Figure 5. Plots of a) fRH versus the sub 10um scattering coefficient Angstrom exponent and b) fRH versus the sub 10um absorption coefficient Angstrom exponent. Box and whisker plots showing the median, 5,25,75 and 95 th percentiles of the data. Introduction Aerosol growth from the uptake of water substantially influences direct radiative forcing. Enhanced aerosol growth from water absorption increases the aerosol extinction, asymmetry parameter and single scattering albedo. The magnitude of this growth depends on the aerosol composition as well as its phase, size and the ambient RH. The U.S. DOE Atmospheric Radiation Measurement (ARM) program has been conducting measurements of aerosol scattering hygroscopic growth, fRH, at its Southern Great Plains (SGP) facility in Oklahoma since December of Sheridan et al. (2001) reported on these measurements for one year of operation in They noted a range of fRH values from 1.0 to 3.3 with an average value of Here fRH is the ratio of the humidified scattering at 85% RH to a reference humidity of 40%. The scattering growth with RH is often expressed as a semi-empirical power law fit parameter. Measurements Acknowledgements We like to acknowledge the dedication of the SGP ARM technical staff, particularly Patrick Dowell and Ken Teske, for maintaining the AOS measurement quality and Annette Koontz for her dedication in ingesting and processing the data for the archive. Six Years of Aerosol Optical Hygroscopic Growth Measurements from the ARM Southern Great Plains Site: Parameter Variability and Uncertainty Anne Jefferson 1, Hadi Morrow 2, Derek Hagemen 1,3 and John Ogren 3 1.Cires, University of Colorado, Boulder, CO 2. Appalachian State University, Boone, NC 3. NOAA, ESRL, Global Monitoring and Diagnostic Laboratory, Boulder, CO 1) Use of the above equation assumes the aerosol is an aqueous droplet. This may be a good assumption for a highly oxidized, mostly organic aerosol. Single particle and narrow size diameter growth studies show a more complex aerosol with multiple phases within a single particle or size range (Martin et al. 2008, Bertram et al., 2011). This poster examines 6 years of aerosol scattering hygroscopic growth measurements from SGP from over a period when the system design and the humidity sensor calibrations were consistent. Aerosol scattering hygroscopic growth measurements were performed with two tandem TSI nephelometers (model 3563) separated by a humidifier that ramped the RH between 40 to 80% RH hourly. Hourly scan profiles are shown below. The RH difference across humidified wet neph is due to 2.5 o C heating inside instrument. Figure 1.a) Hourly RH scans of RH at wet neph exit (red) and inside wet neph (blue). b) Ratio of wet:dry scattering for sub 10 um (light green) and sub um (dark green) aeroso l. a)a) b) Assumptions: 1) Hygroscopic growth follows the form given in Equation 1. 2) The propagation of uncertainty assumes that the variables Equation 1 are independent and error is the sq. root of the sum of squares of the partial derivatives 3) There are no size-dependent losses in the humidifier 4) The effects of residence time and RH gradient inside Nephs are minimal Sources of Uncertainty Nephs: drift, noise, calibration, truncation, STP correction RH and T calibration: ~3% for RH, assume T uncertainty is negligible Noise and goodness of fit in power law fit parameter γ Uncertainty Analysis σ w is the end product or ambient state scattering used in ADRF models. What we calculate is a fit parameter “ γ ”. Uncertainty in γ is calculated using a Monte Carlo simulation of the data. The standard deviation of γ didn’t vary much with γ over a range of 0.2 to 0.8. d γ values are 0.31, 0.03 and 0.01 for dry σ of 1, 10 and 100, respectively. The largest contribution to the total error was from the dry σ at low RH(45%) and low scattering(1Mm -1 ). At high RH(85%) and high scattering (100 Mm -1 ) the largest contribution to the total error was from the RH. Table 1 Calculated humidified scattering coefficients at 3 RH values (45, 60 and 85%) and for a given dry scattering coefficient (1,10 and 100 Mm -1 ) along with the associated errors for 3 values of the power law fit parameter gamma (γ=0.2, 0.5 and 0.8). Figure 2 Times series of sub 10um fRH at 550 nm from January 2009 to December The ambient as well as minimum instrument RH at SGP varies diurnally and seasonally with higher RH values at night and in the summer months. Summer months have higher rates of photochemical oxidation. Lower RH values in winter may result in a higher percent of the aerosol as solid or effloresced and not metastable. The hygroscopic growth curves don’t display a distinct deliquescent behavior. However, variability of fRH with the ambient RH is evident in Figure 3. Figure 3. FRH versus the ambient RH from at SGP. Figure 4. Plots of a) fRH versus the sub 10um scattering coefficient at 550 nm and b) fRH versus the sub 10um single scatter albedo at 550 nm. Box and whisker plots showing the median, 5,25,75 and 95 th percentiles of the data. fRH vs scattering at 550 nmfRH vs single scatter albedo fRH vs absorption Angstrom fRH vs scattering Angstrom Summary Hygroscopic growth doesn’t show much variance with the aerosol scattering coefficient or amount. It does increase with increasing single scatter albedo or less absorbing aerosol. fRH covaries with aerosol size, increasing with the scattering Angstrom exponent or smaller aerosol. fRH is shown to decrease with the absorption Angstrom, which correlates to higher organic composition(private communication, Bob Cary). Further work is needed to investigate the dependence of hygroscopic growth with composition and also the aerosol phase. The uncertainty calculations show that regions with low aerosol loading near 1 Mm -1 have an uncertainty in fRH that exceeds 100%. The lowest uncertainty in fRH was for high aerosol loading and low hygroscopic growth. Uncertainty in the nephelometer scattering coefficient dominated the fRH uncertainty at low and moderate scattering and γ values. This points to the importance in minimizing aerosol loss and increasing calibration accuracy in the nephelometer with regular calibration checks of the instruments. References Bertram, A. K. et al., Atmos. Chem. Phys., 11, 10995–11006, 2011 Martin. S.T. et al., Geophys. Res. Lett., 35, Sheridan, P.J.,et al., J. Geophys. Res., 106(D18), doi: / 2001JD000785, Propagation of error for wet scattering coefficient, σ w