Relations between Sunlight and humic acid

A deceptively simple sounding interaction of sunlight and soil may be turning voluminous quantities of atmospheric nitrogen dioxide (NO₂) into nitrous acid (HONO), a precursor to the atmospheric “cleanser” hydroxyl, which oxidizes numerous pollutants.

This previously unrecognized link between soil and atmospheric chemistry could fill gaps in our understanding of atmosphere and climate and improve models.

The degradation of certain soils produces humic acid, a complex mélange of macromolecular organics whose chemistry is difficult to characterize. Atmospheric chemists Konrad Stemmler and Markus Ammann at the Paul Scherrer Institute in Villigen, Switzerland, and coworkers in France and Germany have found that reactions of NO₂ and humic acid with light produce HONO in quantities that could account for the levels observed in daytime measurements (Nature 2006, 440, 195).

This could be a major daytime source of HONO, says Barbara J. Finlayson-Pitts, a chemistry professor at the University of California, Irvine, who has studied atmospheric HONO production.

In the 1970s, scientists first measured HONO in the lower regions of the atmosphere. This compound is produced when NO₂ (a major by-product, along with NO, of combustion) reacts with water or organic compounds on the surface of particles. Scientists believed that HONO is produced in the dark because sunlight triggers its photolysis to OH.

In recent years, with the advent of much more sensitive instruments, scientists found that HONO is also produced during the day. Though there were suggestions as far back as the 1980s that light could play a role in the conversion of NO₂ to HONO, only a few models and experiments have been put forth to explain the phenomenon.

Ammann and colleagues had been exploring how, in the dark, phenols can transfer electrons to NO₂and thus produce nitrite, which is then protonated to HONO. They further showed that light hastened those reactions. Then they realized that decomposition of the plant-based compound lignin produces monomeric phenols, which are among the building blocks of humic matter.

The group exposed thin films of humic acid to NO₂ and light and found that HONO was produced at the same rate that NO₂ disappeared in quantities 30 times greater than could be produced at night. The reaction occurs under a wide range of frequencies, including visible light.

Xianliang Zhou, a professor at the State University of New York, Albany, who has studied photosensitized HONO production, calls the paper “interesting and exciting.” He notes that the effect is likely to be more significant in urban environments, as concentrations of nitrogen oxides are low in rural regions.

This chemistry could also affect other trace gases in the atmosphere, such as ozone, Ammann says. And F. Sherwood Rowland, Nobel laureate and chemistry professor at UC Irvine, observes that “the role of HONO as an oxidant in the lower atmosphere is starting to draw considerable attention.”