One-year observation of the mixing states of oxygenated organics-containing single particles in Guangzhou, China

Abstract

Oxygenated organic molecules (OOMs) play an important role in the formation of secondary organic aerosols (SOAs), but the mixing states of OOMs are still unclear. This study investigates the mixing states of OOM-containing single particles from the measurements taken using a single particle aerosol mass spectrometer in Guangzhou, China in 2022. Generally, the particle counts of OOM particles and the mass concentration of secondary organic carbon (SOC) exhibited similar temporal trends throughout the entire year. The OOM particles were consistently enriched in secondary ions, including ¹⁶O⁻, ²⁶CN⁻, ⁴⁶NO₂⁻, ⁶²NO₃⁻, and ⁹⁷HSO₄⁻. In contrast, the number fractions and diurnal patterns of OOM particles among the total detected particles showed similar distributions in August and October; however, the SOC ratios in fine particulate matter were quite different, suggesting that there were different mixing states of single-particle oxygenated organics. In addition, further classification results indicated that the OOM particles were more aged in October than August, even though the SOC ratios were higher in August. Furthermore, the distribution of hydrocarbon fragments exhibited a notable decrease from January to October, emphasizing the more aged state of the organics in October. In addition, the sharp increase in elemental carbon (EC)-OOM particles in the afternoon in October suggests the potential role of EC in the aging process of organics. Overall, in contrast to the bulk analysis of SOC mass concentration, the mixing states of the OOM particles provide insights into the formation process of SOAs in field studies.