PPWD-SDEP-IC monitoring system for atmospheric precursor inorganic gases and PM2.5 water-soluble ions

To resolve the potential evaporation loss of about 14% NH4+ in the particle into liquid sampler (PILS), a novel two-stage semi-dry electrostatic precipitator (SDEP) was developed successfully for monitoring atmospheric PM2.5 water-soluble inorganic ions (WSIIs) with the high collection efficiency and good accuracy. The number-based collection efficiency was >95% for particle sizes ranging from 14 nm to 11 μm, and the mass-based collection efficiency for PM2.5 was also as high as 94.4–98.1%. With an efficient pulse water jet system, the SDEP had a high ion recovery rate of 93 ± 10% for Na+, 103 ± 15% for NH4+, 92 ± 5.6% for SO42−, and 96 ± 8% for NO3−, respectively. The SDEP was coupled with the parallel plate wet denuder (PPWD) and ion chromatography (IC) as the PPWD-SDEP-IC semi-automatic monitoring system for the hourly precursor gases and PM2.5 WSII concentrations. The field comparison test for three-month measurements showed that the daily average results of the PPWD-SDEP-IC had a good agreement with those of the porous-metal denuder sampler (PDS), showing the linear regression slopes of 0.99–1.05 and 0.98–1.05, respectively, for precursor gases (NH3, HONO, SO2 and HNO3) and PM2.5 WSIIs (Na+, NH4+, K+, F−, Cl−, NO3− and SO42−) with high R2 values of 0.92–0.99 and 0.96–0.99, respectively. Moreover, the results of the hourly concentration comparison between the PPWD-SDEP-IC and the PPWD-PILS-IC showed linear regression slopes of 0.90–0.99 and 0.84–0.97 for precursor gases and WSIIs with high R2 values of 0.93–0.96 and 0.90–0.96, respectively. The PILS showed an underestimation of NH4+ concentration by about 15.6 ± 6.3% when compared to NH4+ from the SDEP owing to the volatilization loss of NH4+ caused by the mixing of aerosols with high-temperature steam in the PILS. Thus, the current PPWD-SDEP-IC system can be a viable tool for accurate semi-continuous monitoring of precursor gases and PM2.5 WSII concentrations.