Organic compounds that absorb sunlight are produced on the wet surfaces of atmospheric particles

Heterogeneous oxidation reactions can occur when mixing chemicals that are in two different physical states, such as liquid and gas. for example, in the atmosphere the reaction of nitrate gases (NO₃) free radicals that reach the liquid surface of aerosol particles containing aromatic pollutants from fires.

In such a situation, the heterogeneous reaction converts phenols into nitroaromatic compounds that absorb sunlight. The process should be important at night, when the concentration of nitrate radicals peaks and hazardous pollutants can be transformed to the surface of suspended particles by mechanisms not previously considered by atmospheric scientists.

Given the vast distribution of airborne particles, this nocturnal process should be quite common in areas affected by forest fire pollution or fossil fuel burning. Therefore, phenolic molecules can play an active role in changing the absorption of sunlight by atmospheric particles and chemical structure, which in turn affect air quality and climate.

“Previous textbook material generally explained that nitroaromatic compounds are formed by gas-phase nitration reactions, but our new work shows that reactions on wet aerosol surfaces are very efficient for the production of nitrophenols,” said Professor Marcelo Guzman from the Department of Chemistry, University of Kentucky.

“There is no previous scientific examination of the nitrification reactions that occur at the water-air interface, how such processes are initiated, or the mechanisms by which nitrate radicals can contribute to such reactions.”

Phenols are primary pollutants released into the air during fires or produced in the atmosphere when solvents from fossil fuel refineries and other industries leak into the air and oxidize. The study, published in Environmental Science & Technologystates that phenols favorably transfer an electron to nitrate radicals or temporarily attach to the molecule.

These mechanisms should work for other atmospheric compounds that react with nitrate radicals. For example, during the breakdown of aromatic substances exposed to ozone, the mucilage acid molecule is formed. The study also compared the results of nitrification reactions with those caused by ozone at the water-air interface.

Related studies have shown that the new yellowish molecules produced absorb more sunlight, increasing the absorption properties of atmospheric particles. Based on the products’ greater absorption of sunlight, the term brown coal is often used to refer to this yellowish material.

Professor Guzman concluded, “The formation of yellow organic nitrate products increases the absorption of atmospheric aerosols, possibly affecting particle radiation, a factor that has been overlooked for a long time.”