INVITED ABSTRACT
Compositional and mixing state impacts, including the effects of new particle formation, on size-resolved CCN concentrations in Atlanta, Georgia during 2009

RICHARD H. MOORE (1), Molly McLaughlin (1), Athanasios Nenes (1), Jacob Scheckman (2), Jingkun Jiang (2), Modi Chen (2), Peter McMurry (2), Jun Zhao (3), Jim Smith (3), Fred Eisele (3)

(1) Georgia Institute of Technology, Atlanta, GA (2) University of Minnesota, Minneapolis, MN (3) National Center for Atmospheric Research, Boulder, CO

     Abstract Number: 566
     Last modified: May 13, 2010

Abstract
The effect of organic species on cloud droplet formation remains a large source of uncertainty in aerosol-cloud-climate interactions studies. Consequently, there is a need for in-situ, size-resolved measurements of Cloud Condensation Nuclei (CCN) to unravel these complex effects. The city of Atlanta provides a unique environment for exploring the interplay between the anthropogenic emissions of a major metropolitan area with biogenic emissions from surrounding forest and farmland and their relative contribution to the aerosol size distribution and chemical mixing state, and, thus, CCN. Additionally, recent work has suggested that CCN activity decreases after new particle formation (NPF) events$^1, and this effect is evaluated for Atlanta aerosol.

We present a comprehensive characterization of aerosol and CCN sampled in Midtown Atlanta during 2009, which includes continuous observations over a nine-month period on the Georgia Tech Rooftop Measurement Platform and also during the month-long NCCN intensive campaign at the Jefferson Street monitoring site. CCN were measured with a DMT Continuous-Flow Streamwise Thermal-Gradient Chamber, operated in spectrometer mode using Scanning Flow CCN Analysis (SFCA)$^2, which was 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. Continuous measurements of aerosol size distribution (1-460 nm) and chemical composition were also conducted during the NCCN campaign.

The daily trend of size-resolved CCN activity, mixing state, and droplet growth kinetics are presented and compared to chemical composition measurements. CCN activity was found to vary diurnally and reached a minimum during the morning rush hour, coincident with an increased externally-mixed, unactivated aerosol fraction of ~0.4-0.5. NPF events were observed to increase CCN, but this was delayed relative to total particle concentration. The effect of NPF on size-resolved CCN activity will be discussed.

$^1Dusek et al., Geophys. Res. Lett., 2010
$^2Moore, R.H, Nenes, A. Aerosol Sci. Technol., 2009