The influence of the tropical rainforest on atmospheric CO and CO2 as measured by aircraft over Surinam, South America

Abstract

Gradients of CO and CO₂, taken between 12:00 and 15:00 local time, from the boundary layer over the tropical rainforest in Surinam were determined as 29 pmol/mol km⁻¹ and −8.9 nmol/mol km⁻¹, respectively, with a distance of south from the coast. For one CO₂ molecule fixed in tropical forests 0.33% CO was produced. From an extrapolation of the CO gradient to the global scale we deduce that approximately 19% of C emitted as isoprene from tropical forests is converted to C in CO. From an extrapolation of the CO₂ gradient we estimate that approximately 1.2% of the global atmospheric CO₂ is converted each year into tropical seasonal and rainforests.

CO production from isoprene was calculated using an explicit gas-phase photochemical model but was found to be insufficient to account for the gradients measured. Diurnal variation in CO was controlled by a complex interplay between advection, chemical formation from natural NMHCs, direct soil emissions, removal by HO, and possible night-time uptake by soil. Diurnal variations of CO₂ in the boundary layer were controlled by vegetation.

Over the 12.5 km altitude range of the aircraft, a high degree of variability was observed in CO, CO₂ and northerly wind components. Two biomass burning events were identified and the ratio of delta CO and delta CO₂ was determined in each case with a two-sided linear regression. A ratio of 12.1% was found in plumes from smouldering cooking or clearing fires at low-altitude. A ratio of 5.9% was found for a plume encountered between 10–12 km. The lower ratio of delta CO and delta CO₂ indicates hotter, more complete, flame burning than the cooking or clearing fires. Emissions from savanna fires in the Brazil, Colombia and Venezuela regions coupled with deep convection and long-range advection are proposed to explain the observations.