Tower- and drone-based BVOC observations in a suburban Tokyo forest: methodological insights and MEGAN comparison

Abstract

Biogenic volatile organic compounds (BVOCs) substantially influence regional photochemical air pollution, climate, and the carbon cycle. However, observational constraints on BVOC emissions from urban or suburban forests in Asian megacity regions under humid subtropical climates remain limited. In this study, we conducted multi-year intermittent, multi-height BVOC observations at a 30 m flux tower in a suburban Tokyo forest dominated by Quercus serrata . Spatial variability was examined by combining tower measurements with supplemental drone-based sampling. The measurements were compared with estimates from the Model of Emissions of Gases and Aerosols from Nature (MEGAN). Isoprene volume mixing ratios increased during the warm-season observations from May to October, accounting for over 90 % of the measured BVOC composition during peak summer, while monoterpenes remained low with weak vertical gradients. Isoprene exhibited distinct vertical volume mixing ratio gradients peaking within the canopy, with the daily average emission fluxes ranging from −0.05 to 15.30 mgm-2h-1. Drone-based measurements indicated horizontal variability in isoprene volume mixing ratios of approximately 10 %–30 % within 30 m of the tower. Flux estimates derived from tower and drone measurements differed by approximately 30 %, suggesting that small-scale spatial heterogeneity and height differences can affect gradient-based flux estimates. MEGAN generally overestimated the observed fluxes, particularly during the warm-season observations. These results demonstrate the potential of combining tower-based vertical profiling with supplemental drone-based horizontal sampling to evaluate BVOC fluxes and their spatial representativeness. Although the intermittent sampling design limits comprehensive seasonal and interannual interpretations, this study provides methodological insights for future tower- and drone-based BVOC observations and emission model evaluation based on canopy-scale measurements.