Long-Term Measurements of Volatile Organic Compound Fluxes and Concentrations By Proton Transfer Reaction-Mass Spectrometry from an Amazonian Terra Firme Ecosystem (CLAIRE-UK)

Abstract:

Tropical broad leaf species are the highest contributors to biogenic volatile organic compounds (BVOCs) globally making the Amazon tropical forest a major global source. BVOCs can affect atmospheric chemistry, air quality and climate by influencing the oxidative capacity and radiative balance of the atmosphere. Isoprene is the main constituent of total BVOC emissions, however, a wide suite of compounds such as methyl vinyl ketone and methacrolein (MVK and MACR), methyl ethyl ketone, acetone, and monoterpenes play an important role. Due to the remoteness of tropical background sites, there are few continuous long-term canopy scale BVOC measurements and more are needed to improve global atmospheric chemistry models.

Approximately one year of continuous high temporal resolution BVOC measurements were made during 2013-2014 as part of the CLAIRE-UK project. Measurements were carried out from the top of a tower above a primary terra firme forest canopy situated approximately 60km north of Manaus, Brazil. A high sensitivity proton transfer reaction-(quadrupole) mass spectrometer (PTR-(Q)MS) was deployed alongside a sonic anemometer to quantify BVOC fluxes using disjunct eddy covariance. Mixing ratios of a range of compounds were measured for 45 minutes at a frequency of 0.5 Hz for flux calculation.

Here we present the first results of BVOC flux and mixing ratio measurements from September 2013 to July 2014. Diurnal variability, seasonal differences and possible driving factors will be discussed. In example, positive isoprene fluxes were observed during the day, closely following light intensity and temperature. Diurnal maxima, typically in the order of 5-15 mg isoprene m^-2 h^-1 were observed between 11:00 and 14:00 local time. Higher emissions occurred during the drier and warmer months from September to December. Preliminary analyses suggest deposition of isoprene oxidation products MVK and MACR, though there is evidence of emission at higher temperatures during some afternoons.