Scientists studying a class of atmospheric pollutants known as aerosols now have a new tool at their disposal – an instrument that automates the collection of air samples for analysis with sensitive ion chromatography equipment.
By automating the collection of data, the Particle-Into-Liquid-Sampler (PILS) developed at the Georgia Institute of Technology can measure particulate pollutants several times an hour, giving atmospheric scientists detailed time-dependent information not previously available.
In a recent study, this ability to make frequent measurements revealed previously unknown morning and afternoon peaks in the levels of two key pollutants.
‘Chemists have made significant advances in measuring trace species,’ said Rodney Weber, assistant professor in Georgia Tech’s School of Earth and Atmospheric Sciences. ‘We are applying those technologies to measure the composition of atmospheric particles. In combination with the ion chromatograph, this instrument provides some unique insights that could not be obtained before.’
Developed by Weber and colleague Douglas Orsini with help from Brookhaven National Laboratory, the PILS system uses small quantities of steam to form water droplets on individual aerosol particles entering the instrument. The water droplets containing the dissolved aerosols can then be captured and analysed by ion chromatography techniques to detect as many as 15 different chemical species. The instrument can operate unattended for extended periods of time on the ground or in research aircraft, and can take samples as often as every four minutes.
Scientists analysing airborne particulates had previously relied on filters that collected the aerosol particles over a long period of time, usually 24 hours. The particles were then removed from the filters and dissolved in water for ion chromatograph analysis. It is these long time-integrated measurements that have provided the air quality data to study the effects of aerosols on human health, and which are the basis for current US Environmental Protection Agency (EPA) particulate matter standards. This, however, is likely to change since the EPA is now moving toward more real-time measurements for assessing air quality, Weber said.
The PILS system is said to have shown 97 percent efficiency at collecting particles with diameters of between 0.03 and 10 microns – which includes the PM 2.5 particles of concern for their health effects.
Joining the instrument to an ion chromatograph is said to allow rapid analysis of aerosol ionic components of interest. Georgia Tech has focused on nine major chemical species, including sodium, ammonium, potassium, oxides, calcium, magnesium, chloride, nitrates and sulphates. With longer analysis times – approximately 15 minutes – the combined system can also analyse acetate, formate and oxylate.
The limits of detection for the cation and anion species are, respectively, 50 nanograms per cubic millimetre and 10 nanograms per cubic millimetre. Testing against other measurement systems has validated the accuracy of the system, Weber noted.
‘With the stock ion chromatograph, our instrument makes all of these measurements online and there is no manual labour involved,’ Weber noted. ‘We can make the measurements more quickly than with previous techniques, and with higher sensitivity. We no longer have to dissolve these particles in large volumes of water and don’t have to be concerned with contamination during sample handling.’
Because of its water-based process, the PILS system measures only aerosols that are soluble in water. But combined with other detection techniques that measure the size, volume and mass of particles, the instrument gives air quality researchers a better view of what’s fouling the air – and clues about how to make it cleaner.