Transport and Transformation of Chemicals in the Atmosphere

Tuesday, July 3, 2018

By Hélène Angot, Project 2 Postdoc

Have you ever wondered how we can trace the fate of toxic pollutants in the atmosphere and forecast air pollution in a given region? If so, this blog post is for you! We are going to focus on polycyclic aromatic hydrocarbons (PAHs), one of the MIT Superfund Research Program’s chemicals of interest, but keep in mind that the principles discussed here can be applied to other pollutants.

Let’s start with a review of PAHs. What are these compounds, where do they come from, and why are they of concern to public health? PAHs are hydrocarbons (i.e., they only contain carbon and hydrogen atoms) that are composed of fused “aromatic” carbon rings (aromatic is a chemical designation here that describes the ring structure, not a reference to smell). For example, naphthalene has two fused rings, pyrene has four, and benzo[a]pyrene has five. (More info regarding their structure can be found here.) 

They are primarily emitted to the atmosphere by combustion of carbon-containing fuels, and can come from industrial, commercial, vehicular, and residential sources, as well as natural sources, like forest fires and volcanoes. Industrial processes produce most atmospheric PAH emissions, but individuals can also directly expose themselves to PAHs from sources like wood stoves, cigarettes, or grilled food. These compounds are potent carcinogens, and the U.S. Environmental Protection Agency (EPA) has established a list of 16 priority PAHs.

So, PAHs are emitted to the atmosphere. Then what? Now is a good time to describe their atmospheric life cycle. PAHs entering the atmosphere can be transported over very long distances (as far as pole-to-pole!) before deposition (through rain, for instance) to environmental surfaces (e.g., soils, vegetation, waters). Upon deposition, they can return to vapor form and re-enter the atmosphere, a process called “re-emission”. PAHs can also react with other atmospheric pollutants (such as ozone, nitrogen oxides, and sulfur dioxides) to form so-called degradation products, which can be even more toxic than the original chemicals. How far a given PAH can travel and how quickly it deposits or is re-emitted depends, among other things, on its volatility (i.e., it tendency to evaporate at ambient temperature). In general, low-weight PAHs (2, 3, or 4 rings) are more volatile and exist mainly in the gas phase, while heavier PAHs occur mainly in the particulate phase (i.e., they adhere to the surface of atmospheric particles) and are deposited more readily. However, the tendency of a given PAH to be in the gas or in the particle phase also depends on temperature: as temperature increases, it is more likely to be found in the gas phase.

As you can see, the atmospheric life cycle of PAHs is rather complex. Now the more pressing question: how can we tell if the atmospheric concentration of PAHs in a given area is of concern? The easiest way to answer that question is to use a sensor to analyze the local concentration, which gives a measurement for a certain place at a certain time. However, concentration is expected to vary with time of year and location, and collecting readings from sensors requires time (and therefore money). As we increase the temporal and spatial resolution of measurements, this approach quickly becomes too expensive. The other (and complementary!) option is to use a chemical transport model.

A chemical transport model is a numerical model used to simulate the fate of compounds in the atmosphere. Using mathematical equations, the model represents the entire cycle for the chemical species of interest, such as transport, production/loss, and deposition. The model is driven by meteorology (e.g., wind speed and direction) to simulate transport. We also rely on emissions inventories, databases that list the amount of PAHs emitted each month or year from each country and sector of activity. The atmosphere is divided into grid boxes, which are like 3D latitude, longitude, altitude boxes that define portions of the atmosphere. The atmospheric concentration of PAHs in a given box depends on the magnitude of local and foreign emissions, the relative importance of deposition and re-emissions, and the local composition of the atmosphere that degrades PAHs. 

If we increase the number of grid boxes for a given area, the size of each box becomes smaller, and the model better represents smaller-scale features (i.e., the horizontal resolution of the model improves). However, it also takes more time to calculate the model results. A compromise between resolution and computation time has to be made! The example below shows three typical horizontal resolutions that can be achieved with one of the most widely used chemical transport models, GEOS-Chem (4x5°, 2x2.5°, and 0.5x0.667°, for the atmospheric scientists out there). In these three simulations, the 48 contiguous states of the U.S. are represented by approximately 70, 290, or 4,325 boxes, respectively, and you can see that fine-scale details become clearer with increased resolution.

Here at the MIT SRP Center, Project 2 is investigating the fate of the EPA’s 16 priority PAHs and their degradation products. Using the chemical transport model GEOS-Chem, we can predict the presence and relative abundance of the various compounds over regions of interest and identify priority species for further exposure studies (such as the biological consequences that will be determined in Projects 3 and 5). Knowing when and where specific chemicals are found allows scientists, environmental groups, and the government to prioritize research and cleanup efforts and helps inform new policies to address air pollution.