Concentration Characteristics and Wavelet Analysis of Ambient NH3 in Kitakyushu, Japan over the Period of 2018–2022

The concentration of ambient ammonia gas (NH₃(g)) in Kitakyushu, Japan was determined by a passive method for four years, with a mean value of 4.7 ppb. On a weekly basis, the NH₃(g) concentration fluctuated greatly; however, by applying a low-pass filter of 1 Hz/yr, a common feature in seasonal variation, a relative maximum for one year appeared in late spring to summer, and a relative minimum appeared in late autumn to winter. The two outliers with high NH₃(g) concentrations (16.5, 10.5 ppb) were observed in July and September in the second year, 2019. By using the backward trajectory analysis as well as comparing with other air pollutants from the viewpoint of time series and correlation coefficient, the high NH₃(g) concentration was considered to be due mainly to local meteorology and emissions. Five cases in which the NH₃(g) concentration was N.D. (not detected) reflected the low level in actual atmospheric condition; the PM_2.5 concentration also decreased synchronously during corresponding period. Comprehensively taking into account not only the large fluctuation but also the evaluation of high and low NH₃(g) concentrations, a statistically significant increasing trend of ca. 0.24 ppb/yr, providing an increasing rate of ca. 5.1%/yr during the study period which was comparable to the studies by satellite data analyses, was clarified. Wavelet analysis further showed three oscillation periods in NH₃(g) concentration on the scales of 30–50 weeks (w), 60–80 w, and 85–128 w. Moreover, the strongest oscillation occurred at 111 w with the variance of 3990, and three down-up conversions for the real part of wavelet coefficients were found, providing an overall cognition of NH₃(g) concentration variation to predict further changes.