应用扩散管测量霾污染期间大气氮硫化合物浓度的方法

Observations of Reactive Nitrogen and Sulfur Compounds During Haze Episodes Using a Denuder-based System

活性氮和硫化合物在大气颗粒物形成过程中扮演重要角色,但对它们气相/颗粒相的同步观测结果比较缺乏.本研究尝试基于扩散管的DELTA系统测量氮和硫化合物短时累积浓度,以期捕捉它们在霾污染期间的演变规律.结果表明,DELTA系统收集气态污染物的扩散管中以及颗粒物滤膜上NH_4^+和NO-3空白干扰较小,适用于研究NH_3、HNO_3、NH_4^+和NO-3的日均浓度,可以作为城市环境空气质量监测参数的有效补充;但采样系统中SO_2-4背景含量较高,仅适合监测48 h以上时间尺度的SO_2浓度和周~月尺度SO_2-4浓度,用于大气硫沉降观测.北京2016年5月9日~6月7日观测期间,大气NH_3、HNO_3、NH_4^+和NO-3浓度具有明显的逐日演变规律,呈现出随着风向转变而发生周期性波动的典型特征;这些含氮污染物与PM_(2.5)、CO、SO_2和NO_2浓度的变化规律一致,其来源可能与化石燃料燃烧源有关.污染天NH_3、HNO_3、NH_4^+和NO-3浓度约为清洁天的2倍,但还原性氮和氧化性氮的相态分布在清洁天和污染天无明显差异;整个观测期间,HNO_3/NO-3约为1.2,NH_3/NH_4^+为4.5,春夏之交较高的温度有利于活性氮在气粒平衡过程中偏向于气态形式存在.

Reactive nitrogen and sulfur compounds are chemically active in the atmosphere and play an important role in secondary particle formation. Among them,sulfate,nitrate,and ammonium（ SNA） are important components of particulate matter（ PM） that account for approximately one-third of fine particles. The precursors of SNA including HNO3,SO2,and NH3 are all involved in haze formation in China. To date,the concurrent measurements of SNA and their precursors have been limited to single sites and short terms because of the high cost of the instruments. This study aimed to use DELTA（ Denuder for Long-Term Atmospheric sampling） to characterize the daily concentrations of reactive nitrogen and sulfur species during haze episodes. The results showed that the background interface of NH4^＋,NO3^-,and SO4^2- in the denuder was minor and could be used to determine 24-48 h concentrations of NH3,HNO3,SO2,NH4^＋,and NO3^-. However,the SO4^2-concentrations in the blank filter was so high that they could only be used for sampling weekly or for longer periods of time. During the campaign between May 9 and June 7,2016 in urban Beijing,the concentrations of NH3,HNO3,NH4^＋,and NO-3showed distinct daily variations at different wind directions,i. e.,higher values were observed during southerly winds and lower values during northerly winds. The time series of these reactive nitrogen compounds coincided with that of PM2.5,CO,SO2,and NO2,indicating the combustion of fossil fuels. The mean concentrations of NH3,HNO3,NH4^＋,and NO3^- were twice the concentrations during clean days,further highlighting the effect of local emissions on the urban environment. The ratios of HNO3/NO3^- and NH3/NH4^＋ were 1. 2 and 4. 5,respectively,without significant differences between hazy and clean days. The findings demonstrated that both the reduced and oxidized nitrogen were preferred in gaseous phase rather than particulate phase due to relatively high temperatures during the transition season of spring/summer.