Evidence of a Large Bias in Rooftop Measurements of Atmospheric Ammonia

In contrast to Europe and the U.S., human excreta in urban China are mainly stored in septic tanks beneath buildings, and the generated odors (including NH3) are released into the atmosphere through connecting pipes on the rooftops, that is, ceiling ducts. Here, we show that the levels of NH3 in ceiling ducts were 3 to 5 orders of magnitude higher than urban background concentrations. Unfortunately, urban air quality monitoring sites in China are mostly set on the roofs of various buildings. As the first attempt to investigate the influence of this overlooked NH3 source on the measurements of ambient NH3, hourly measurements of NH3 were performed on the rooftops of four buildings in Nanjing and Shanghai megacities. Although the two buildings in Nanjing are located in the same university campus (1 km apart), NH3 concentrations measured on the rooftop of a teaching building with dense ceiling ducts were on average 2.8 times higher than those of another building without ceiling ducts. In Shanghai, the exhaust vents of two ceiling ducts at a supersite were deliberately extended to a higher position and bent toward a direction downwind of our NH3 instrument. The resulting ambient NH3 concentration was 40% lower than that of another academic building without modification of its ceiling ducts. Besides, the measured NH3 peaks were consistently associated with air plumes from ceiling ducts. Realizing this strong NH3 source is important to avoid its significant influences on the measurement of NH3 and to interpret a large spatial heterogeneity of NH3 concentrations in urban China.