The Relationship between Molecular Size and Polarity of Atmospheric Organic Aerosol in Singapore and Its Implications for Volatility and Light Absorption Properties

Organic aerosol, which is a complex mixture, has been categorized by both chemical (e.g., degree of oxygenation and molecular weight (MW)) and physical (e.g., water solubility and optical) characteristics. However, the inter-relationships among these characteristics have not been well-established yet. This study developed a novel framework on the inter-relationship among volatility, water solubility, and MW. The framework suggests that water-soluble organic matter (WSOM) can have relatively small MW (<200) because highly polar species tend to have lower volatility. On the other hand, water-insoluble organic matter (WISOM) needs to have larger MW than the threshold, as less polar species tend to be highly volatile. The idea was tested by analyzing aerosol samples collected at Singapore. Both WSOM and WISOM were extracted from the samples. The WSOM fraction was further classified by polarity using the 1-octanol-water method. The samples were analyzed by atmospheric pressure chemical ionization mass spectrometry (APCI-MS). APCI-MS is a soft-ionization technique that dominantly provides [M-H]- ions. The APCI-MS data validated the newly developed idea on the MW distributions for WSOM and WISOM. The positive matrix factorization (PMF) analysis was applied to the APCI-MS data. The analysis for WSOM also supported the inverse relationship between polarity and MW. The PMF outputs were combined with the ultraviolet-visible absorption data for obtaining absorption Ångstrom exponents (AAE) for each factor. The result suggests that highly polar WSOM factors have relatively small values of average MW (≤205) and higher values of AAE (>8.5), connecting polarity and optical properties of OM.