南昌市PM_(2.5)中铵盐及其气态前体物的分布特征与转化机制

Distribution and conversion mechanism of ammonium in PM_(2.5) and gaseous precursors in Nanchang

在南昌市三种类型环境区域(混合区、道路区域以及郊区)开展PM_(2.5)中NH_4^+和空气中NH_3的同步采样监测,分析PM_(2.5)中铵盐及其气态前体物的分布特征,探讨氨气的转化与细粒子铵盐的形成机制,结果显示:2014-2015年采样期间南昌不同区域空气中NH_3浓度和PM_(2.5)中NH_4^+浓度较高;NH_3浓度存在空间分布差异,反映了不同环境区域NH_3源强的差异;NH_4^+浓度的空间分布表现为道路区域、郊区高于混合区,源于道路区域和郊区的气态前体物浓度高;NH_3/NH_4^+比值郊区<道路区<混合区,反映NH_4^+的形成受前体物(SO_2、NO_x)影响大;NH_3浓度春>秋>冬>夏,说明NH_3源强受各季气象条件的影响大;NH_4^+浓度呈现秋冬高、春夏低的特征,反映不同季节的气象条件对铵盐的生成、清除和分解的影响不同;然而,NH_3/NH_4^+比值春>夏>秋>冬,NH_3/NH_4^+比值季节分布与NH_4^+浓度季节分布呈相反的趋势;NH_3浓度昼、夜分布有差异,受昼夜间温差、太阳辐射、源强、逆温等多种因素的影响;NH_4^+浓度日变化各季节有差异;NH_3/NH_4^+比值日分布与NH_3浓度日分布相似;不同季节PM_(2.5)中铵盐形成的受控因素有差异,主要影响因素是气态前体物和温度、湿度;NH_4^+/SO_4^(2-)的比值(>1.5),表明铵盐充足;铵盐形式主要为硫酸铵、硝酸铵,硫酸氢铵较少。

Background,aim,and scope Ammonium is one of the most important water-soluble ions,which is the main constituent of PM2.5.Ammonium has strong hygroscopicity and reduces visibility,and the wetdry deposition of ammonium could lead to nitrogen saturation in a local ecosystem.We could understand the formation mechanism of fine particles,especially ammonium in fine particles by sampling and monitoring ammonium in PM2.5 and ammonia in the air simultaneously,that would provide scientific basis to control scientifically and reduce the pollution of fine particles as well as prevention and control the pollution of regional atmospheric environment.Materials and methods Sampling and monitoring the NH1 in PM2.5 and the NH3 in air in three types of environment regions in Nanchang simultaneously,including mixed areas,road areas and suburban areas respectively.The method of analysis and determinations mainly performed in accordance with Ambient air and exhaust gas-Determination of ammonia by Nessler’s reagent spectrometry.Results During the sampling period in 2014-2015,the concentration of NH3 in air and NH1 in PM2.5 were at high level in different regions of Nanchang.The spatial distribution difference of NH3 reflects the difference of source-strength of NH3 in different environmental regions.The concentration of NH1 in road areas and suburban areas were higher than mixed areas because of the concentration of gaseous precursors in road areas and suburban areas were higher than mixed areas.The order of value of NH3/NH1 in different environmental regions was mixed areas>road areas>suburban areas,which shows that the formation of NH1 was greatly affected by gaseous precursors.And the concentration of NH3 was spring>autumn>winter>summer,which shows that the source-strength of NH3 was greatly influenced by seasonal meteorological conditions.The concentration of NH1 was high in autumn and winter while low in spring and summer,and it indicated that the meteorological conditions in different seasons have different effects on the generation,removal and decomposition of ammonium.Discussion The daily distribution of value of NH3/NH1 have the same trend of daily distribution of concentration of NH3.The formation of ammonium in PM2.5 were affected by different factors in different seasons,and the main influencing factors were gaseous precursors,temperature and humidity.We can infer that ammonium is sufficient from the value of NH1/SO1^-exceeds 1.5.The forms of ammonium are mainly ammonium sulfate and ammonium nitrate.Conclusions The order of value of NH3/NH1 in seasonal distribution was spring>summer>autumn>winter,while the concentration of NH1 in seasonal distribution was the opposite.The concentration of NH3 between day and night distribution were different,and it was influenced by many factors,such as temperature difference between day and night,solar radiation,source-strength,temperature inversion and so on.The concentration of NH1 changes in different seasons.Recommendations and perspectives On the study of concentration distribution of NH1 in atmospheric PM2.5 and NH3 in air between different environment regions,or different seasons,or different periods of the same day.We could understand that source-strength of gaseous precursors,seasonal variation,temperature and humidity change,solar radiation intensity and inversion phenomenon all play an important role in the formation of ammonium.And it leads to the results,fine particle pollution could be controlled and reduced fundamentally only by reducing and avoiding the emissions of gaseous precursors while meteorologic condition beyond our control.