Seasonal changes in chlorine and methoxyl content of leaves of deciduous trees and their impact on release of chloromethane and methanol at elevated temperatures

Abstract

Environmental context Chloromethane is the most abundant naturally produced chlorine-containing organic compound, responsible for ~16% of chlorine-catalysed stratospheric ozone destruction. A significant source of this gas is emission from biomass by reaction between chloride ion and methoxyl groups of the biopolymers pectin and lignin. The seasonal changes in the chlorine and methoxyl pools observed in leaves of several deciduous tree species have implications for understanding chlorine volatilisation during biomass burning and estimation of the global chloromethane budget. Abstract Atmospheric chloromethane (CH3Cl) plays a role in the destruction of stratospheric ozone. Previous studies suggest an important source of this gas is emission from leaves and leaf litter at ambient and elevated (150–350°C) temperatures. In this study, the total chlorine and OCH3 content of leaves of the deciduous temperate tree species ash, beech, Norway maple and oak were measured throughout the 2004 and 2005 growing seasons. The total chlorine content increased with leaf age. The overall seasonal accumulation varied between five- and twenty-one fold, dependent on both year and species. Throughout the 2004 growing season, the OCH3 pool and the release of CH3Cl and methanol (CH3OH) from leaves of ash and Norway maple were monitored on heating to 350°C. The amounts of CH3Cl released increased linearly as leaf chlorine accumulated whereas emissions of CH3OH did not substantially change. Conversion of chlorine to CH3Cl was lower in the spring than during the summer and autumnal senescence period, ranging from 22 to 58%. No correlation was found between leaf OCH3 content and either CH3Cl or CH3OH release. The percentage conversion of OCH3 to the summed concentrations of CH3OH and CH3Cl ranged from 41 to 66%. The plant components pectin and lignin were identified as two major sources of the CH3 group in CH3Cl and CH3OH and emissions ceased when the OCH3 pool contributing the methyl moiety was exhausted (>350°C). These findings have implications for estimation of CH3Cl release during biomass burning and for our understanding of chlorine volatilisation during energy production from biomass.