Abstract
Abstract. Local meteorological conditions and natural and anthropogenic sources affect atmospheric NH3 concentrations in urban areas. To investigate potential sources and processes of NH3 variation in urban areas, hourly NH3 and NH4+ concentrations were measured during November 2017–October 2019 in Nagoya, a central Japanese megacity. Average NH3 concentrations are high in summer and low in winter. Daily minimum NH3 concentrations are linearly correlated with daily minimum air temperatures. By contrast, daily maximum NH3 concentrations increase exponentially with temperature, suggesting that different nighttime and daytime processes and air temperatures affect concentrations. Short-term increases in NH3 concentrations of two types were examined closely. Infrequent but large increases (11 parts per billion (ppb) for 2 h) occurred after mist evaporation during daytime. During 2 years of observations, only one event of this magnitude was identified in Nagoya, although evaporation of mist and fog occurs frequently after rains. Also, short-term increases occur with a large morning peak in summer. Amplitudes of diurnal variation in NH3 concentration (daily maximum minus minimum) were analyzed on days with nonwet and low wind conditions. Amplitudes were small (ca. 2 ppb) in winter, but they increased from early summer along with new leaf growth. Amplitudes peaked in summer (ca. 20 ppb) because of droppings from hundreds of crows before roosting in trees on the campus. High daily maximum NH3 concentrations were characterized by a rapid increase occurring 2–4 h after local sunrise. In summer, peak NH3 concentrations at around 08:00 local time (LT) in sunny weather were greater than in cloudy weather, suggesting that direct sunlight particularly boosts the morning peak. Daily and seasonal findings related to the morning peak imply that stomatal emission at the site causes the increase. Differences between daily amplitudes during the two summers was explained by the different input amounts of reactive nitrogen from bird droppings and rain, suggesting that bird droppings, a temporary rich source of NH3, affected the small forest canopy.
Highlights
Ammonia (NH3) plays an important role in various atmospheric chemical processes (Behera et al, 2013)
A surge event (11 ppb during 2 h) was observed after mist evaporation during daytime, which was very rare at Nagoya, even though the evaporation of mist or fog droplets is expected to be frequent after rain
Large diurnal variation in NH3 concentration was characterized by a peak at 2–4 h after sunrise
Summary
Ammonia (NH3) plays an important role in various atmospheric chemical processes (Behera et al, 2013). A mixture of various small nonagricultural sources is expected to be the main contributor to local atmospheric NH3, which potentially acts as a precursor of aerosol particles. Hu et al (2014) reported green spaces in downtown Toronto, Canada, as being a potential source of ambient NH3, based on analyses of local, regional, and temporal variations in NH3 concentrations. The data were analyzed by examining various timescales and the amplitude of diurnal variation in relation to potential reactive nitrogen sources and plant physiology near the site. These data were expected to elucidate the effects of large amounts of bird droppings on ambient NH3 concentrations in urban areas with scattered forests
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