New particle formation and growth during summer in an urban environment: a dual chamber study

Nucleation and subsequent growth are a major source of new particles in many environments, but the pollutants involved, and the details of the corresponding processes are still under debate. While sulfuric acid has a major role in new particle formation under a lot of conditions, the role of ammonia, amines and organic vapors is less clear. In most continental areas new particle formation is quite frequent especially in relatively clean, sunny days when there is some sulfur dioxide available. In parts of Eastern Mediterranean even if all the previous requirements are satisfied, new particle formation events are relatively rare during summertime.

In this work, we take advantage of this unexpected low new particle formation frequency in Greece and use a dual atmospheric simulation chamber system with starting point ambient air in an effort to gain insights about the chemical species that is limiting nucleation in this area. A potential nucleation precursor, ammonia, was added in one of the chambers while the other one was used as a reference. Three different types of outcomes were observed: new particle formation only in the perturbed chamber, new particle formation in both chambers, and no observed new particle formation. The addition of ammonia assisted in new particle formation in almost 50% of the conducted experiments. The growth rate of the newly formed particles ranged from 3 – 11 nm h^-1 with particles reaching a diameter of 20-25 nm after a few hours. The nucleation rate was estimated using an aerosol dynamics model and was found to be in the range of 500 to 25000 particles cm^-3 h^-1 for the different experiments. These results support the hypothesis that ammonia at levels of several ppb can accelerate or even cause new particle formation at least in the environment of the Eastern Mediterranean.