Ammonia (NH
3) in ambient air was measured at Cape Hedo, Okinawa, in spring 2008 using a photo-acoustic spectroscopy method (TGA310, Omnisens). Average concentration and standard deviation (1σ) of NH
3 was 0.56±0.50 ppbv. It is considered that the variation in NH
3 concentration does not come from the long range transport. The concentration and standard deviation (1σ) of ammonium ion (NH
4+) measured using an aerosol mass spectrometer (Q-AMS, Aerodyne) was 2.2±1.4 µgm
-3 and NH
4+ was more than 80% for NH
3+NH
4+ when NH
4+ was transported over long-range associated with sulfate (SO
42-). NH
4+ was deficient when SO
42-/(SO
2+SO
42-) was large in air masses.
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