Characteristics of ammonia gas and fine particulate ammonium from two distinct urban areas: Osaka, Japan, and Ho Chi Minh City, Vietnam.

Continuous and simultaneous measurements of ammonia gas (NH ₃ ) and fine particulate ammonium (PM _2.5 NH ₄ ⁺ ) were performed in two distinct urban areas: Osaka, Japan, and Ho Chi Minh City (HCMC), Vietnam. Measurements were performed using a new online instrument. Two measurement periods were conducted during February 11-March 12, 2015 (cold period), and July 1-September 14, 2015 (warm period), at the urban site in Osaka, while 17 days of measurements, from May 21 to June 8, 2015, were conducted at the urban site in HCMC. The average NH ₃ concentration at the HCMC site was much higher than that at the Osaka site. The differences in the NH ₃ levels between the two cities are a result of their different emission sources. Traffic emission is a significant contributor to the NH ₃ levels within the urban area in Osaka. Conversely, the contribution of traffic emission to the NH ₃ levels in the HCMC urban area is negligible. With a population of around 8.5 million people living in the urban area of HCMC, the high NH ₃ level is due to human sources and poor waste management systems, especially because of the high temperature (30 °C) and dense population of the city (density up to 42,000 inhabitants per km ² ). In contrast to the NH ₃ levels, the highest PM _2.5 NH ₄ ⁺ level occurred during the cold period at the Osaka site, and the average level at this site was higher than that at the HCMC site. The availability of atmospheric acids, low temperature, and high humidity facilitates the formation of ammonium. Our results indicate that NH ₃ plays a key role in secondary inorganic aerosol formation; therefore, it contributes to a significant amount of PM _2.5 at the Osaka site. In contrast, the high levels of PM _2.5 observed at the HCMC site are likely from road traffic emission, mainly motorcycles, rather than secondary inorganic aerosol formation.