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<p>Local meteorological conditions and natural and anthropogenic sources affect atmospheric <span class="inline-formula">NH₃</span> concentrations in urban areas. To investigate potential sources and processes of <span class="inline-formula">NH₃</span> variation in urban areas, hourly <span class="inline-formula">NH₃</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="dd01b19a584d9ff35339d41174090f98"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-11941-2020-ie00001.svg" width="24pt" height="15pt" src="acp-20-11941-2020-ie00001.png"/></svg:svg></span></span> concentrations were measured during November 2017–October 2019 in Nagoya, a central Japanese megacity. Average <span class="inline-formula">NH₃</span> concentrations are high in summer and low in winter. Daily minimum <span class="inline-formula">NH₃</span> concentrations are linearly correlated with daily minimum air temperatures. By contrast, daily maximum <span class="inline-formula">NH₃</span> concentrations increase exponentially with temperature, suggesting that different nighttime and daytime processes and air temperatures affect concentrations. Short-term increases in <span class="inline-formula">NH₃</span> concentrations of two types were examined closely. Infrequent but large increases (11 parts per billion (ppb) for 2&thinsp;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 <span class="inline-formula">NH₃</span> concentration (daily maximum minus minimum) were analyzed on days with nonwet and low wind conditions. Amplitudes were small (ca. 2&thinsp;ppb) in winter, but they increased from early summer along with new leaf growth. Amplitudes peaked in summer (ca. 20&thinsp;ppb) because of droppings from hundreds of crows before roosting in trees on the campus. High daily maximum <span class="inline-formula">NH₃</span> concentrations were characterized by a rapid increase occurring 2–4&thinsp;h after local sunrise. In summer, peak <span class="inline-formula">NH₃</span> 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 <span class="inline-formula">NH₃</span>, affected the small forest canopy.</p>