A comparison of HONO budgets for two measurement heights at a field station within the boreal forest in Finland

Abstract

Abstract. Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, significantly exceed the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. This study presents budget calculations of HONO based on simultaneous measurements of all relevant species, including HONO and OH at two different measurement heights, i.e. 1 m above the ground and about 2 to 3 m above the canopy (24 m above the ground), conducted in a boreal forest environment. We observed mean HONO concentrations of about 6.5 × 10 8 molecules cm −3 (26 ppt) during daytime, more than 20 times higher than expected from the PSS of 0.2 × 10 8 molecules cm −3 (1 ppt). To close the budgets at both heights, a strong additional source term during daytime is required. This unidentified source is at its maximum at noon (up to 1.1 × 10 6 molecules cm −3 s −1 , 160 ppt h −1 ) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO 3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.

Diurnal fluxes of HONO above a crop rotation

“…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. humic acids (George et al, 2005;Stemmler et al, 2006;2007, Sosedova et al, 2011Han et al, 2016), (ii) the photolysis of adsorbed nitric acid (Zhou et al, , 2011Laufs and Kleffmann, 2016), (iii) biogenic production of nitrite in soil (Su et al, 2011;Ostwald et al, 2013;Maljanen et al, 2013;Oswald et al, 2015;Scharko et al, 2015;Weber, 2015) and (iv) release of adsorbed HONO from soil surfaces after deposition of strong acids (VandenBoer et al, 2013(VandenBoer et al, , 2014(VandenBoer et al, , 2015Donaldson et al, 2014). Another discussed source, the reaction of excited gaseous NO 2 with water (Li et al, 2008), is of minor importance as demonstrated by laboratory (Crowley and Carl, 1997;Carr et al, 2009;Amedro et al, 2011) and modelling studies (Sörgel et al, 2011b;Czader et al, 2012).…”

“…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.…”

Emission of nitrous acid from soil and biological soil crusts represents an important source of HONO in the remote atmosphere in Cyprus

“…This can be ruled out in this study, as pure air (no NO 2 ) was used to purge the chambers and no light was applied. While in Cyprus the observed soil emissions can explain high amounts of atmospheric HONO, other studies excluded soil emission as a dominant source (Oswald et al, 2015;Laufs et al, 2017). Oswald et al (2015) studied soil samples from a boreal forest in Finland and observed HONO emissions below the detection limit.…”

“…While in Cyprus the observed soil emissions can explain high amounts of atmospheric HONO, other studies excluded soil emission as a dominant source (Oswald et al, 2015;Laufs et al, 2017). Oswald et al (2015) studied soil samples from a boreal forest in Finland and observed HONO emissions below the detection limit. However, those samples had very low nutrient contents and were highly acidic (pH ≈ 3), for which microbial activity is supposed to be low (Fierer and Jackson, 2006;Persson and Wirén, 1995;Ste-Marie and Pare, 1999;Šimek and Cooper, 2002).…”

“…Currently, the HONO formation processes, especially during daytime, are still not fully understood. Recent ground-based field measurements showed unexpectedly high daytime concentrations of HONO, which could not be solely explained by atmospheric gas-phase reactions (R1-R3) Su et al, 2008a;Sörgel et al, 2011a;Su et al, 2011;Michoud et al, 2014;Czader et al, 2012;Wong et al, 2013;Tang et al, 2015;Oswald et al, 2015, Meusel et al, 2016 …”