The tropical forest and fire emissions experiment: Emission, chemistry, and transport of biogenic volatile organic compounds in the lower atmosphere over Amazonia

Abstract

[1] Airborne and ground-based mixing ratio and flux measurements using eddy covariance (EC) and for the first time the mixed layer gradient (MLG) and mixed layer variance (MLV) techniques are used to assess the impact of isoprene and monoterpene emissions on atmospheric chemistry in the Amazon basin. Average noon isoprene (7.8 ± 2.3 mg/m 2 /h) and monoterpene fluxes (1.2 ± 0.5 mg/m 2 /h) compared well between ground and airborne measurements and are higher than fluxes estimated in this region during other seasons. The biogenic emission model, Model of Emissions of Gases and Aerosols from Nature (MEGAN), estimates fluxes that are within the model and measurement uncertainty and can describe the large observed variations associated with land-use change in the region north-west of Manaus. Isoprene and monoterpenes accounted for $75% of the total OH reactivity in this region and are important volatile organic compounds (VOCs) for modeling atmospheric chemistry in Amazonia. The presence of fair weather clouds (cumulus humilis) had an important impact on the vertical distribution and chemistry of VOCs through the planetary boundary layer (PBL), the cloud layer, and the free troposphere (FT). Entrainment velocities between 10:00 and 11:30 local time (LT) are calculated to be on the order of 8-10 cm/s. The ratio of methyl-vinyl-ketone (MVK) and methacrolein (MAC) (unique oxidation products of isoprene chemistry) with respect to isoprene showed a pronounced increase in the cloud layer due to entrainment and an increased oxidative capacity in broken cloud decks. A decrease of the ratio in the lower free troposphere suggests cloud venting through activated clouds. OH modeled in the planetary boundary layer using a photochemical box model is much lower than OH calculated from a mixed layer budget approach. An ambient reactive sesquiterpene mixing ratio of 1% of isoprene would be sufficient to explain most of this discrepancy. Increased OH production due to increased photolysis in the cloud layer balances the low OH values modeled for the planetary boundary layer. The intensity of segregation (I s ) of isoprene and OH, defined as a relative reduction of the reaction rate constant due to incomplete mixing, is found to be significant: up to 39 ± 7% in the $800-m-deep cloud layer. The effective reaction rate between isoprene and OH can therefore vary significantly in certain parts of the lower atmosphere.

Biogenic emission measurement and inventories determination of biogenic emissions in the eastern United States and Texas and comparison with biogenic emission inventories

“…The entrainment flux out of the boundary is estimated to be 30% of the emission flux as was recently found for isoprene flux measurements from an aircraft over the Amazonian rain forest [Karl et al, 2007] using the mixed layer gradient method. This might not be representative for the forest in the United States and could therefore contribute to the error in this calculation as discussed below.…”

“…Another large error involves the entrainment flux from the boundary layer. Here a constant flux of 30% from the emissions was used [Karl et al, 2007]. The entrainment flux certainly will not be a constant fraction of the emissions and will be larger or smaller than the 30% in different areas and time of day.…”

“…Only recently have results been published that use aircraft measurements on a somewhat larger spatial scale to quantitatively derive isoprene emissions and compare to emission inventories. For example, Karl et al [2007] found measured isoprene fluxes derived from aircraft measurements about 40% higher than predicted by MEGAN2 over Amazonia.…”

Observed and modeled ecosystem isoprene fluxes from an oak-dominated temperate forest and the influence of drought stress

“…On average, the model values were about 47.5% higher than observation-based results over the measuring period. This difference can be explained by (1) observation-based fluxes having strong sudden decreases, which are related to the measured approach (Karl et al, 2007); (2) low observation-based fluxes occurring on some days, due to the low mixing layer height and low measured isoprene concentrations on these days. Given the uncertainties in this approach, this comparison indicates that the local constrained MEGAN tends to predict the isoprene emission reasonably well for this location and time.…”

Cloud Shading Effects on Characteristic Boundary-Layer Length Scales

“…The initial atmospheric conditions used are based on the TROpical Forest and Fire Emissions Experiment (TROFFEE) (Karl et al 2007;Vilà-Guerau de Arellano et al 2011), adapted by Ouwersloot et al (2013) to ensure that shallow Cu convection occurs with no deep convection. The initial conditions follow Amazonian surface and upper-air conditions: the initial boundary-layer height is 200 m with a potential temperature (θ ) of 300 K and a specific humidity (q) of 16 × 10 −6 kg kg −1 .…”