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The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest

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The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest. / Pugh, Thomas; Mackenzie, Rob; Langford, Ben; Nemitz, E.; Misztal, P. K.; Hewitt, C. N.

In: Atmospheric Chemistry and Physics , Vol. 11, No. 9, 2011, p. 4121-4134.

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Pugh, Thomas ; Mackenzie, Rob ; Langford, Ben ; Nemitz, E. ; Misztal, P. K. ; Hewitt, C. N. / The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest. In: Atmospheric Chemistry and Physics . 2011 ; Vol. 11, No. 9. pp. 4121-4134.

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@article{7088c9243ad14b288c1ea8fceecf3089,
title = "The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest",
abstract = "Biogenic volatile organic compounds (BVOCs) such as isoprene constitute a large proportion of the global atmospheric oxidant sink. Their reactions in the atmosphere contribute to processes such as ozone production and secondary organic aerosol formation. However, over the tropical rainforest, where 50% of the global emissions of BVOCs are believed to occur, atmospheric chemistry models have been unable to simulate concurrently the measured daytime concentration of isoprene and that of its principal oxidant, hydroxyl (OH). One reason for this model-measurement discrepancy may be incomplete mixing of isoprene within the convective boundary layer, leading to patchiness or segregation in isoprene and OH mixing ratios and average concentrations that appear to be incompatible with each other. One way of capturing this effect in models of atmospheric chemistry is to use a reduced effective rate constant for their reaction. Recent studies comparing atmospheric chemistry global/box models with field measurements have suggested that this effective rate reduction may be as large as 50%; which is at the upper limit of that calculated using large eddy simulation models. To date there has only been one field campaign worldwide that has reported co-located measurements of isoprene and OH at the necessary temporal resolution to calculate the segregation of these compounds. However many campaigns have recorded sufficiently high resolution isoprene measurements to capture the small-scale fluctuations in its concentration. Assuming uniform distributions of other OH production and loss processes, we use a box model of atmospheric chemistry, constrained by the spectrum of isoprene concentrations measured, as a virtual instrument, to estimate the variability in OH at a point and hence, to estimate the segregation intensity of isoprene and OH from high-frequency isoprene time series. The method successfully reproduces the only directly observed segregation, using measurements made in a deciduous forest in Germany. The effective rate constant reduction for the reaction of isoprene and OH over a South-East Asian rainforest is calculated to be typically <15%. Although there are many unconstrained uncertainties, the likely nature of those processes suggests that this value represents an upper limit. The estimate is not sensitive to heterogeneities in NO at this remote site, unless they are correlated with those of isoprene, or to OH-recycling schemes in the isoprene oxidation mechanism, unless the recycling happens in the first reaction step. Segregation alone is therefore unlikely to be the sole cause of model-measurement discrepancies for isoprene and OH above a rainforest.",
keywords = "CONVECTIVE BOUNDARY-LAYER, VOLATILE ORGANIC-COMPOUNDS, BOX MODEL, NONMETHANE HYDROCARBONS, HYDROXYL RADICALS, NORTH-ATLANTIC, TROPOSPHERE, SIMULATION, OZONE, OH",
author = "Thomas Pugh and Rob Mackenzie and Ben Langford and E. Nemitz and Misztal, {P. K.} and Hewitt, {C. N.}",
year = "2011",
doi = "10.5194/acp-11-4121-2011",
language = "English",
volume = "11",
pages = "4121--4134",
journal = "Atmospheric Chemistry and Physics ",
issn = "1680-7316",
publisher = "Copernicus GmbH (Copernicus Publications) on behalf of the European Geosciences Union (EGU)",
number = "9",

}

RIS

TY - JOUR

T1 - The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest

AU - Pugh, Thomas

AU - Mackenzie, Rob

AU - Langford, Ben

AU - Nemitz, E.

AU - Misztal, P. K.

AU - Hewitt, C. N.

PY - 2011

Y1 - 2011

N2 - Biogenic volatile organic compounds (BVOCs) such as isoprene constitute a large proportion of the global atmospheric oxidant sink. Their reactions in the atmosphere contribute to processes such as ozone production and secondary organic aerosol formation. However, over the tropical rainforest, where 50% of the global emissions of BVOCs are believed to occur, atmospheric chemistry models have been unable to simulate concurrently the measured daytime concentration of isoprene and that of its principal oxidant, hydroxyl (OH). One reason for this model-measurement discrepancy may be incomplete mixing of isoprene within the convective boundary layer, leading to patchiness or segregation in isoprene and OH mixing ratios and average concentrations that appear to be incompatible with each other. One way of capturing this effect in models of atmospheric chemistry is to use a reduced effective rate constant for their reaction. Recent studies comparing atmospheric chemistry global/box models with field measurements have suggested that this effective rate reduction may be as large as 50%; which is at the upper limit of that calculated using large eddy simulation models. To date there has only been one field campaign worldwide that has reported co-located measurements of isoprene and OH at the necessary temporal resolution to calculate the segregation of these compounds. However many campaigns have recorded sufficiently high resolution isoprene measurements to capture the small-scale fluctuations in its concentration. Assuming uniform distributions of other OH production and loss processes, we use a box model of atmospheric chemistry, constrained by the spectrum of isoprene concentrations measured, as a virtual instrument, to estimate the variability in OH at a point and hence, to estimate the segregation intensity of isoprene and OH from high-frequency isoprene time series. The method successfully reproduces the only directly observed segregation, using measurements made in a deciduous forest in Germany. The effective rate constant reduction for the reaction of isoprene and OH over a South-East Asian rainforest is calculated to be typically <15%. Although there are many unconstrained uncertainties, the likely nature of those processes suggests that this value represents an upper limit. The estimate is not sensitive to heterogeneities in NO at this remote site, unless they are correlated with those of isoprene, or to OH-recycling schemes in the isoprene oxidation mechanism, unless the recycling happens in the first reaction step. Segregation alone is therefore unlikely to be the sole cause of model-measurement discrepancies for isoprene and OH above a rainforest.

AB - Biogenic volatile organic compounds (BVOCs) such as isoprene constitute a large proportion of the global atmospheric oxidant sink. Their reactions in the atmosphere contribute to processes such as ozone production and secondary organic aerosol formation. However, over the tropical rainforest, where 50% of the global emissions of BVOCs are believed to occur, atmospheric chemistry models have been unable to simulate concurrently the measured daytime concentration of isoprene and that of its principal oxidant, hydroxyl (OH). One reason for this model-measurement discrepancy may be incomplete mixing of isoprene within the convective boundary layer, leading to patchiness or segregation in isoprene and OH mixing ratios and average concentrations that appear to be incompatible with each other. One way of capturing this effect in models of atmospheric chemistry is to use a reduced effective rate constant for their reaction. Recent studies comparing atmospheric chemistry global/box models with field measurements have suggested that this effective rate reduction may be as large as 50%; which is at the upper limit of that calculated using large eddy simulation models. To date there has only been one field campaign worldwide that has reported co-located measurements of isoprene and OH at the necessary temporal resolution to calculate the segregation of these compounds. However many campaigns have recorded sufficiently high resolution isoprene measurements to capture the small-scale fluctuations in its concentration. Assuming uniform distributions of other OH production and loss processes, we use a box model of atmospheric chemistry, constrained by the spectrum of isoprene concentrations measured, as a virtual instrument, to estimate the variability in OH at a point and hence, to estimate the segregation intensity of isoprene and OH from high-frequency isoprene time series. The method successfully reproduces the only directly observed segregation, using measurements made in a deciduous forest in Germany. The effective rate constant reduction for the reaction of isoprene and OH over a South-East Asian rainforest is calculated to be typically <15%. Although there are many unconstrained uncertainties, the likely nature of those processes suggests that this value represents an upper limit. The estimate is not sensitive to heterogeneities in NO at this remote site, unless they are correlated with those of isoprene, or to OH-recycling schemes in the isoprene oxidation mechanism, unless the recycling happens in the first reaction step. Segregation alone is therefore unlikely to be the sole cause of model-measurement discrepancies for isoprene and OH above a rainforest.

KW - CONVECTIVE BOUNDARY-LAYER

KW - VOLATILE ORGANIC-COMPOUNDS

KW - BOX MODEL

KW - NONMETHANE HYDROCARBONS

KW - HYDROXYL RADICALS

KW - NORTH-ATLANTIC

KW - TROPOSPHERE

KW - SIMULATION

KW - OZONE

KW - OH

U2 - 10.5194/acp-11-4121-2011

DO - 10.5194/acp-11-4121-2011

M3 - Journal article

VL - 11

SP - 4121

EP - 4134

JO - Atmospheric Chemistry and Physics

JF - Atmospheric Chemistry and Physics

SN - 1680-7316

IS - 9

ER -