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| Titel |
The relationship between nighttime formation of gaseous HONO and nocturnal stability in an urban environment |
| VerfasserIn |
Robert McLaren, Patryk Wojtal, Peter Taylor |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099635
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| Publikation (Nr.) |
 EGU/EGU2014-15440.pdf |
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| Zusammenfassung |
| Nitrous acid (HONO) is an important radical precursor in the troposphere that accumulates
overnight giving rise to a significant photolytic production of the hydroxyl radical, OH, in the
boundary layer during early morning hours the next day. It is understood that HONO is
formed in the dark through the heterogeneous hydrolysis of NO2 on surfaces (2 NO2+ H2O
-> HONO + HNO3) in a first order process, largely dominated by hydrolysis on ground
surfaces and a smaller contribution from aerosol surfaces. Despite progress, the dark
heterogeneous mechanism of HONO formation is still not well understood, mirroring our
lack of consensus on the daytime production of HONO. We have measured HONO at night in
an urban area (York University, Toronto, Canada) by DOAS for over one year. This
rich dataset was analyzed with a view to understanding the nocturnal formation
mechanism, and possible links to the daytime HONO formation mechanism. Frequently,
"steady-states" of HONO are observed at night; d[HONO]/dt ~ 0, which follow
after a rapid buildup of HONO during sunset at rates of several ppb hr-1. These
steady-state levels of HONO are found to be independent of the mixing ratio of NO2
throughout the night. On other occasions, steady-states are not observed and HONO
continues to increase throughout the night, highly correlated with the levels of NO2
d([HONO]/[NO2])/dt ~ 0). We have found that a very significant predictor of the type of
behavior is the nocturnal stability of the atmosphere, measured by the thermal gradient,
ΔT=T9.5m-T1.0m, and wind speed. The steady-state behavior is found to occur
almost exclusively on unstable nights with higher wind speeds and ΔT ~ 0, when
mixing of air in the lower atmosphere is more efficient. The non steady-state behavior
of HONO is observed on stable nights with low wind speeds and large thermal
gradients, ΔT > 2oC indicating limited vertical mixing. The observation of NO2
independent steady-states of HONO under conditions of efficient nocturnal mixing
suggest that the steady-state may result from an equilibrium between a surface
reservoir source of HONO, reminiscent of those observed in the polluted marine
boundary layer at night (Wojtal et al., Atmos. Chem. Phys., 11, 3243-3261, 2011). |
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