An amorphous solid state of biogenic secondary organic aerosol particles

A phased approach to aerosols Secondary organic aerosol (SOA) particles can scatter radiation and act as cloud condensation nuclei, and thereby influence Earth's radiation balance. It is generally assumed that such particles are liquid, but this experimental study shows that they can adopt an amorphous — probably glassy — solid state under ambient conditions. The findings of the study — which monitored SOA particles formed from oxidation products of volatile organic compounds from seedlings grown in plant chambers, and used measurements made in forests following atmospheric oxidation events — challenge traditional views of the kinetics and thermodynamics of SOA formation and transformation in the atmosphere. Secondary organic aerosol (SOA) particles can scatter radiation and act as cloud condensation nuclei, and thereby influence the Earth's radiation balance. It is generally assumed that SOA particles are liquid, but these authors show that they can adopt an amorphous solid state under ambient conditions. The findings challenge traditional views of the kinetics and thermodynamics of SOA formation and transformation in the atmosphere. Secondary organic aerosol (SOA) particles are formed in the atmosphere from condensable oxidation products of anthropogenic and biogenic volatile organic compounds (VOCs) 1 , 2 , 3 , 4 , 5 , 6 , 7 . On a global scale, biogenic VOCs account for about 90% of VOC emissions 1 , 8 and of SOA formation (90 billion kilograms of carbon per year) 1 , 2 , 3 , 4 . SOA particles can scatter radiation and act as cloud condensation or ice nuclei, and thereby influence the Earth’s radiation balance and climate 1 , 2 , 5 , 9 , 10 . They consist of a myriad of different compounds with varying physicochemical properties, and little information is available on the phase state of SOA particles. Gas–particle partitioning models usually assume that SOA particles are liquid 1 , 5 , 11 , but here we present experimental evidence that they can be solid under ambient conditions. We investigated biogenic SOA particles formed from oxidation products of VOCs in plant chamber experiments and in boreal forests within a few hours after atmospheric nucleation events. On the basis of observed particle bouncing in an aerosol impactor and of electron microscopy we conclude that biogenic SOA particles can adopt an amorphous solid—most probably glassy—state. This amorphous solid state should provoke a rethinking of SOA processes because it may influence the parti