Water uptake of soot particles emitted from a Jing-CAST soot generator

Hygroscopic growth of soot particles is of importance in atmospheric science since hydration may result in an increase in the specific absorption of the particles. Generally it is believed that black carbon is hydrophobic and does not take up water at saturations found in the atmosphere. However, it seems that even a thin organic film on the black carbon surface can render it hygroscopic.

We show an experiment where soot particles are generated with a CAST soot generator. The CAST (Combustion Aerosol STandard) of Jing-CAST is a propane burner and produces suspended and active combustion soot particles in a broad submicron size- and concentration range. The characteristics of the soot particles are similar to those emitted from real combustion processes like diesel engines, wood- or coal combustion. Combined with high precision flow control, the instrument allows the generation of soot particle with high accuracy, stability and repeatability.

When operating the CAST, the gas flux of oxygen was varied. Stable burning conditions could be obtained withing a certain range for oxygen flow and soot with different composition of organic and elemental carbon (OC/EC) could be produced. Samples for OC/EC analysis were collected right behind the exhaust on quartz fibre filters and were analysed with a thermal method. Aerosol number concentrations were obtained with two condensation particle counters (CPC), one working with butanol and one with water. The OC/EC ratio could be varied from about 0.1 (with high oxygen flux) to 1 (with low oxygen flux), number concentrations range from roughly 2 to 4 times 10E7 particles per cubic centimeter and the mean mobility diameter varies between 18 and 160 nm.

When comparing the number counts of the two different CPCs the following behaviour of the soot particles may be observed: When the oxygen flux is reduced slowly, a threshold is reached where the number counts, as measured with the water CPC, plunge dramatically and virtually no particles are counted anymore while the butanol CPC still counts more than 10E7 particles. Therefore, soot aerosols with a predominant fraction of EC (produced with a high oxygen flux) can be activated under laboratory conditions with water vapor whereas soot aerosols with a high OC content (produced with a low oxygen flux) cannot be activated anymore. This stands in direct contrast to the general belief that organics enhance the potential of soot particles for water sorption.