An assessment of hygroscopic growth factors for aerosols in the surface boundary layer for computing direct radiative forcing

Aerosol optical properties in the southeastern United States were measured at two research sites in close horizontal proximity but at different altitu des at Black Mountain (35.66 degreesN, 82.38 degreesW, 951 m msl) and Mount Gibbes (35.78 degreesN, 82.29 degreesW, 2006 in msl) to estimate the direc t radiative forcing in the lowest 1 km layer of the troposphere during the summer of 1998. Measurements of light scattering and light absorption at am bient relative humidity (RH) are categorized by air mass type (polluted con tinental, marine with some continental influence, continental) according to 48-hour back-trajectory analysis. At a wavelength of 530 nm the average to tal scattering coefficient (sigma (sp)) measured at the valley site was 1.4 6 x 10(-4) m(-1) for polluted continental air masses, 7.25 x 10(-5) m(-1) f or marine air masses, and 8.36 x 10(-5) m(-1) for continental air masses. T he ratio of sigma (sp) at the mountain site to sigma (sp) at the valley sit e was 0.64, 0.58, and 0.45 for polluted continental, marine, and continenta l air masses, respectively. The hygroscopic growth factor (sigma (sp)(RH = 80%)/sigma (sp)(RH = 30%)) was calculated to be almost a constant value of 1.60 +/- 0.01 for polluted continental, marine, and continental air masses. As the RH increased from 30% to 80%, the backscatter fraction decreased by 23%. On the basis of these measurements, direct radiative climate forcing (DeltaF(R)) by aerosols in the lowest 1 km layer of the troposphere was est imated. The patterns of DeltaF(R) for various values of RH were similar for the three air masses, but the magnitudes of DeltaF(R)(RH) were larger for polluted continental air masses than for marine and continental air masses by a factor of about 2 due to higher sulfate concentration in polluted cont inental air masses. The average value of DeltaF(R)(RH = 80%)/DeltaF(R)(RH = 30%) was calculated to be almost a constant value of 1.45 +/- 0.01 for all three types of air masses. This implies little dependence of the forcing r atio on the air mass type. The averaged DeltaF(R) for all the observed ambi ent RHs, in the lowest 1 km layer during the 3-month summer period, was -2. 95 W m(-2) (the negative forcing of -3.24 W m(-2) by aerosol scattering plu s the positive forcing of +0.30 W m(-2) by aerosol absorption) for polluted continental air masses, -1.43 W m(-2) (-1.55 plus +0.12) for marine air ma sses, and -1.50 W m(-2) (-1.63 plus +0.14) for continental air masses. The DeltaF(R) for polluted continental air masses was approximately twice that of marine and continental air masses. These forcing estimates are calculate d from continuous in situ measurements of scattering and absorption by aero sols without assumptions for Mie calculations and global mean column burden of sulfates and black carbon (in g m(-2)) used in most of the model comput ations.