Growth of atmospheric nano-particles by heterogeneous nucleation of organic vapor J. Wang, R. L. McGraw, and C. Kuang Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, NY 11973-5000, USA
Abstract. Atmospheric aerosols play critical roles in air quality, public health, and
visibility. In addition, they strongly influence climate by scattering solar
radiation and by changing the reflectivity and lifetime of clouds. One major
but still poorly understood source of atmospheric aerosols is new particle
formation, which consists of the formation of thermodynamically stable
clusters from trace gas molecules (homogeneous nucleation) followed by
growth of these clusters to a detectable size (~3 nm).
Because freshly nucleated clusters are most susceptible to loss due to high
rate of coagulation with pre-existing aerosol population, the initial growth
rate strongly influences the rate of new particle formation and ambient
aerosol population. Whereas many field observations and modeling studies
indicate that organics enhance the initial growth of the clusters and
therefore new particle formation, thermodynamic considerations would suggest
that the strong increase of equilibrium vapor concentration due to cluster
surface curvature (Kelvin effect) may prevent ambient organics from
condensing on these small clusters. Here, the contribution of organics to the
initial cluster growth is described as heterogeneous nucleation of organic
molecules onto these clusters. We find that the strong gradient in cluster
population with respect to its size leads to positive cluster number flux.
This positive flux drives the growth of clusters substantially smaller than
the Kelvin diameter, conventionally considered the minimum particle size
that can be grown through condensation. The conventional approach neglects
the contribution from the cluster concentration gradient, and
underestimates the cluster survival probabilities by a factor of up to 60 if
early growth of clusters is due to both condensation of sulfuric acid and
heterogeneous nucleation of organic vapors.
Citation: Wang, J., McGraw, R. L., and Kuang, C.: Growth of atmospheric nano-particles by heterogeneous nucleation of organic vapor, Atmos. Chem. Phys., 13, 6523-6531, doi:10.5194/acp-13-6523-2013, 2013.