3B.2 Atmospheric NH3 Dynamics in Relation to PM2.5 Pollution in the North China Plain (Invited Presentation)

Monday, 8 January 2018: 10:45 AM
Room 18CD (ACC) (Austin, Texas)
Xuejun Liu, China Agricultural Univ., Beijing, Beijing, China; and W. Xu, Y. Zhang, and A. Tang

Air pollution is one of the most serious environmental problems in China due to its rapid economic development alongside a very large consumption of chemical fertilizers, livestock production and fossil fuels, particularly in the North China Plain (NCP). In historic studies, major emphasis was made on traffic and industrial emission induced air pollution; while the role of agricultural emissions such as ammonia (NH3) emissions were paid less attention in China. That is why China has experienced a continuous decrease in SO2 emission, a slight decrease in NOx emission but a stable increase in NH3 emission. Under this background, we made an attempt to quantify the seasonal dynamics of ammonia (NH3) concentration as well as its contribution (through gas to particle transformation) to PM2.5 in the NCP.

During the period 2011–2014, we integrated active and passive sampling methods to perform continuous measurements of NH3, HNO3, NO2, and PM2.5 at two urban, one suburban, and two rural sites in the NCP. The annual average concentrations of NH3, NO2, and HNO3 across the five sites were in the ranges 8.5–23.0, 22.2–50.5, and 5.5–9.7 μg m-3, respectively, showing no significant spatial differences for NH3 and HNO3 but significantly higher NO2 concentration at the urban sites. At each site, annual average concentrations of NH3 and NO2 showed increasing and decreasing trends, respectively, while there was no obvious trend in annual HNO3 concentrations. Daily PM2.5 concentrations ranged from 11.8 to 621 μg m-3 at the urban site, from 19.8 to 693 μg m-3 at the suburban site, and from 23.9 to 754 μg m-3 at the two rural sites, with more than 70% of sampling days exceeding 75 μg m-3. Concentrations of water-soluble ions in PM2.5 ranked differently between the non-rural and rural sites. The three dominant ions were NH4+, NO3, and SO42− and mainly existed as (NH4)2SO4, NH4HSO4, and NH4NO3, and their concentrations averaged 48.6±44.9, 41.2±40.8, and 49.6±35.9 μg m-3 at the urban, suburban, and rural sites, respectively. The direct contribution of NH4+ to PM2.5 was around 10% but the NH4+ salts (e.g. (NH4)2SO4 and NH4NO3) contribution was higher than 30% during the haze days in the NCP. Ion balance calculations indicated that PM2.5 was neutral at the non-rural sites but acidic at the rural sites. Seasonal variations of NH3 and PM2.5 showed clear trade-off relationships, that is, NH3 peaked in summer (strong agricultural emission) and lowest in winter while PM2.5 peaked in winter (strong acid gas emission from heating and bad weather conditions) and lowest in summer, reflecting the complicated interactions between source emission strength and meteorological conditions. Our results suggest that a feasible pathway to control PM2.5 pollution in the NCP should target ammonia and acid gases together.

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