NO 2 and OH reactivity (used as a proxy for organic material), with little correlation seen. Much better correlation is observed with the product of these species with j (NO 2 ), in particular NO 2 and the product of NO 2 with OH reactivity. This suggests the missing HONO source is in some way related to NO 2 and also requires sunlight. Increasing the photosensitized surface conversion rate of NO 2 by a factor of 10 to a mean daytime first-order loss of ∼ 6 ×10 −5 s −1 (but which varies as a function of j (NO 2 )) closes the daytime HONO budget at all times (apart from the late afternoon), suggesting that urban surfaces may enhance this photosensitized source. The effect of the missing HONO to OH radical production is also investigated and it is shown that the model needs to be constrained to measured HONO in order to accurately reproduce the OH radical measurements.

Diurnal fluxes of HONO above a crop rotation

“…Correlations of HONO with its precursors are expected to become worse when measurements are carried out above high trees as at the PROPHET site, which are able to fully decouple the ground surface from the air above the canopy (Sörgel et al, 2011a;Foken et al, 2012). The results from the present study are qualitatively also in good agreement with former studies in which the daytime source of HONO was quantified using the PSS approach and in which also a strong correlation of the daytime source with radiation and/or NO 2 was observed (Elshorbany et al, 2009;Sörgel et al, 2011b;Villena et al, 2011;Wong et al, 2012;Lee et al, 2016;Meusel et al, 2016). This observation may imply a mechanism of HONO formation by the reduction of NO 2 with organic photosensitizer materials like humic acids as proposed in laboratory studies (George et al, 2005;Stemmler et al, 2006Stemmler et al, , 2007Han et al, 2016).…”

“…In these studies, calculated daytime HONO sources, determined from HONO levels exceeding theoretical photostationary state (PSS) values, showed high correlations with the photolysis rate coefficient J (NO 2 ) or the irradiance and NO 2 concentration Su et al, 2008;Elshorbany et al, 2009;Sörgel et al, 2011b;Villena et al, 2011;Wong et al, 2012;Lee et al, 2016;Crilley et al, 2016). However, concentrations are not only controlled by the local ground surfaces source processes but also depend on the convective mixing in the atmosphere, leading to a potential misinterpretation of the correlation results.…”

“…During the last decades, many field measurement campaigns have reported unusually high nitrous acid (HONO) concentrations during daytime for remote (Zhou et al, 2002;Acker et al, 2006a;Sörgel et al, 2011a;Villena et al, 2011;Oswald et al, 2015;Meusel et al, 2016), semi-urban (Neftel et al, 1996;Staffelbach et al, 1997;Kleffmann et al, 2005;Su et al, 2008;Li et al, 2012;Yang et al, 2014) and urban regions (Kleffmann et al, 2002, Ren et al, 2003Acker et al, 2006b;Elshorbany et al, 2009Elshorbany et al, , 2010Hou et al, 2016;Lee et al, 2016). These results stimulated laboratory investigations on potential HONO precursors from which the most frequently discussed mechanisms are (i) the photosensitized reduction of nitrogen dioxide (NO 2 ) by organic material, e.g.…”

Higher measured than modeled ozone production at increased NO<sub><i>x</i></sub> levels in the Colorado Front Range

“…Therefore, model underprediction of HONO mixing ratios in the morning can be one cause for modeled versus measured HO 2 / OH disagreement. Lee et al (2016) indicate that, even after additional gasphase and heterogeneous HONO sources were added to MCMv331, HONO was still underestimated relative to models on average and that a missing HONO source was correlated with J NO 2 , NO 2 , and the product of NO 2 and OH reactivity for an urban area. Furthermore, only model results using measured HONO were able to replicate observed OH levels.…”

“…It is known that NO 2 adsorption onto the chamber walls can result in the heterogeneous formation of nitrous acid (HONO) through the reaction of NO 2 with water vapor ad-sorbed onto surfaces (Finlayson-Pitts et al, 2003); a photolytic HONO source has also been previously reported (Rohrer et al, 2005;George et al, 2005;Stemmler et al, 2006;Langridge et al, 2009;Lee et al, 2016;Crilley et al, 2016). During the 2013 DISCOVER-AQ study in Houston, TX, excess HONO of up to 5 times ambient values was measured in the MOPS chambers, which can thus create excess OH and positively bias the MOPS P(O 3 ).…”