Diurnal variations of biogenic volatile organic compounds and their role in secondary pollutant formation in the Huai Hong Khrai subtropical forest, Thailand.

Understanding the emissions and atmospheric impact of biogenic volatile organic compounds (BVOCs) in subtropical forests remains limited despite their role in secondary pollutant formation. This study presents the first comprehensive BVOC dataset from the Huai Hong Khrai subtropical forest, Northern Thailand, during the dry season (March 4-9, 2024). Using thermal desorption gas chromatography-mass spectrometry (TD-GC-MS), we identified 45 BVOCs, dominated by monoterpenes, sesquiterpenes, and oxygenated derivatives. Unlike temperate forests where sesquiterpenes exhibit stable emissions, this subtropical ecosystem exhibited pronounced nighttime monoterpene accumulation (50% increase), probably affected by temperature-driven volatilization and reduced atmospheric mixing. Peak area normalization was used to provide a semi-quantitative assessment, with internal standard correction ensuring comparability. However, the absence of ozone scrubbers may have resulted in partial oxidation of reactive BVOCs, and isoprene underestimation was observed due to Tenax-TA's low retention efficiency. Strong correlations were found between α-pinene and environmental drivers, including temperature (r = 0.84), PM2.5 (r = 0.94), and relative humidity (r = -0.83), highlighting its role in secondary organic aerosol (SOAP, 10 μg/m at night) and ozone formation potential (OFP, 200 μg/m). These results highlight the influence of environmental factors on atmospheric BVOC concentrations and their implications for secondary pollution in subtropical forests. Expanding BVOC research in these ecosystems is crucial for refining atmospheric models, improving air quality management, and developing climate resilience strategies in regions where biogenic and anthropogenic emissions interact.