Reply to ASK-AN-EARTH-SCIENTIST

Subject: Acid rain in Hawaii

We are doing a project in my chem class about acid rain and I would like to know how has acid rain affected the rain forests in Hawaii.

Acid rain from man-made causes is very rare in Hawaii, because our geographic isolation in the central north Pacific is such that we are removed from most of the problematic point-sources of acid precurser gasses (oxides of nitrogen and sulfur) to the atmosphere. In addition, our abundant rainfall and fairly constant trade winds diminish those local anthropogenic sources (e.g. car exhaust) by dilution in the resultant precipitation. Dilution is particularly strong in our rain forests, where 150 to 300 inches of rain can fall in a typical year.
We do, however, have a source of acid precurser gasses in Hawaii that are not found in most areas: active volcanoes. Kilauea volcano has been actively erupting since 1983, spewing SO2, SO3 and HCl into the atmosphere all the while. Even during times of non-eruption, fumaroles on the volcano vent these gasses to the atmosphere. The effect of this input is phenomenal. One need only spend a few hours on Kilauea volcano to be struck by the stark contrast between its lushly vegitated northern and eastern slopes and the near desert conditions that exist in its summit area and in a triangular swath leading southwest to the sea from this area. This is due to intense volcanic acid rain fallout following a pattern governed by the prevailing northeasterly trade winds. It is rare, however, for these intense effects to be observed outside of the volcano region.
Whether or not there has/been some long-term effect on neighboring and distant forests in Hawaii due to this volcanic input of gasses to the atmosphere is difficult to determine, as Kilauea is the most active volcano on Earth and has been very active for many tens of thousands of years. In fact, certain plants may have evolved here to deal with this constant acid fallout, although I am not aware of any studies that prove or disprove this (botany is way outside of my area of expertise).

Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822