Chemistry
The Interaction of Polycyclic Aromatic Hydrocarbons with Atmospheric Aerosols
Quick fact
Polycyclic aromatic hydrocarbons (PAHs) with four or more rings are almost entirely bound to particulate matter in the atmosphere, making them a key component of the toxic cocktail in fine particles that can penetrate deep into the lungs and are linked to cancer.
Why this is interesting
You've probably heard of PM2.5, but did you know that some of the most toxic chemicals in the air hitch a ride on these tiny particles? How do polycyclic aromatic hydrocarbons—sprawling carbon rings—bind to atmospheric aerosols, and why does that matter for your health?
Read the full explanation
Understanding The Interaction of Polycyclic Aromatic Hydrocarbons with Atmospheric Aerosols
Atmospheric aerosols are tiny solid or liquid particles suspended in air. They come from natural sources like sea spray and wildfires, and from human activities like burning coal, diesel, or biomass. Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds made of fused benzene rings, produced during incomplete combustion of carbon-containing materials. Many PAHs are known carcinogens. This interaction is governed by gas–particle partitioning: PAHs are 'semivolatile'—meaning they exist in both gas and particle phases depending on temperature, humidity, and the nature of the particles. When they adsorb onto a particle, they coat its surface or dissolve into its organic matter. Think of the air as a busy highway where PAH molecules are like passengers deciding whether to walk (gas phase) or board a bus (aerosol particle). The bus can take them much further and into your lungs much more efficiently. The process depends on the aerosol's properties: carbonaceous particles (like soot) are excellent at adsorbing PAHs due to their strong hydrophobic surface interactions. In contrast, mineral dust or sea salt particles are less efficient. Over time, particles undergo 'aging'—chemical reactions in the atmosphere—which can either release PAHs or transform them into even more toxic products.
A deeper explanation
The mechanism behind PAH–aerosol interaction is primarily physical adsorption onto the particle surface, driven by van der Waals forces and hydrophobic effects. For particles containing organic matter, absorption—like a sponge soaking up liquid—is also significant. The equilibrium partitioning can be described by absorption or adsorption models, which relate the fraction in the particle phase to the aerosol organic matter content and the PAH's vapor pressure. Chemically, PAHs are nonpolar and have low vapor pressures, especially as molecular weight increases. As a result, lighter PAHs (e.g., naphthalene) remain mostly as gases, while heavier PAHs (e.g., benzo[a]pyrene) condense onto particles. Once on the surface, PAHs can undergo photochemical reactions—oxidized by ozone, hydroxyl radicals, or nitrate radicals—forming nitro-PAHs or quinones, which are often more mutagenic and toxic than their parent compounds. This interaction has profound consequences: particulate-bound PAHs have longer atmospheric lifetimes, allowing long-range transport across continents. They also become a serious health hazard, as fine particles (PM2.5) carry PAHs deep into the respiratory system. Moreover, deposition of PAHs onto soil and water bodies affects ecosystems. Understanding these interactions is therefore crucial for predicting PAH fate, exposure risks, and designing effective control strategies.