Effect of chemical degradation on fluxes of reactive compounds - A study with a stochastic Lagrangian transport model
In the analyses of VOC fluxes measured above plant canopies, one usually\nassumes the flux above canopy to equal the exchange at the surface. Thus\none assumes the chemical degradation to be much slower than the\nturbulent transport. We used a stochastic Lagrangian transport model in\nwhich the chemical degradation was described as first order decay in\norder to study the effect of the chemical degradation on above canopy\nfluxes of chemically reactive species. With the model we explored the\nsensitivity of the ratio of the above canopy flux to the surface\nemission on several parameters such as chemical lifetime of the\ncompound, friction velocity, stability, and canopy density. Our results\nshow that friction velocity and chemical lifetime affected the loss\nduring transport the most. The canopy density had a significant effect\nif the chemically reactive compound was emitted from the forest floor.\nWe used the results of the simulations together with oxidant data\nmeasured during HUMPPA-COPEC-2010 campaign at a Scots pine site to\nestimate the effect of the chemistry on fluxes of three typical biogenic\nVOCs, isoprene, alpha-pinene, and beta-caryophyllene. Of these, the\nchemical degradation had a major effect on the fluxes of the most\nreactive species beta-caryophyllene, while the fluxes of alpha-pinene\nwere affected during nighttime. For these two compounds representing the\nmono- and sesquiterpenes groups, the effect of chemical degradation had\nalso a significant diurnal cycle with the highest chemical loss at\nnight. The different day and night time loss terms need to be accounted\nfor, when measured fluxes of reactive compounds are used to reveal\nrelations between primary emission and environmental parameters.