Role of canopy‐scale photochemistry in modifying biogenic‐atmosphere exchange of reactive terpene species: Results from the CELTIC field study

Abstract

[1] A one-dimensional canopy model was used to quantify the impact of photochemistry in modifying biosphere-atmosphere exchange of trace gases. Canopy escape efficiencies, defined as the fraction of emission that escapes into the well-mixed boundary layer, were calculated for reactive terpene species. The modeled processes of emission, photochemistry, diffusive transport, and deposition were highly constrained based on intensive observations collected in a Loblolly Pine plantation at Duke Forest, North Carolina, during the CELTIC field study. Canopy top fluxes for isoprene and a,b-pinene were not significantly altered by photochemistry as calculated escape efficiencies were greater than 0.90 for both species. b-caryophyllene emission and photochemistry were added to the canopy model as a surrogate for the reactive sesquiterpene class of species. b-caryopyllene escape efficiencies of 0.30 were calculated for midday summertime conditions. Urbanization scenarios were also performed to assess the impact of pollution on modifying biosphere-atmosphere exchange. Modest changes in escape efficiencies were calculated for a wide range of anthropogenic hydrocarbon and NO x mixing ratios suggesting a simple parameterization of escape efficiency in terms of grid cell NO x may be possible for incorporating the impact of canopy scale photochemistry within biogenic emission processing systems for regional air quality and climate models. The inferred magnitude of sesquiterpene ozonolysis reactions has important implications on both daytime and nighttime radical formation in the canopy.

Canopy level emissions of 2-methyl-3-buten-2-ol, monoterpenes, and sesquiterpenes from an experimental Pinus taeda plantation

“…However, the PTR-MS used in that study could not distinguish MBO from isoprene (Karl et al, 2012). Based on our above canopy isoprene and MBO observations, we suspect that a significant fraction of the isoprene reported by Stroud et al (2005) may have actually been MBO, although their vertical mixing ratio profiles (Fig. 5 of Stroud et al, 2005) suggest that isoprene maxima correspond to leaf area profiles of L. styraciflua (a high isoprene emitter, but not known to emit MBO) as well as leaf area profiles of P. taeda.…”

“…Based on our above canopy isoprene and MBO observations, we suspect that a significant fraction of the isoprene reported by Stroud et al (2005) may have actually been MBO, although their vertical mixing ratio profiles (Fig. 5 of Stroud et al, 2005) suggest that isoprene maxima correspond to leaf area profiles of L. styraciflua (a high isoprene emitter, but not known to emit MBO) as well as leaf area profiles of P. taeda. The corresponding above-canopy isoprene flux from Stroud et al (2005) was 550 μg C m −2 h −1 (under sunny to partly cloudy skies and air temperatures of 26-30°C) which compares to our late spring through summer MBO flux mean of 530 μg C m −2 h −1 .…”

“…5 of Stroud et al, 2005) suggest that isoprene maxima correspond to leaf area profiles of L. styraciflua (a high isoprene emitter, but not known to emit MBO) as well as leaf area profiles of P. taeda. The corresponding above-canopy isoprene flux from Stroud et al (2005) was 550 μg C m −2 h −1 (under sunny to partly cloudy skies and air temperatures of 26-30°C) which compares to our late spring through summer MBO flux mean of 530 μg C m −2 h −1 .…”

“…From this tower on July 24, 2003, Stroud et al (2005) observed midday (10 AM to 2 PM) above-canopy isoprene mixing ratios of 1.2-1.6 ppbv. Their total within-canopy vertical profile isoprene mixing ratios ranged from 1.5-2.1 ppbv, and isoprene oxidation products (methyl vinyl ketone + methacroleine) ranged from 2.0-1.7 ppbv.…”

“…CEE is the percentage of leaf emission of a given BVOC which escapes the canopy into the atmosphere above, and account for potential losses due to deposition and reactions with oxidants, primarily O 3 , NO 3 •, and OH•. These values range from approximately 10% for very reactive compounds such as β-caryophyllene to over 90% for the pinenes, isoprene and MBO (Stroud et al, 2005;Guenther et al, 2012). Since the (2) and (3)) used to describe the seasonal LAI pattern.…”