Experimental Confirmation of van der Waals-Enhanced Growth of Sulfuric Acid/Water Nanoparticles

Since the size of an atmospheric particle determines many of its effects, we conducted experiments to better understand their rate of growth. Seed particles composed of sulfuric acid and water were exposed to photolytically generated H₂SO₄ molecules and their change in size was monitored with a mobility particle system. H₂SO₄ production rates were held steady while the seed particle diameter was varied from 3 to 25 nm to explore how growth is affected by size. The growth rate of 25 nm diameter particles was about 50% less than that for 3 nm diameter particles. Gas-kinetic hard-sphere growth rates decline only 18% over this size range, but a decrease of 35-to-50% in growth over this range is expected according to theories that include the effects of a van der Waals interaction between gaseous H₂SO₄ and the small particles. The size-dependence of the measured growth rates, which does not require knowledge of the H₂SO₄ gas concentration, suggests that the attractive force between hydrated H₂SO₄ and small sulfuric acid particles leads to a significant enhancement of the collision rate; this force depends strongly on particle size below 10 nm in diameter. Recent calculations based on a central field approximation for the van der Waals interaction are consistent with the measurements, although empirical enhancement factors better explain the data for some conditions. Nucleation experiments were also performed with H₂SO₄ detection, and simulations of these nucleation experiments required similar van der Waals enhancements to secure agreement between measured H₂SO₄ vapor and the size of the nucleated particles.