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Composition of Ultrafine Particles in Urban Beijing: Measurement Using a Thermal Desorption Chemical Ionization Mass Spectrometer
journal contribution
posted on 2021-02-12, 21:13 authored by Xiaoxiao Li, Yuyang Li, Michael J. Lawler, Jiming Hao, James N. Smith, Jingkun JiangUltrafine particles (UFPs) dominate
the particle number population
in the urban atmosphere and revealing their chemical composition is
important. The thermal desorption chemical ionization mass spectrometer
(TDCIMS) can semicontinuously measure UFP composition at the molecular
level. We modified a TDCIMS and deployed it in urban Beijing. Radioactive
materials in the TDCIMS for aerosol charging and chemical ionization
were replaced by soft X-ray ionizers so that it can be operated in
countries with tight regulations on radioactive materials. Protonated N-methyl-2-pyrrolidone ions were used as the positive reagent
ion, which selectively detects ammonia and low-molecular weight-aliphatic
amines and amides vaporized from the particle phase. With superoxide
as the negative reagent ion, a wide range of inorganic and organic
compounds were observed, including nitrate, sulfate, aliphatic acids
with carbon numbers up to 18, and highly oxygenated CHO, CHON, and
CHOS compounds. The latter two can be attributed to parent ions or
the decomposition products of organonitrates and organosulfates/organosulfonates,
respectively. Components from both primary emissions and secondary
formation of UFPs were identified. Compared to the UFPs measured at
forest and marine sites, those in urban Beijing contain more nitrogen-containing
and sulfur-containing compounds. These observations illustrate unique
features of the UFPs in the urban environment and provide insights
into their origins.