Nitrous acid formation in a snow-free wintertime polluted rural area
Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH) in the lower atmosphere, in particular in winter when other OH sources are less efficient. The nighttime formation of HONO and its photolysis in the early morning have long been recognized as an important contributor...
| Main Authors: | , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/1977/2018/acp-18-1977-2018.pdf |
| Summary: | Nitrous acid (HONO) photolysis is an important source of
hydroxyl radicals (OH) in the lower atmosphere, in particular in winter when
other OH sources are less efficient. The nighttime formation of HONO and its
photolysis in the early morning have long been recognized as an important
contributor to the OH budget in polluted environments. Over the past few
decades it has become clear that the formation of HONO during the day is an
even larger contributor to the OH budget and additionally provides a pathway
to recycle NO<sub><i>x</i></sub>. Despite the recognition of this unidentified HONO
daytime source, the precise chemical mechanism remains elusive. A number of
mechanisms have been proposed, including gas-phase, aerosol, and ground
surface processes, to explain the elevated levels of daytime HONO. To
identify the likely HONO formation mechanisms in a wintertime polluted rural
environment we present LP-DOAS observations of HONO, NO<sub>2</sub>, and
O<sub>3</sub> on three absorption paths that cover altitude intervals from 2 to
31, 45, and 68 m above ground level (a.g.l.) during the UBWOS 2012
experiment in the Uintah Basin, Utah, USA. Daytime HONO mixing ratios in the
2–31 m height interval were, on average, 78 ppt, which is lower than HONO
levels measured in most polluted urban environments with similar NO<sub>2</sub>
mixing ratios of 1–2 ppb. HONO surface fluxes at 19 m a.g.l., calculated
using the HONO gradients from the LP-DOAS and measured eddy diffusivity
coefficient, show clear upward fluxes. The hourly average vertical HONO flux
during sunny days followed solar irradiance, with a maximum of
(4.9 ± 0.2) × 10<sup>10</sup> molec. cm<sup>−2</sup> s<sup>−1</sup> at
noontime. A photostationary state analysis of the HONO budget shows that the
surface flux closes the HONO budget, accounting for 63 ± 32 % of the
unidentified HONO daytime source throughout the day and 90 ± 30 %
near noontime. This is also supported by 1-D chemistry and transport model
calculations that include the measured surface flux, thus clearly identifying
chemistry at the ground as the missing daytime HONO source in this
environment. Comparison between HONO surface flux, solar radiation, NO<sub>2</sub>
and HNO<sub>3</sub> mixing ratios, and results from 1-D model runs suggest that,
under high NO<sub><i>x</i></sub> conditions, HONO formation mechanisms related to
solar radiation and NO<sub>2</sub> mixing ratios, such as photo-enhanced conversion
of NO<sub>2</sub> on the ground, are most likely the source of daytime HONO. Under
moderate to low NO<sub>2</sub> conditions, photolysis of HNO<sub>3</sub> on the ground
seems to be the main source of HONO. |
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| ISSN: | 1680-7316 1680-7324 |