Burning brimstone: the why and how of measuring sulfur dioxide

980x480-air

Factory smokestacks Image courtesy of Reuters

Carbon dioxide gets a lot of press as a human-generated atmospheric pollutant, but while we have not yet even doubled its concentration in the atmosphere over the past three hundred years of industrial activity we have managed close to a hundred-fold increase in sulfur dioxide during this same period. Some sulfur dioxide gets produced deliberately with such activities as the manufacture of sulfuric acid, but the bulk of it is a byproduct of burning oil and coal or in refining some metals from their ore.  In short, it is the industrial output gas.  It is also the source of acid rain as atmospheric sulfur dioxide from industrial centers reacts with water vapor to produce sulfuric acid.  In the concentrations often found in urban environments near industrial activity it can cause elevated rates of asthma and other respiratory illness.

Density of ground-based sulfur dioxide emissions in the continental United States.

Our interest then has been to find sensors for measuring sulfur dioxide which can fulfill the following criteria: 1. Detection of concentrations down to the parts per billion (ppb) range.  Long term exposure above even 30 ppb is thought to trigger adverse health effects. 2. Corrosion resistant.  Sulfur dioxide will quickly corrode, even over a few months, many types of sensor elements. 3. Stable.  With any gas we want to monitor we’re always aiming for reproducible data on the absolute concentration.  Many sensor types, such as those used in alarm systems, will give reliable readings for the change in gas concentration but have a drifting value for the absolute concentration. With these criteria in mind we have recently chosen to incorporate the sulfur dioxide sensors from Alphasense into our latest round of air quality monitor prototypes.  More updates to come on data collected in the field with these sensors incorporated into our monitors.