ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-22395-2010 Ternary homogeneous nucleation of H<sub>2</sub>SO<sub>4</sub>, NH<sub>3</sub>, and H<sub>2</sub>O under conditions relevant to the lower troposphere Benson D. 1 Markovich A. 1 Lee S.-H. 1 Kent State University, Department of Chemistry, Kent, Ohio 44240, USA 29 09 2010 10 9 22395 22414 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/22395/2010/acpd-10-22395-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/22395/2010/acpd-10-22395-2010.pdf

Ternary homogeneous nucleation (THN) of H<sub>2</sub>SO<sub>4</sub>, NH<sub>3</sub> and H<sub>2</sub>O has been used to explain new particle formation in various atmospheric regions, yet laboratory measurements have failed to reproduce atmospheric observations. Here, we report laboratory observations of THN made under conditions relevant to the lower troposphere (H<sub>2</sub>SO<sub>4</sub> of 10<sup>6</sup>–10<sup>7</sup> cm<sup>−3</sup>, NH<sub>3</sub> of 0.08–20 ppbv, and 288 K). Our observations show that NH<sub>3</sub> can enhance atmospheric H<sub>2</sub>SO<sub>4</sub> aerosol nucleation and the enhancement factor (EF) in nucleation rate due to NH<sub>3</sub> increases linearly with increasing NH<sub>3</sub> and increases exponentially with decreasing H<sub>2</sub>SO<sub>4</sub> and RH. The critical clusters of ternary homogeneous nucleation contain 3–5 molecules of H<sub>2</sub>SO<sub>4</sub>, 1–4 molecules of H<sub>2</sub>O, and only 1 molecule of NH<sub>3</sub>. The composition of H<sub>2</sub>SO<sub>4</sub> and H<sub>2</sub>O in critical clusters and the threshold of H<sub>2</sub>SO<sub>4</sub> concentrations required for the unit nucleation rate both do not vary in the presence and absence of NH<sub>3</sub>. These observations can be directly used to improve aerosol nucleation models to correctly assess how man-made SO<sub>2</sub> and NH<sub>3</sub> affect aerosol formation and CCN production at the global scale.

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