Impacts of an unknown daytime HONO source on the mixing ratio and budget of HONO, and hydroxyl, hydroperoxyl, and organic peroxy radicals, in the coastal regions of China
Many field experiments have found high nitrous acid (HONO) mixing ratios in both urban and rural areas during daytime, but these high daytime HONO mixing ratios cannot be explained well by gas-phase production, HONO emissions, and nighttime hydrolysis conversion of nitrogen dioxide (NO<sub>2&l...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/9381/2015/acp-15-9381-2015.pdf |
| Summary: | Many field experiments have found high nitrous acid (HONO) mixing ratios in
both urban and rural areas during daytime, but these high daytime HONO
mixing ratios cannot be explained well by gas-phase production, HONO
emissions, and nighttime hydrolysis conversion of nitrogen dioxide
(NO<sub>2</sub>) on aerosols, suggesting that an unknown daytime HONO source
(<i>P</i><sub>unknown</sub>) could exist. The formula <i>P</i><sub>unknown</sub> ≈ 19.60[NO<sub>2</sub>] ·
<i>J</i>(NO<sub>2</sub>) was obtained using observed data from 13 field experiments across
the globe. The three additional HONO sources (i.e., the <i>P</i><sub>unknown</sub>,
nighttime hydrolysis conversion of NO<sub>2</sub> on aerosols, and HONO emissions)
were coupled into the WRF-Chem model (Weather Research and Forecasting model
coupled with Chemistry) to assess the <i>P</i><sub>unknown</sub> impacts on the
concentrations and budgets of HONO and peroxy (hydroxyl, hydroperoxyl, and
organic peroxy) radicals (RO<sub><i>x</i></sub>) (= OH + HO<sub>2</sub> + RO<sub>2</sub>) in
the coastal regions of China. Results indicated that the additional HONO
sources produced a significant improvement in HONO and OH simulations,
particularly in the daytime. High daytime average <i>P</i><sub>unknown</sub> values were
found in the coastal regions of China, with a maximum of 2.5 ppb h<sup>−1</sup> in
the Beijing–Tianjin–Hebei region. The <i>P</i><sub>unknown</sub> produced a
60–250 % increase of OH, HO<sub>2</sub>, and RO<sub>2</sub> near the ground in the
major cities of the coastal regions of China, and a 5–48 % increase
of OH, HO<sub>2</sub>, and RO<sub>2</sub> in the daytime meridional-mean mixing ratios
within 1000 m above the ground. When the three additional HONO sources were
included, the photolysis of HONO was the second most important source in the
OH production rate in Beijing, Shanghai, and Guangzhou before 10:00 LST with
a maximum of 3.72 ppb h<sup>−1</sup> and a corresponding <i>P</i><sub>unknown</sub>
contribution of 3.06 ppb h<sup>−1</sup> in Beijing, whereas the reaction of
HO<sub>2</sub> + NO (nitric oxide) was dominant after 10:00 LST with a maximum
of 9.38 ppb h<sup>−1</sup> and a corresponding <i>P</i><sub>unknown</sub>
contribution of 7.23 ppb h<sup>−1</sup> in Beijing. The whole RO<sub><i>x</i></sub> cycle was accelerated by the
three additional HONO sources, especially the <i>P</i><sub>unknown</sub>. The daytime
average OH production rate was enhanced by 0.67 due to the three additional
HONO sources; [0.64], due to the <i>P</i><sub>unknown</sub>, to 4.32 [3.86] ppb h<sup>−1</sup>, via
the reaction of HO<sub>2</sub> + NO, and by 0.49 [0.47] to 1.86 [1.86] ppb h<sup>−1</sup>, via the photolysis of HONO. The OH daytime average loss rate
was enhanced by 0.58 [0.55] to 2.03 [1.92] ppb h<sup>−1</sup>, via the reaction of
OH + NO<sub>2</sub>, and by 0.31 [0.28] to 1.78 [1.64] ppb h<sup>−1</sup>, via the
reaction of OH + CO (carbon monoxide) in Beijing, Shanghai, and Guangzhou.
Similarly, the three additional HONO sources produced an increase of 0.31
[0.28] (with a corresponding <i>P</i><sub>unknown</sub> contribution) to
1.78 [1.64] ppb h<sup>−1</sup>, via the reaction of OH + CO, and 0.10 [0.09] to 0.63 [0.59] ppb h<sup>−1</sup>, via the reaction of CH<sub>3</sub>O<sub>2</sub> (methylperoxy
radical) + NO in the daytime average HO<sub>2</sub> production rate, and 0.67 [0.61] to 4.32
[4.27] ppb h<sup>−1</sup>, via the reaction of HO<sub>2</sub> + NO in the daytime
average HO<sub>2</sub> loss rate in Beijing, Shanghai, and Guangzhou. The above
results suggest that the <i>P</i><sub>unknown</sub> considerably enhanced the RO<sub><i>x</i></sub>
concentrations and accelerated RO<sub><i>x</i></sub> cycles in the coastal regions of
China, and could produce significant increases in concentrations of
inorganic aerosols and secondary organic aerosols and further aggravate haze
events in these regions. |
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| ISSN: | 1680-7316 1680-7324 |