High-resolution observation of stable carbon isotope ratios of water-soluble organic carbon in particle/gas phases at an urban site in China: Using an improved isotope ratio mass spectrometry method.

A high time resolution synchronous sampling method al ong with determination of stable carbon isotopes of gaseous water-soluble organic carbon (WSOCg) and particulate water-soluble organic carbon (WSOCp) was realized in this research through equipment modification and method improvement. It was found that WSOCg has significant higher concentration than WSOCp and a more depleted δ¹³C. Both concentrations of WSOCp and WSOCg have the seasonal variation characteristics of high in winter (WSOCp=15.7±3.9µg/m³; WSOCg=42.4±6.0µg/m³) and low in summer (WSOCp=5.9±1.8µg/m³; WSOCg=25.2±5.2µg/m³), with greater increase in WSOCp (167%) than that in WSOCg (68%). During wintertime, WSOCp and WSOCg had similar daily variation characteristics of concentration, and opposite daily variation characteristics of δ¹³C. WSOCp had a bimodal distribution with obvious low value at sunrise and sunset, while δ¹³C WSOCp had a unimodal distribution with low in daytime (-24.6±1.1‰) and high in nighttime (- 22.3±1.7‰). WSOCg and δ¹³C-WSOCg had same distribution with high in daytime and (49.3±8.8µg/m³; -27.9±1.1‰) low in nighttime (38.3±4.6µg/m³; -29.9±0.4‰). Combining the δ¹³C variation characteristics with the synchronous observation results of meteorological conditions, gaseous precursor pollutants, gaseous oxidants, gaseous acids and fine particle components, the restriction factors of WSOC gas-particle distribution mechanism were discussed. The presence of radiation rather than its intensity decided whether generations process of WSOCp and WSOCg are divided, for δ¹³C-WSOC of two phases showed significant correlation only during daytime. Meteorological conditions, gaseous precursor pollutants, gaseous oxidants and gaseous acids restrict the gas particle distribution of WSOC by affecting the aging process of WSOCp and WSOCg, gas particle conversion ratio of semi-volatile organic compounds (SVOC) and the gas phase and liquid phase generation ratio of WSOCp. At the same time, the gas-particle distribution process of WSOC is strongly related to the formation of secondary inorganic ions (nitrate, sulfate, ammonium), and the gas-particle distribution between gaseous nitrous acid and nitrite. [ABSTRACT FROM AUTHOR]