What are Secondary Aerosols?

When particles and other substances are emitted, e.g. via vehicle exhaust or powerplants, these primary emission go through several chemical changes in the atmosphere under the influence of sunlight, ozone and humidity and ultimately transform into so-called “secondary aerosols”. Volatile compounds originated from the primary emissions react in the atmosphere with highly reactive oxidative species to form new compounds. In turn, these new compounds might have a lower volatility and hence lead to new particle formation through nucleation, or they can condense on existing particles. This process is also called “ageing” of the aerosol. In both cases the process generates secondary aerosol mass, which can have a substantial impact on the environment and health. Recent studies have also indicated that secondary aerosols may have an even stronger correlation with health and environmental effects than primary emissions. Consequently, the research of secondary aerosols is rapidly gaining importance, driving the need for dedicated measurement solutions.

Studying Atmospheric Ageing

The atmosphere contains a wide variety of volatile compounds that have potential to form particulate mass, referred to as precursors. Precursors are commonly divided into two subgroups, organic and inorganic precursors. The vast number of different organic precursors makes the secondary organic aerosol chemistry very complex. The aging process of these precursors takes from hours to days in the atmosphere and it is challenging to trace their origin. Therefore, researchers use reactors to simulate the atmospheric processes. These reactors enable controlled aging studies of specific emission sources or precursors to determine their potential impact on the environment or human health.

Oxidation Flow Reactor

To study the potential of different sources to form secondary aerosols in lab or field conditions, the ageing process can be accelerated by exposing the source emissions to elevated concentrations of those chemical species that cause atmospheric ageing. These fast atmospheric ageing simulating devices are called “Oxidation Flow Reactors” (OFR). Especially in cases where the emission source is highly transient or time dependent, the fast flow through the reactors offers a high time resolution to study the changes.

OFRs can use different chemical ageing species.  The Dekati Oxidation Flow Reactor (DOFR™) uses a combination of water vapor, ozone, and UV light to accelerate the ageing process. In the DOFR™ solution, firstly, ozone and water vapor are mixed to the sample in controlled concentrations. This conditioned sample is then exposed to UV light inside the quartz made reactor tube. The UV light dissociates ozone, and through this reaction a single oxygen molecule is formed. This oxygen molecule further reacts with water molecules and as a result of this reaction hydroxyl radicals (OH) are formed. These OH-radicals are the ageing compound of the DOFR™. Hydroxyl radicals are also naturally present in the atmosphere but in the DOFR™ their concentration can be boosted up to four orders of magnitude higher than in troposphere, thereby substantially accelerating the ageing processes. As such, ageing processes that may take up to several weeks’ time in the atmosphere can be simulated in a one-minute time frame in the DOFR™.

Measurement of secondary aerosols generated by vehicle exhaust gas using DOFR in combination with sample conditioning unit.

Oxidation flow reactor simulates atmospheric ageing of aerosols