Development and application of a mass closure PM<sub>2.5</sub> composition online monitoring system

Abstract. Online instruments have been widely applied for the measurement of PM2.5 and its chemical components; however, these instruments have a major shortcoming in terms of the lack or limited number of species in field measurements. To this end, a new mass closure PM2.5 online integrated system was developed and applied in this work to achieve more comprehensive information on chemical species in PM2.5. For the new system, one isokinetic sampling system for PM2.5 was coupled with an aerosol chemical speciation monitor (Aerodyne, ACSM), an Aethalometer (Magee, AE-31), an automated multimetals monitor (Cooper Corporation, Xact-625), and a hybrid synchronized ambient particulate real-time analyzer monitor (Thermo Scientific, SHARP-5030i) to enable high-resolution temporal (1 h) measurements of organic matter, SO42−, NO3−, Cl−, NH4+, black carbon and important elements as well as PM2.5 mass concentrations. The new online integrated system was first deployed in Shenzhen, China to measure the PM2.5 composition from 25 September to 30 October 2019. Our results showed that the average PM2.5 concentration in this work was 33 μg m−3, and the measured species well-reconstructed the PM2.5 and almost formed a mass closure (94 %). The multilinear engine (ME-2) model was employed for the comprehensive online PM2.5 chemical dataset to apportion the sources with predetermined constraints, in which the organic ion fragment m/z 44 in ACSM data was used as the tracer for secondary organic aerosol (SOA). Nine sources were determined and obtained reasonable time series and diurnal variations in this study, including identified SOA (23 %), secondary sulfate (22 %), vehicle emissions (18 %), biomass burning (11 %), coal burning (8.0 %), secondary nitrate (5.3 %), fugitive dust (3.8 %), ship emissions (3.7 %), and industrial emissions (2.1 %). The potential source contribution function (PSCF) analysis indicated that the major source areas could be the region north of the sampling site. To the best of our knowledge, this is the first system that can perform online measurements of PM2.5 components with a mass closure, thus providing a new powerful tool for PM2.5 long-term daily measurement and source apportionment.