High biogenic volatile organic compound emissions but NOx-constrained ozone formation over a tropical island: A case study of Hainan Island, China

Abstract

Biogenic volatile organic compounds (BVOCs) are important precursors of tropospheric ozone (O₃), yet their emissions and photochemical effects remain poorly characterized in tropical island regions of China. Using the Model of Emissions of Gases and Aerosols from Nature (MEGAN), driven by satellite-based land cover, Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation products, and Weather Research and Forecasting (WRF) meteorology, we developed a high-resolution BVOC emission inventory for Hainan Island in 2019. The WRF model coupled with the Comprehensive Air Quality Model with Extensions (WRF-CAMx) was further applied to quantify summertime BVOC contributions to ozone formation through emission sensitivity simulations and photochemical regime diagnostics. Total BVOC emissions were estimated at 5.51 × 10⁵ tons (t), with an annual mean emission intensity of 16.02 tons per square kilometer per year (t·km⁻²·yr⁻¹), indicating strong biogenic emission potential under tropical conditions. Emissions peaked in summer (42.1%) and were lowest in winter (8.5%), but showed a weaker seasonal contrast than many continental regions of China, reflecting persistent evergreen vegetation activity and relatively small seasonal variations in temperature and radiation. Although BVOCs dominated the regional volatile organic compound (VOC) budget, their contribution to surface ozone was moderate. In July 2019, BVOCs increased regional mean ozone by 1.2 parts per billion (ppb), accounting for 3.67% of total ozone, with localized enhancements mainly in northern and northeastern Hainan. Empirical Kinetic Modeling Approach (EKMA) and relative incremental reactivity (RIR) analyses showed that ozone formation was predominantly nitrogen oxides (NOx)-limited across most of the island, with transitional NOx–VOC co-control in northern urbanized areas. These results reveal a high-BVOC but NOx-constrained photochemical environment, where abundant biogenic emissions do not necessarily cause strong island-wide ozone enhancement because ozone production is limited by NOx availability and modulated by regional transport. This study provides a baseline assessment of BVOC emissions and ozone impacts in tropical island ecosystems and suggests that ozone mitigation in Hainan should prioritize anthropogenic NOx reduction, with coordinated NOx–VOC control in northern urbanized areas.