American rain loses its acidity

Rain in North Carolina, US, is less acidic than it was 20 years ago, report researchers in the state. The changes could trigger significant environmental effects, they say. 

Joan Willey and her team at the University of North Carolina at Wilmington (UNCW) have been collecting precipitation on the university campus since 1985. They found that the hydrogen ion concentration in that water, which relates to acidity, decreased by about half in that time. They also found that the dissolved organic carbon (DOC) concentration fell by more than 50 per cent between 1995-2005, after making the first long-term study of DOC in precipitation.

DOC is a measure of all organic compounds found in rain and is linked to hydrogen ion concentration. ’Small organic acids, primarily formic and acetic acids, are water-soluble and tend to accumulate in rain,’ said Willey. ’They contribute approximately 25 per cent to the total DOC in our rain. These organic acids add to the hydrogen ion concentration, making rain more acidic.’

Falling levels of small organic acids in the atmosphere are a major factor in why rain in Wilmington is getting less acidic. They are falling, Willey thinks, because emissions of volatile organic compounds (VOCs) have dropped. A fraction of VOCs oxidise in air to become small organic acids. ’Logically, if less VOCs are emitted, then less formic and acetic acids are formed, DOC and acidity decreases, and pH increases,’ explains Willey. VOCs have fallen since the introduction of a cleaner-burning fuel called reformulated gasoline in 1996 in the northeastern US. Wilmington receives just under 40 per cent of its rain from storms with a northeast trajectory.

The researchers argue that these changes in the chemical composition of precipitation are so profound they will have significant, mostly positive, effects on the environment. For example, less acidic rain is delivered to lakes, including mountain lakes that are particularly vulnerable to acidification. Certain chemical forms of the trace metal aluminium, which can be toxic to freshwater fish, are less soluble in less acidic solutions. And shallow-water marine corals may be another beneficiary. Some of these corals make their shells out of a material called aragonite. If rainwater adds acid to seawater, for example after a large rainstorm, this could dissolve the aragonite. 

However, on the negative side, Willey points out that rain with a higher pH tends to contain higher levels of ammonium, which has increased due to agricultural activities. Ammonium is a plant nutrient that can contribute to eutrophication (excess plant growth) in lakes and coastal waters.

Maria Burke

Science in the sky

Compounds emitted by plants during West Africa’s monsoon are thought to have a profound impact on global atmospheric chemistry.