Measurement and interpretation of isoprene fluxes and isoprene, methacrolein, and methyl vinyl ketone mixing ratios at the PROPHET site during the 1998 Intensive

Abstract

[1] Mixing ratios of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) were determined continuously during an 8-day period in the summer of 1998 at a rural forested site located within the University of Michigan Biological Station (UMBS). The measurements were obtained as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) study. Fluxes of isoprene were concurrently measured at a nearby tower (AmeriFlux, located 132 m north-northeast of the PROPHET tower). Following the study, 1-kmresolution emission estimates were derived for isoprene within a 60-km radius of the tower using forest density estimates (Biogenic Emissions Inventory System (BEIS3) model). Measured isoprene fluxes at the site compared well with modeled isoprene fluxes when using BEIS3 and a detailed leaf litter-fall data set by tree species from the UMBS site. Mean midday (1000 -1400 LT) mixing ratios for isoprene, MACR, and MVK were 1.90 ± 0.43, 0.07 ± 0.01, and 0.14 ± 0.04 ppbv, respectively. Median midday mixing ratios of these compounds were 1.96 ± 0.26, 0.06 ± 0.02, and 0.10 ± 0.02 ppbv, respectively. Ratios of the isoprene oxidation products to isoprene are understood in the context of previous laboratory and field measurement studies of these compounds and a simple consecutive reaction scheme model. Results of the model indicate that the air masses studied represented relatively fresh emissions with a photochemical age of measured isoprene between 3.6 and 18 min, which is significantly less than the photochemical lifetime of isoprene (t = 45 min at [OH] = 3.35 Â 10 6 molecules cm À3 ). Thus a large portion of the isoprene that reaches the manifold has not had time to react completely with OH, yielding lower than expected ratios based on model calculations that do not explicitly take this into account. A rapid decrease in isoprene mixing ratios was observed soon after sunset, followed by a slower decay throughout the rest of the night. Emission maps were generated indicating that isoprene fluxes are highest in the immediate vicinity of the tower compared to the surrounding area of the site. Thus vertical diffusion and advection from the surrounding region are postulated to cause the observed initial rapid decrease in isoprene at the site. The second isoprene decay may be due to chemistry and/or dynamics, but the effects cannot be separated with the available data.

Impact of isoprene and HONO chemistry on ozone and OVOC formation in a semirural South Korean forest

“…In addition, the ratios of MVK + MACR, major isoprene oxidation products and isoprene at this period, are significantly lower than those of late morning to early afternoon. The enhanced isoprene levels in the late afternoon or early evening have been also reported in previous studies (Apel et al, 2002;Bryan et al, 2012). The branch enclosure observations demonstrate that isoprene is not emitted from the pine plantation but rather transported from surrounding broadleaf forests, because right outside of the pine plantation (200 m × 200 m) is a forested area dominated by oak trees.…”

Peroxy radical chemistry and OH radical production during the NO<sub>3</sub>-initiated oxidation of isoprene

“…Thus, the isoprene:NO 3 ratios during the experiment, while variable, are likely on the order of ∼ 1000 when most of the reaction takes place. Apel et al (2002) and Brown et al (2009) find isoprene concentrations ∼ 1 ppb at sunset in Northern Michigan and New England, respectively, so NO 3 levels of ∼ 1 ppt would yield similar ratios. Studies of daytime NO 3 oxidation find even higher ratios, with isoprene concentrations ∼ 10 ppb and NO 3 concentrations ∼ 0.5 ppt, or a ratio of ∼ 20 000 (Brown et al, 2005;Fuentes et al, 2007).…”

Biomass burning emissions disturbances on the isoprene oxidation in a tropical forest

“…[MVK+MACR+ISOPOOH] + OH (6.1  10 -11 cm 3 molecules -1 sec -1 ), and we are assuming a total yield of 0.55 of MVK+MACR+ISOPOOH from the OH + Isoprene reaction (Apel, 2002;Karl et al, 2007). We estimated the processing time as …”

“…The OH concentrations from the [MVK+MACR+ISOPOHH]/[Isoprene] ratio using the sequential reaction model were originally developed by Apel, 2002 andStroud et al, 2001 and modified according to the approach of Karl et al (2007). This method can be used to investigate the impact of vertical transport, representing the processing time of the isoprene and its oxidation products 205 from the surface to the atmosphere through the ratio of PBL depth and the convective velocity scale.…”

“…Motivated by the discrepancies between model and observation of OH production in the atmosphere and the influence of the biomass burning plumes in the isoprene reactivity with OH during the day, we propose in this study a refinement in the OH estimation method that has been applied by several previous studies (Apel, 2002;Karl et al, 2007;Kuhn et al, 2007;Stroud et al, 2001). …”