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

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 PM_2.5 in the NCP.

During the period 2011–2014, we integrated active and passive sampling methods to perform continuous measurements of NH₃, HNO₃, NO₂, and PM_2.5 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 PM_2.5 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 PM_2.5 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 PM_2.5 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 PM_2.5 was neutral at the non-rural sites but acidic at the rural sites. Seasonal variations of NH₃ and PM_2.5 showed clear trade-off relationships, that is, NH₃ peaked in summer (strong agricultural emission) and lowest in winter while PM_2.5 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 PM_2.5 pollution in the NCP should target ammonia and acid gases together.