If you need the recap, you can find Susan Goldhaber's article here.
National Acid Precipitation Assessment Program NAPAP
NAPAP began in the mid-1980s and was reauthorized in conjunction with the 1990 Clean Air Amendments [1]. It was a huge operation involving four national Laboratories, several Federal agencies, and countless academics. Its assessment dealt with effects of acid rain on aquatic and terrestrial ecosystems, air and water pollution, fossil fuel emissions, corrosion, and human health. Looking back, it now seems that health effects got the short straw.
NAPAP found that SO2 emissions could indeed hydrolyze in the atmosphere to form sulfuric acid that may or may not be neutralized by atmospheric ammonia (NH3). Such neutralization could also occur in terrestrial or aquatic environments, indoors, and, more importantly, in human breath. This last process seems to have been mainly overlooked by epidemiologists. The NAPAP assessment was necessarily concerned with impacts that could happen (experimental studies, toxicology) rather than with what had happened (surveys, epidemiology).
My role with NAPAP was to assess the role of atmospheric acidity on rates of metal corrosion and calcareous stone (marble) dissolution - the latter dubbed and still cited, “Lipfert function.” The low rates of material damage that we found were of little lasting consequence.
Ms. Goldhaber included a history of annual SO2 emissions to illustrate the significant improvements since NAPAP. However, emission reductions per se do not constitute environmental benefits, which require reductions in environmental impacts. I added aquatic acidity (H+) information to her graph to compare trends in exposures with those for responses [2], the percentage of surveyed water bodies that have insufficient acid-neutralizing capacity. That report also noted that central and western regions of the US were not at risk from acid deposition, leading to the conclusion that aquatic acidification may not be very important on the national scale.
The question of associations with human health deserves a bit more scrutiny. The 1992 NAPAP report devoted only one page to it, with a single journal citation. Ms. Goldhaber cited the notorious 1952 London fog episode, described as ”acid fog”, as supporting the hypothesis of excess mortality from acid aerosols. However, no direct measurements of acidity were made during this episode, and black smoke particles were found to be responsible for the increased mortality.
The 1993 Harvard Six Cities Study reported no significant association of long-term mortality with aerosol acidity, in stark contrast with other pollutants. No further studies of long-term acid aerosol health effects have been published, and no air pollution mortality study has considered indoor environments, which likely include enough ammonia to neutralize ambient acids.
The 1990 Clean Air Act Amendments aimed to reduce SO2 emissions from power plants, without regard to NAPAP findings. The emissions reduction from 1990 to ~2000 may have resulted from regulation, but the subsequent much more significant drop was more likely due to natural gas becoming cheaper than coal. In any event, ambient SO2 concentrations are now too low to warrant continued monitoring.
In summary, NAPAP did its job, produced at least 27 multi-chapter technical reports, hundreds of scientific journal articles, and employment for thousands of scientists and technicians. It found that atmospheric acidity could indeed exert harmful effects in various media, depending on the innate neutralizing capacity, but the long-term consequences of these effects on the national scale remain uncertain. The basic chemistry remains valid; the premise of long-term human health effects did not.
[1] National Acid Precipitation Assessment Program, 1992 Report to Congress. University of California reprint collection.
[2] Acidity in Lakes and Streams, EPA Report on the Environment. https://www.epa.gov/roe/
