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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/acp-2020-244
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-244
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Apr 2020

14 Apr 2020

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A revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Measurement report: Short-term variation of ammonia concentration in an urban area: contributions of mist evaporation and emissions from a forest canopy with bird droppings

Kazuo Osada Kazuo Osada
  • GSES, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan

Abstract. Short-term variations of NH3 concentrations in the urban atmosphere are affected by local meteorological conditions and variations of natural and anthropogenic sources. To investigate potential sources and processes of NH3 variation in an urban area, hourly NH3 and NH4+ concentrations were measured from November 2017 through October 2019 in Nagoya, a megacity located in central Japan. Monthly averages of NH3 concentrations were high in summer and low in winter. Daily minimum NH3 concentrations were almost linearly correlated with daily minimum air temperature. In contrast, daily maximum NH3 concentrations revealed an exponential increase with temperature, suggesting that different processes with air temperature acted during the nighttime and daytime. Short-term increases of NH3 concentrations of two types were examined closely. The first is a rare but large increase (11 ppb for 2 hr) after mist evaporation during daytime. It is noteworthy that an event of this magnitude was identified only once during two years of observations at Nagoya even though evaporation of mist or fog droplets is expected to be frequent after rain. The second short-term increase was a large morning peak in summer. After selected days were fulfilled with non-wet and weak wind conditions, the amplitude of diurnal variation of NH3 concentration (daily maximum minus minimum) was analyzed: the amplitude was small (ca. 2 ppb) in winter but it increased from early summer along with new leaf growth. It peaked in summer (up to ca. 20 ppb) during intense addition of droppings from hundreds of crows on trees in the campus assembled before roosting. The high daily maximum NH3 concentration was characterized by a rapid increase occurring 2–4 hr after local sunrise. Daily and seasonal findings related to the morning peak implied that stomatal emission at the site was responsible for the increase. The yearly difference between daily amplitudes during the two summers was explained by the difference in the input amounts of reactive nitrogen derived from bird droppings and some rain, suggesting that the canopy of a small forest affected by the bird droppings might act as a temporary but strong source of NH3.

Kazuo Osada

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Kazuo Osada

Kazuo Osada

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Latest update: 20 Oct 2020
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Short summary
Various sources and meteorological conditions affect the short-term variation of NH3 concentrations in the urban atmosphere. Analysis of 2 years of hourly data suggests that mist evaporation and stomata exchange of tree leaves after effects of bird droppings engender a rapid increase of NH3 concentrations. Emissions from urban tree canopies are a new mode of passing reactive nitrogen that has never been described as an important source in the literature.
Various sources and meteorological conditions affect the short-term variation of NH3...
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