Characteristic trends and source shifts of n-alkanes in a multi-year study: Insights from gas-particle partitioning in a central Chinese city.

Understanding the carbon distribution and source shifts of n-alkanes, which have considerable environmental and public health implications, is vital for assessing the long-term atmospheric behaviour of organic compounds. Here, we combined seasonal data of PM bounded n-alkanes (C-C) collected in a Chinese city across three time periods (2014-2015, 2019, 2022) to investigate their characteristics and source proportions over time. The annual average concentration of particulate n-alkanes in 2022 was 118 ng m⁻³ , showing a 41 % decline from 2019 and a 50 % reduction from 2014 to 2015. Diagnostic ratio analysis (carbon preference index and plant wax ratio) revealed that anthropogenic sources dominated in all three periods. Positive Matrix Factorisation (PMF) model of PM bounded n-alkanes identified a shift from coal combustion to motor vehicle emissions as the primary source, while contributions from plant waxes increased in 2022 (22.6 %) compared to 2014-2015 (18.8 %) and 2019 (19.6 %). A comparative analysis of the source of intermediate and semi-volatile n-alkanes (C-C, including measured particle-phase and predicted gas-phase) against particle-phase n-alkanes (C-C) demonstrated notable discrepancies, highlighting the significant role of gas-particle partitioning in source attribution. These insights deepen the understanding of long-term n-alkane trends and are critical for designing effective air pollution control strategies.