Compositional and mixing state impacts on CCN concentrations in an heterogeneous urban environment

The effect of organic species on the cloud droplet formation process constitutes a large source of uncertainty in aerosol-cloud-climate interactions studies. In-situ size-resolved measurements of the Cloud Condensation Nucleus (CCN) activity of ambient aerosol can unravel such complex chemical effects. The city of Atlanta provides a unique environment for exploring the interplay between the anthropogenic emissions of a major metropolitan area (believed to be predominantly SO₂ and (NH₄)₂SO₄ with some organic contribution) with biogenic emissions from surrounding forest and farmland (believed to be comprised of mostly organics), and their relative contribution to the aerosol size distribution and chemical mixing state, and, thus, cloud condensation nuclei (CCN).

We present a comprehensive characterization of aerosol and CCN sampled in Midtown Atlanta during the period of August 2008 to August 2009, which includes continuous observations over a nine-month period on the Georgia Tech Rooftop Measurement Platform (in midtown Atlanta) and also during the month-long AMIGAS 2008 intensive campaign at the Jefferson Street monitoring site. CCN were measured with a Droplet Measurement Technologies Continuous-Flow Streamwise Thermal-Gradient Chamber^1,2, operated in spectrometer mode using Scanning Flow CCN Analysis³ (SFCA), or size-resolved counter mode using Scanning Mobility CCN Analysis⁴ (SMCA). SFCA and SMCA were used to provide size-resolved CCN concentrations over a variety of particle sizes (40-120 nm) and supersaturations (0.1-1%) with high temporal resolution. A TSI 3936 Scanning Mobility Particle Sizer (SMPS) was used to obtain measurements of aerosol size distribution, and chemical composition was obtained from with a Particle-Into-Liquid Sampler coupled with online ion chromatography (PILS-IC).

The daily trend of the mixing state, CCN activity, and droplet growth kinetic of the size-resolved CCN are presented and compared to chemical composition measurements. Inferences about the impact of photochemistry and mixing on CCN activity, hygroscopic uptake and droplet activation kinetics are carried out. We also quantify the predictive uncertainty associated with simplifying compositional assumptions (e.g., size-invariant composition, organic insoluble/soluble with constant hygroscopicity) used for predicting CCN concentrations in global climate models.

¹Roberts, G. and A. Nenes. Aerosol Sci. Technol., 2005

²Lance et al., Aerosol Sci. Technol., 2006

³Moore, R.H. and A. Nenes. Aerosol Sci. Technol., in press

⁴Nenes, A. and J. Medina. Aerosol Sci. Technol., in review