Biogenic-volatile-organic-compound-profiles in the amazon rainforest
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Abstract
Biogenic volatile organic compounds (BVOC) released from the Amazon rainforest play a key role in the local, regional, and global atmospheric chemistry. A huge variety of BVOC is emitted from the Amazonian biosphere into the atmosphere and even more are formed via subsequent oxidative reactions. This makes the canopy and adjacent atmospheric layers a chemically complex interface of great importance. BVOC observations at this interface link biological metabolic mechanisms and chemical communication to oxidative atmospheric chemistry, and to turbulent boundary layer dynamics. This doctoral thesis focuses on the vertical and temporal distribution of selected BVOC, measured from a tower at the heights of 80, 150, and 325 m with a proton transfer time-of-flight mass spectrometer (PTR-ToF-MS). With this in-situ measurement technique, many of the most atmospherically relevant BVOC can be detected with a high time resolution. It has been found that ozone, which is ubiquitous in the troposphere can cause interferences in the detection of some BVOC. The interfering signals can be generated within the inlet tubing or inside the instrument. A laboratory experiment to comprehensively investigate this effect and the influence of an ozone scrubber under varying humidity was performed with a PTR-ToF-MS and a gas chromatograph-mass spectrometer (GC-MS). It showed that ozone can induce a decrease in the signals of certain unsaturated species, including terpenes, but it can also artificially enhance the signals of most tested carbonyl compounds. A thiosulfate ozone scrubber was applied to reduce the interferences, which was found to not influence the detection of the tested VOC itself. It successfully inhibited the interference for all tested compounds, except for sesquiterpenes, and its performance and lifetime improved under humid conditions. It was also shown that the ozone levels in an unpolluted environment such as the remote parts of the Amazon (O3 < 40 ppb) are not sufficient to induce significant interferences for all tested compounds except for sesqui- and to a smaller extent monoterpenes, due to their high ozone reactivity.
Measurement of the vertical distribution of BVOC led to two main studies that contribute to the characterization of the canopy-atmosphere interface region of the Amazon Forest. In the first, the vertical gradients of isoprene and its combined oxidation products methyl ethyl ketone, methacrolein, and isoprene hydroxy hydroperoxide were scrutinized. The observations were used to constrain the Dutch Large Eddy Simulation (DALES), which was applied to investigate the sensitivity of the isoprene gradient towards chemical loss, dominated by reaction with the OH radical, and turbulent transport leading to dilutive mixing. Moreover, the impact of segregation through inhomogeneous mixing, a small-scale phenomenon influencing the rate of atmospheric reactions was also considered. The simulation showed that more than 50 % of the decrease in isoprene over the first 325 m of the atmosphere was determined by dilutive mixing from above, while the residual loss was attributed to the reaction with OH. This relation was used to infer a concentration of the OH radical ranging from 0.2 (0.1, 7.4) × 105 to 2.2 (0.2, 3.8) × 106 molecules cm-3 s-1, necessary to account for the observed residual loss. For the calculation of the OH concentration, an estimation of the mixing timescale between 80 and 325 m was crucial. Therefore a method from the field of speech recognition (Dynamical Time Warping) was utilized to determine a mixing timescale of 105 minutes in the morning which decreases to 15 minutes at 15:00 local time, based on the observed heating at two heights in the lowermost atmosphere. Those results were useful in examining the vertical distribution of carbonyl compounds at the same measurement site, which forms the second study of BVOC above the Amazon Forest. Usually, isomeric carbonyl compounds, i.e. aldehydes and ketones cannot be measured separately by conventional PTR-ToF-MS using H3O+ as the reagent ion. Their distribution in the atmosphere is expected to be different though, since ketones have significantly longer atmospheric lifetimes (days to weeks) than aldehydes (hours to days). Biogenic emission ratios of these compounds are poorly known. To overcome this limitation for carbonyl compounds, NO+ chemical ionization has been shown to be effective. The diel variability and vertical dispersion between 80 and 325 m of the ketones indicated a strong source close to the forest’s canopy. The vertical distribution of aldehydes varied from compound to compound depending on their chemical precursors, direct emission sources, and atmospheric lifetimes. At nighttime, an overall loss of carbonyl species was found and mainly attributed to canopy deposition and uptake by leaves.