Gaseous fluxes of peroxyacetyl nitrate (PAN) into plant leaves

Abstract

Peroxyactyl nitrate (PAN) is the most abundant of the gaseous organic nitrates produced from the photochemistry of hydrocarbons and NOx (i.e. ozone and smog production). PAN is known to be toxic to plants and also as a reservoir for the transport nitrogen dioxide in the troposphere. Here, the effect of vegetation on PAN deposition was investigated in four plant species by measuring leaf fluxes of PAN in a dynamic leaf chamber using atmospheric PAN fumigations between 0.7 and 18 nmol mol-1. A linear relationship was observed between PAN flux and ambient PAN mixing ratio for all species. Depending on the species, measured PAN flux varied between 11 and 24 pmol m -2 s-1. Measured fluxes of PAN accounted for 12-48% of the PAN flux predicted solely from modelled stomatal conductance to PAN, suggesting the presence of a mesophyllic resistance to PAN uptake. The brief (approximately 5-10 min) exposure to PAN during uptake measurements did not affect photosynthesis, transpiration or conductance to water vapour. Increasing stomatal resistance by varying the vapour pressure gradient between the leaf chamber and leaf internal air space led to a corresponding drop in PAN uptake. Varying leaf nitrogen and total leaf-ascorbate concentrations did not appear to influence PAN uptake as had been reported for other reactive odd-nitrogen gases. Measured and model-predicted PAN fluxes were offset, but correlated suggesting that PAN flux could be estimated using established stomatal conductance algorithms.