Comparing the loss of polycyclic hydrocarbons (PAHs) in soil and the atmosphere

Gases can partition between soil and the atmosphere, depending on their physicochemical properties. They can be degraded in both compartments by chemical or microbial processes. Polycyclic aromatic hydrocarbons (PAHs) are among such gases that are ubiquitous in the atmosphere and soils. They result mainly from the incomplete combustion or pyrolysis of organic matter; they are of concern due to their toxic, mutagenic and carcinogenic properties. PAHs are semivolatile, highly lipophilic and persistent organic pollutants.

The biodegradation of PAHs and other semivolatile compounds in soil has been extensively studied. Their chemical processing in the atmosphere has also been addressed by means of many model studies. However, even though the existence of bacteria in the atmosphere is well known, the role of atmospheric biodegradation has not been extensively studied.

We present a new model that includes explicit chemical processes in the atmospheric gas and aqueous phases, as well as by biodegradation in fog droplets, coupled to a soil module by volatilization and deposition processes. PAHs in soil are transported between three aerobic layers where they can be degraded by chemical and bacterial processes.

Even though PAH lifetimes in soil and the atmosphere might be similar, our initial model results suggest that degradation in soil represents the major sink for PAHs. While biodegradation in soil is predicted to be the main loss process of PAHs, oxidation processes in the atmospheric gas phase also contribute significantly to overall PAH loss.

Our model represents a new tool to assess the relative and absolute sink strengths of pollutants and other compounds in soil and the atmosphere. We will give an outlook on planned model studies for other compounds and discuss reasons for their choice.