Showing papers by "Michael H. Bergin published in 1998"

Models overestimate diffuse clear‐sky surface irradiance: A case for excess atmospheric absorption

Abstract: Radiative transfer models consistently overestimate surface diffuse downward irradiance in cloud-free atmospheres by 9 to 40% at two low altitude sites while correctly calculating direct-normal Solar irradiance. For known systematic and random measurement errors and for realistic aerosol optical properties, the discrepancy can be resolved by a reduction in the vertical aerosol optical thickness (AOT) inferred from sunphotometric measurements by an average 0.02 ± 0.01 for 32 cases examined, together with a compensating increase in a continuum-like atmospheric absorptance over the solar spectrum of ∼5.0% ± 3.0%. This phenomenon is absent at two high altitude sites, where models and measurements agree to within their mutual uncertainties. Examination of apparent AOT at several locations around the globe also indicates presence of such excess atmospheric absorption. The proposed absorption and corresponding reduction in AOT would have important consequences for climate prediction and remote sensing.

Apportionment of light scattering and hygroscopic growth to aerosol composition

Abstract: During a recent campaign at the NOAA CMDL monitoring station on Sable Island, Canada (43.93° N, 60.01° W) a dual-nephelometer humidigraph measured the hygroscopic growth factor of aerosol scattering, f RH (σ sp ), one of the key parameters necessary for estimating short-wave aerosol radiative forcing. Measurements revealed less growth for anthropogenically influenced aerosols than for marine, f RH (σ sp ) of 1.7 ± 0.1 vs. 2.7 ± 0.4, where f RH (σ sp ) = σ sp(85%) /σ sp(40%) . A combined measurement-modeling approach was used to estimate σ sp and its RH-dependence, based on the measured particle size distribution and composition. The model suggested that differences in the particle size distribution, assuming the same aerosol composition, could not explain the observed differences in f RH (σ sp ). We have confirmed with individual particle analysis, that aerosol composition was indeed responsible for the difference in f RH (σ sp ). As well, the scattering contribution of organic carbon for the influenced case is at least as much as sulfate aerosol.

Biomass burning signatures in the atmosphere of central Greenland

Abstract: Daily atmospheric concentrations of participate oxalate measured at the Summit of the Greenland Ice Sheet are presented for the summers 1992–1995. We believe that four episodes of elevated concentrations are due to biomass burning plumes passing over the site. In at least two cases the source regions of the fires are located in northern Canada. Further characteristics of the aerosol are examined during one of these events. A large increase of particle number concentrations in the accumulation mode can be observed, while the increase is much more limited for total particle number. The suite of chemical species enriched in the aerosol includes typical biomass burning tracers like fine K, large concentrations of ammonium, particulate formate and acetate, as well as other organic species like glycolate. The size distributions of K, oxalate, and glycolate are skewed toward the accumulation mode and exhibit the very same shape as sulfate, suggesting internal mixing of these species in the same particles. Molar ratios S/K indicate incorporation of S during transport, most probably by production of sulfate. Concentrations of these species were measured in fog samples for radiative events that occurred during the plume passage. There is a good agreement in the relative variation of concentrations between the aerosol and fog for oxalate and glycolate, while the gas phase probably dominates incorporation in the fog droplets for acetate, formate, chloride, nitrate, and sulfate (incorporated as SO2, which is further oxidized). The complexity of the transfer of the organic acids from the atmosphere to fog is underlined.

Relationship between Continuous Aerosol Measurements and Firn Core Chemistry over a 10‐year Period at the South Pole

Abstract: Before ice core chemistry can be used to estimate past atmospheric chemistry it is necessary to establish an unambiguous link between concentrations of chemical species in the air and snow. For the first time a continuous long-term record of aerosol properties (aerosol light scattering coefficient, σ sp , and Angstrom exponent, a) at the South Pole are compared with the chemical record from a high resolution finn core (∼10 samples per year) covering the period from 1981 to 1991. Seasonal signals in a, associated with winter minima due to coarse mode seasalt and summer maxima due to accumulation mode sulfate aerosol, are reflected in the fim core SO 4 2- /Na + concentration ratio. Summertime ratios of σ sp and aerosol optical depth, τ, to corresponding firn core sulfur concentrations are determined and the 'calibrations' are applied to sulfur concentrations in snowpits from a previous study. Results show that σ sp estimates from snowpit sulfur concentrations are in agreement with atmospheric measurements while τ estimates are significantly different, which is likely due to the lack of understanding of the processes that mix surface air with air aloft.

Effects of Non-Equilibrium Hygroscopic Growth of (NH4)2SO4 on Dry Deposition to Water Surfaces

Abstract: Growth of hygroscopic aerosols near water surfaces is believed to enhance dry deposition rates, which are a strong function of particle size. Previous dry deposition models estimate hygroscopic growth by assuming equilibrium between aerosols and water vapor (Williams, R. M. Atmos. Environ. 1982, 16, 1933-1938). A model is presented that combines the relative humidity profile above water surfaces with hygroscopic growth rates for (NH 4 ) 2 SO 4 , assuming cases for a deliquescing and metastable aerosol. Model results show that particles greater than 0.1 μm in diameter do not grow to their equilibrium size before depositing to a hypothetical water surface. As a consequence, equilibrium models overpredict the effects of hygroscopic growth on deposition velocities by as much as a factor of 5. In addition, model results suggest a significant difference in the deposition velocities of metastable and deliquescing aerosols. Based on measured (NH 4 ) 2 SO 4 size distributions, overall deposition velocities calculated from a thermodynamic equilibrium model, a mass transfer limited non-equilibrium model with a deliquescing aerosol, and a mass transfer limited non-equilibrium model with a metastable aerosol are 0.11, 0.055, and 0.040 cm/s, respectively.