Short communicationStrong HONO formation in a suburban site during snowy days
Introduction
The hydroxyl (OH) radical is the main oxidant of the troposphere under sunlight conditions. Nowadays, numerous studies recognize HONO photolysis as a major daytime source of OH (Stone et al., 2012 as a review; Michoud et al., 2012 for the same measurement site). The contribution of HONO photolysis to the radical budget is even larger during winter (Aumont et al., 2003). Despite its obvious importance for tropospheric chemistry, there are still uncertainties concerning HONO formation processes.
In Polar Regions, following the pioneering experiments conducted by Zhou et al. (2001) numerous studies found unexpectedly high HONO levels (see Villena et al. (2011) and references therein). Though certain measurements of low levels encountered in these regions may have suffered from interferences (Kleffmann et al., 2006), HONO production from the snow surface is suspected (see Grannas et al. (2007) as a review). Flux studies conducted over the snowpack have shown that HONO is sometimes released into the overlying atmospheric boundary layer (Zhou et al., 2001, Honrath et al., 2002) but sometimes not (Beine et al., 2005, Beine et al., 2006). The hypothesis of a photochemical snow source of HONO is also supported by investigations of HONO mixing ratios present in snowpack interstitial air and its change during shading experiments (Dibb et al., 2002). Finally, laboratory experiments conducted by irradiating natural (Beine et al., 2008, Legrand et al., 2014) or artificial (Bartels-Rausch et al., 2010) snow layers tend to support the importance of snow as a source of HONO. While it is well established that the photolysis of nitrate present in snow is the major source of released NOx (via its major channel: + hν → NO2 + O−), for HONO it is still unclear if either the nitrate photolysis directly produces HONO from hydrolysis of NO2- produced by its second channel ( + hν → + O) or if HONO is formed as a secondary product from NO2 (Villena et al., 2011). Among possible secondary production pathways it is generally accepted that the reduction of NO2 on photo-sensitized organic material like humic acid (Bartels-Rausch et al., 2010 and references therein) would proceed more efficiently than the disproportionation reaction of NO2:
Until now, very little work has been performed to evaluate the impact of an urban snowpack on atmospheric chemistry of the overlaying boundary layer. So far, this feedback has mainly been considered as affecting the energy balance and hence air mass dynamics over cities (Lemonsu et al., 2010). More recently, the study of ozone production in oil and gas extraction regions of Western US received increasing attention. An apparent paradox has been detected as the highest ozone events were observed in winter when snow was covering the ground while neither an increase of the albedo in a box model nor a doubling of the HCHO and HONO snowpack emissions could fully reproduce the measurements (Edwards et al., 2013).
At places where the snowpack is permanent, the quantification of the snow contribution to HONO budget is limited to flux studies. At places with intermittent snow layers a direct analysis of the atmospheric HONO variability would permit examination of the significance of emissions from the snowpack. Such an approach is followed in this paper by using the HONO record obtained during the MEGAPOLI winter campaign in the Paris area (Michoud et al., 2014), when ground was covered by snow during several days at the end of the campaign.
Section snippets
Experimental
Ambient measurements were conducted at the SIRTA observatory (48.718°N, 2.207°E), located 14 km south west of Paris (France) in a semi-urban environment (Haeffelin et al., 2005), during the MEGAPOLI winter campaign. This campaign took place from 15th of January to the 15th of February 2010. Nitrous acid (HONO) was measured by deploying a device (NitroMAC) based on wet chemical derivatization and HPLC detection as described in Michoud et al. (2014). During the winter campaign, ancillary
Results and discussion
HONO measurements were conducted from the 20th of January to the 15th of February 2010. During the whole measurement period HONO concentrations were highly variable ranging from few pptv to a maximum of 4 ppbv on the 13th of February (mean 24 h averaged mixing ratio of 600 pptv, see supplementary material S1).
A discussion of observed HONO levels, in particular a comparison of the observations with HONO levels calculated by Photo-Stationary State (PSS) approximation have been conducted for the
Acknowledgements
The research leading to these results has received funding from the European Union's Seventh Framework Programme FP/2007–2011 under grant agreement n°212520 and has been carried out in the framework of a thesis supported by CNRS grant.
The authors also acknowledge the ANR through the MEGAPOLI PARIS and INSU/LEFE through the MEGAPOLI France project for their financial support and Eurochamp-2 (EU-FP7 grant agreement n°228335) for supporting FIONA experiment dedicated to intercomparison of HONO
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