ABSTRACT

Sulfur species have been directly linked to adverse health effects for a long time. They are implicated in the excess mortality rates in the London Fog event of 1952 and in the Six Cities Health study of 1974-1991. Experiments in the 1990s on exposure studies to sulfuric acid mists and measurements of particle strong acidity linked sulfuric acid to adverse respiratory effects. Recent studies show associations of sulfur species with a wider range of adverse effects, from death to dementia.  During this period atmospheric sulfur concentrations have substantially decreased in many regions, yet sulfur’s adverse health associations persisted. A possible explanation is that sulfur species, such as sulfate aerosol, are only a tracer for secondary aged aerosol. But mechanistic reasons are also possible. Sulfate is nonvolatile and highly hygroscopic so it can control particle liquid water concentrations over a range in sulfate concentrations which may influence aerosol toxicity if overall toxic components of PM2.5 are formed in water. Related to this is particle acidity. Particle pH is often dependent on sulfate as the main acid, yet pH is highly buffered by several processes, such as ammonia volatility, leading to a weak sensitivity of pH to sulfate concentration and highly acidic particles over a wide range of sulfate concentrations. Unlike the past view, where health effects were driven by H+ concentration, particle acidity instead can be a key driver in increasing the toxicity of other particle species, such as by solubilization of a host of insoluble metals. These species have persistent sources, including mineral dust and mechanically generated emissions from vehicles (road dust, brake dust and tire wear). I will discuss various measurements in Atlanta, Georgia, and the surrounding region over the past 10 years that provide insights on the possible role of sulfur-driven pH effects on particle toxicity and discuss more recent work involving development of particle water and pH fields for studies of dementia.