Analysis of twenty years (2004–2023) observation of non-methane hydrocarbons in a subtropical coastal environment

A twenty-year (2004–2023) trend analysis of marine background air was conducted to explore potential changes in non-methane hydrocarbons (NMHCs) emissions and atmospheric oxidation capacity using the Propylene Equivalent (Propy-Equiv) concentration. The focus was on C₂-C₆ NMHCs including alkanes, aromatics, acetylene, and isoprene, as those were most frequently found in the air samples. During wintertime, least impacted by photochemical impacts, a clear increase in n-pentane was observed from 2004 to 2023 (2.07 ± 2.26 % year^-1) (statistically significant). Ethane (-3.82 ± 8.65 % year^-1) and n-butane (-1.35 ± 15.62 % year^-1) decreased from 2004 to 2008, but this was not statistically significant, but a statistically significant increase was then observed until 2023 (ethane: 1.05 ± 0.51 % year^-1; n-butane: 1.09 ± 1.26 % year^-1). Iso-pentane decreased (-4.25 ± 1.91 % year^-1) steadily from 2004 to 2011 (statistically significant), then remained constant but with increased variability until 2023 (0.28 ± 2.49 % year^-1). Propane increased (5.51 ± 23 1.35 % year^-1) from 2004 to 2014 (statistically significant) and decreased thereafter until 2023 (-3.63 ± 3.91 % year^-1). Acetylene (-1.67 ± 0.51 % year^-1), benzene (-2.43 ± 0.14 % year^-1), and i-butane (-0.58 ± 25 0.25 % year^-1) showed a steady decreasing (statistically significant) trend from 2004 to 2023. The increasing ethane trend for the last 15 years is due to global oil and natural gas extraction, especially in the US, which began in mid-2009. Improvements in gasoline technologies are causing the decline of acetylene and benzene trends. The slower than expected decreasing trend of acetylene mixing ratio might have been offset by the impact of biomass burning emissions. Other NMHCs show varying trends indicating the merge of different emission sources and strengths in separate time periods. During the summertime, 80–90% Propy-Equiv concentration is due to isoprene, with a statistically significant increasing trend (0.45 ppbC/year) between 2004 and 2023. This increase is largely due to rising temperatures (1.58 ± 0.14 ◦C) leading to increased isoprene emissions (20 ± 1.6%).