2019年国庆节前后北京气态氨和气溶胶铵盐浓度的同步观测

Concurrent Collection of Ammonia Gas and Aerosol Ammonium in Urban Beijing During National Celebration Days Utilizing an Acid-Coated Honeycomb Denuder in Combination with a Filter System

我国自2013年实施《大气污染防治行动计划》以来,大气细颗粒物(PM_(2.5))特别是硫酸盐浓度迅速下降,但硝酸盐浓度降幅较小,大气中过量的氨气(NH_(3))是维持硝酸盐居高不下的主要因素.迄今,我国生态环境部门尚未将NH_(3)纳入常规观测,以往有关NH_(3)和气溶胶铵盐(NH^(+)_(4))的研究多是分别进行的,缺少同步观测.由于NH_(3)和NH^(+)_(4)在大气中可互相转化,只测量其中一种相态很难全面了解它们的动态变化.本研究基于酸涂覆的蜂窝型扩散管和膜采样串联系统,同步测量了2019年国庆节前后北京城区大气NH_(3)和NH^(+)_(4)浓度,时间分辨率为2 h(PM_(2.5)>35μg·m^(-3))~5 h(PM_(2.5)<35μg·m^(-3)).结果表明,采样期间NH_(3)和NH^(+)_(4)平均浓度分别为(4.1±2.9)μg·m^(-3)和(1.7±1.4)μg·m^(-3),且二者均与PM_(2.5)、CO和NO_(2)呈现相似的时间变化规律.NH_(3)浓度在早晨(05:30~08:30)和夜间(21:30~05:30)较高,这种双峰日变化特征在污染天(PM_(2.5)>75μg·m^(-3))最为明显.NH_(3)浓度在污染天17:30~21:30存在明显的低谷,这主要与有利的扩散条件有关(平均风速6 m·s^(-1)).NH^(+)_(4)浓度的日变化特征与NH_(3)差异较大,NH^(+)_(4)浓度在非污染天(PM_(2.5)<75μg·m^(-3))17:30~21:30出现明显峰值,期间NH_(3)浓度较低,而NO_(2)浓度较高.在非污染天,NH_(3)浓度是NH^(+)_(4)的2.8倍;而在污染天,由于气粒转化加速,大气NH_(3)浓度低于NH^(+)_(4)(NH_(3)/NH^(+)_(4)=0.8).国庆节前大气NH_(3)、CO、NO_(2)、SO_(2)和PM_(2.5)浓度超过国庆后的幅度分别为54.2%、40.4%、33.3%、0.0%和49.4%.国庆节前虽然实施了减排行动,但极端不利的静稳天气导致大气环境容量下降,掩盖了污染物减排的效果,导致大部分污染物浓度不降反升.

Since 2013,the Chinese government implemented the Air Pollution Prevention and Control Action Plan.As a result,the atmospheric concentrations of sulfate reduced significantly,whereas the nitrate concentrations remain relatively high due to the excess of ammonia(NH_(3)).To date,there is no official observation network monitoring NH_(3)concentrations in China.Previous studies have focused on NH_(3)or ammonium(NH^(+)_(4))separately.These limitations hinder a complete understanding of their dynamic changes due to the rapid gas-to-particle conversion.In this study,the concentrations of NH_(3)and NH^(+)_(4)were measured concurrently in urban Beijing during autumn 2019 utilizing an acid-coated denuder-filter combination with a time resolution from 2 h(PM_(2.5)>35μg·m^(-3))to 5 h(PM_(2.5)<35μg·m^(-3)).The mean concentrations of NH_(3)and NH^(+)_(4)during the study were(4.1±2.9)μg·m^(-3)and(1.7±1.4)μg·m^(-3),respectively.The temporal patterns of NH_(3)or NH^(+)_(4)coincided with that of PM_(2.5),CO,and NO_(2)throughout the sampling period.The diurnal distributions of NH_(3)were bimodal,both on polluted(PM_(2.5)>75μg·m^(-3))and non-polluted(PM_(2.5)<75μg·m^(-3))days,peaking at 21:30-05:30 and 05:30-08:30,respectively.The NH_(3)concentrations on polluted days were relatively lower during 17:30-21:30,which may be related to higher wind speeds.In contrast to NH_(3),NH^(+)_(4)had an obvious peak during 17:30-21:30 due to the formation of ammonium nitrate.The meteorological conditions favor the gas-to-particle conversion on polluted days,resulting in a lower NH_(3)/NH^(+)_(4)ratio of 0.8.However,this value may reach 2.8 on non-polluted days.The concentrations of NH_(3),CO,NO_(2),SO2,and PM_(2.5)in the emission control period showed a significant increase greater than or comparable to those in the noncontrol period by 54.2%,40.4%,33.3%,0%,and 49.4%,respectively.This result shows that the stagnant conditions offset the benefit of emission control actions implemented during and before the National Celebration Day.