Evaluation of SO42-,NO3- and NH4+ measurements using ambient PM2.5 real-time monitoring instruments

Abstract

通过两套大气细颗粒物(PM2.5)水溶性离子在线监测仪与蜂巢式固气分离器膜采样系统作对比,评估了在线监测仪器对主要水溶性组分SO24-、NO3-和NH4+的测定结果.美国URG公司生产的在线连续监测分析系统(AIM URG-9000B)对NH4+和NO3-的监测结果较好,但对SO24-的测定结果存在明显高估,其原因是AIM的平板溶蚀器系统无法完全去除大气中高浓度SO2,从而对SO24-的测定结果有干扰.为解决这一问题,进行了一系列实验,结果表明采用两个溶蚀器串联并用5 mmol.L-1H2O2+5 mmol.L-1NaOH混合溶液作吸收液时,高浓度的SO2(甚至达到260μg.m-3)可以被完全吸收而对SO24-的测定结果不产生影响.由荷兰能源研究所(ECN)、Metrohm和Applikon共同研制的在线气体组分及气溶胶监测系统(MARGA,ADI2080)对NH4+和SO24-的监测结果较好,可以满足实验要求;但NO3-的测定结果偏高,其准确性需作进一步评估.新型监测仪器在不同大气环境中投入使用前需进行对比测试,以确定其准确性和精确性. Field studies were conducted in Hong Kong to assess two different real-time techniques for measuring SO2-4,NO-3 and NH+4 in PM2.5 by comparison with a honeycomb denuder filter-pack system.An ambient ion monitor(AIM,URG9000B) showed good performance for NO-3 and NH+4,but obvious overestimation for SO2-4,which was attributed to the positive interference by high ambient SO2 concentrations which could not be totally eliminated in the parallel denuder.To overcome this problem,a series of experiments were conducted,which lead to an improved system including two denuders working in series and with the use of a new denuder liquid(5 mmol · L-1 H2O2 +5 mmol · L-1 NaOH).The new system could absorb high concentration of SO2(even up to 260 μg · m-3) so that the SO2-4 measurement results would not be affected.Another monitor for measuring aerosols and gases(MARGA,Model ADI 2080),developed by ECN,Metrohm and Applikon,was also evaluated.MARGA showed excellent performance for the measurements of SO2-4 and NH+4,but exhibited high NO-3 measurement results.The accuracy of NO-3 measurements should be evaluated further.This study demonstrated the need for evaluating new measurement techniques in different atmospheric environments to determine the accuracy and precision of the measurements.