Vertical and seasonal variations in airborne endotoxins in a coastal megacity of North China: insights from 3-hydroxy fatty acids

Abstract. Endotoxins, integral components of Gram-negative bacteria, are released into atmosphere during bacterial fragmentation and pose health risks. This study investigated 3-hydroxy fatty acids (3-OH-FAs, C₈ – C₁₈) in inhalable particles (PM₁₀) from urban Tianjin, a coastal megacity in northern China, to estimate endotoxin levels utilizing UPLC-MS. Results revealed seasonal and altitudinal variations in 3-OH-FAs and endotoxin levels. Total endotoxin concentrations averaged 21.5 ng m^–3 at near ground (2 m) and 16.1 ng m^–3 at a higher altitude (220 m), corresponding to total 3-OH-FAs (C₁₀ – C₁₈) concentrations of 2.75 ng m^–3 and 2.01 ng m^–3, respectively. Maximum endotoxin level (26.5 ng m^–3) occurred near ground during winter, attributed to enhanced near-surface emissions. Bioactive endotoxins peaked at 12.4 ng m^–3 near ground in winter, exceeding the exposure threshold, while averaging 8.12 ng m^–3 in other seasons. Short-chain 3-OH-FAs (C₈ – C₁₃) exhibited significant correlations with meteorological factors (e.g., temperature, humidity, and wind speed) at both altitudes, indicating regulation through microbial growth dynamics and photochemical processes. Long-chain homologues (C₁₄ – C₁₈) were affected by both meteorological conditions and particulate pollutants (e.g., OC, WSOC, K⁺, Ca²⁺), reflecting mixed influences from natural sources (e.g., soils) and anthropogenic activities (e.g., combustion). These findings advance understanding of spatiotemporal variations in airborne endotoxins within complex urban environments, providing critical data for assessing health risks associated with particulate pollution and informing urban air quality management strategies.