Development of analytical method to measure halogenated volatile organic compounds in the Amazon rainforest

Halogenated volatile organic compounds (XVOCs) are volatile hydrocarbons that have at least one halogen atom. These substances are of global importance because their relatively long atmospheric lifetimes enable transport of halogen atoms to the stratosphere leading to stratospheric ozone depletion. Since the regulation of anthropogenic XVOC emissions through the Montreal Protocol and subsequent amendments, the importance of naturally formed XVOCs has been increasing. Chloromethane (CH₃Cl), for example, represents the largest natural source of chlorine in the stratosphere, currently contributing to approximately 16% of stratospheric ozone depletion, with plants suggested to be its major source. Other XVOCs like Chloroform (CHCl₃) can influence atmospheric chemistry more on a local scale due to their shorter atmospheric lifetimes.

In this work, we present the implementation of an instrumentational setup for long-term in-situ XVOC monitoring at the Amazon Tall Tower Observatory (ATTO) site. The 325 m tall tower, located in the pristine Amazon rainforest (circa 150 km NE of Manaus, Brazil), enables measurements of natural XVOC emissions almost free of anthropogenic influence due to its remote location.

The analytical setup consists of the tower inlet system and a Multi-Capillary Column Trapping System (MCCTS) coupled to GC-MS. Here we show several days of preliminary data for methyl chloride, isoprene and several CFC species as a proof of concept. Furthermore, we examine the applicability of simultaneous CFC measurements as a “natural internal standard” for the correction of systematic errors like MS sensitivity changes.

The presented analytical method is able to provide data in form of atmospheric mixing ratios of the desired XVOC compounds at four different heights (ground level, 80 m, 150 m and 320 m) with a time resolution of approximately two hours per height. It has the potential to open up new opportunities to better quantify the XVOC net production and consumption of the Amazon Basin ecosystem and to understand the underlying processes, especially with respect to future ecosystem transformations due to climate and land use changes.