Research output: Contribution to Journal/Magazine › Journal article › peer-review
Research output: Contribution to Journal/Magazine › Journal article › peer-review
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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 -