3.1.6. Special Studies

Apportionment of Light Scattering and Hygroscopic Growth to Chemical Composition at Sable Island

During a recent campaign at the CMDL monitoring station on Sable Island, a dual-nephelometer humidigraph measured the hygroscopic growth factor of aerosol scattering, f_RH(s_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(s_sp) of 1.7 ± 0.1 versus 2.7 ± 0.4, where f_RH(s_sp) = s_sp(85%)/s_sp(₄0%) as shown in Figure 3.8. A combined measurement-modeling approach was used to estimate s_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(s_sp). We have confirmed with individual particle analysis that aerosol composition was indeed responsible for the difference in f_RH(s_sp). In addition, the scattering contribution of organic carbon for the influenced case is at least as much as that of sulfate aerosol [McInnes et al., 1998].

Fig. 3.8. Hygroscopic growth factor of aerosol scattering f_RH (s_sp), measured for the marine and anthropogenically influenced cases at Sable Island.

Barrow Upgrade

As a result of collaboration with the DOE ARM program, NOAA's aerosol measurement system at BRW was modernized and upgraded in September 1997. The original four-wavelength nephelometer was replaced with a modern, high-sensitivity, three-wavelength nephelometer that additionally determines the backwards-hemispheric component of aerosol light scattering. A continuous light absorption photometer, calibrated in terms of aerosol light absorption coefficient, replaced the original aethalometer. The aerosol inlet system was also upgraded to provide careful control of relative humidity and particle size. These upgrades were performed to meet the data requirements of the ARM program, and to provide the same parameters needed to evaluate the direct radiative forcing of climate by aerosols as are measured at CMDL's regional aerosol sites. In order to maintain continuity of the measurements, the new system will be operated in parallel with the old system until it has been proven that the results from the two systems are quantitatively comparable. Figure 3.9 illustrates the relationship between the aerosol absorption coefficients determined with the old aethalometer and new PSAP instrument, based on the first 3 months of simultaneous operation. Although a systematic difference is observed, the high correlation indicates that it will be possible to determine an empirical relationship linking the historical and future data sets.

Fig. 3.9 Comparison of light absorption photometers.

Mexico City Intensive

As an exploratory step towards understanding the evolution of aerosol intensive properties during long-range transport from source regions, radiative properties of atmospheric aerosols in Mexico City were measured during 2 weeks in November 1997. The same instruments and sampling protocols as at the CMDL regional aerosol sites were used. The measurements were made at a site located in the southwest sector of the city, on a hillside 400 m above the elevation of the main metropolis. The hillside location provided a strong diurnal variation in aerosol properties as surface heating during the day raised the top of the polluted mixed layer above the elevation of the site. Urban aerosols in this region contain a large fraction of absorbing material as indicated by an average single-scattering albedo throughout the research period of 0.7. At times, the single-scattering albedo decreased to as low as 0.4, indicating that more than 50% of the aerosol light extinction was accounted for by absorption. The total light scattering during the high pollution days, when ozone typically exceeded 200 ppb, was quite high, with scattering coefficients on the order of 200 Mm^-1. The absorption, however, increased much less than scattering on high pollution days, and the lowest single-scattering albedos occurred on those days when rainfall and clouds kept the pollution levels much lower (Figure 3.10).

Fig. 3.10. Aerosol radiative properties in Mexico City. The solid horizontal bars indicate nighttime periods.

Aircraft Studies of the Vertical and Horizontal Variability of Aerosol Optical Properties

Aerosol optical properties were measured in situ during three recent field experiments from the NOAA WP-3D Orion research aircraft using an aircraft version of the ground-based CMDL aerosol measurement system. The purpose of flying this airborne package was to measure the spatial (i.e., horizontal and vertical) and temporal variability of aerosol optical properties, and thus to substantially increase the existing amount of these data collected at altitude. By performing the measurements both at very low reference altitudes (which we believe to closely reflect surface measurements) and at higher altitudes, the relationship of surface and higher altitude measurements can be made. Aerosol data from surface sites can then be properly evaluated in terms of the extent to which they represent lower column measurements. The geographical coverage of these field experiments included much of the eastern two-thirds of the United States, the Canadian maritime provinces, and portions of the north Atlantic. Inter- and intra-regional variability of the aerosol properties for boundary layer and free tropospheric measurements was determined. As an example, Figure 3.11 shows the aerosol single-scattering albedo measurements grouped according to atmospheric layer and geographic region. The line across each box, the ends of each box, and the extent of the whiskers represent the median, the 25^th and 75^th percentiles, and the 10^th and 90^th percentiles of each distribution. Median aerosol single-scattering albedos were similar in the boundary layer of all three regions, while in the free troposphere, w₀ was lower and showed the most variability in the East and Atlantic regions.

Fig. 3.11. Variability of aerosol single scattering albedo as a function of atmospheric layer and geographic region. Numbers along x-axis indicate the number of level flight segments included in each data set.

Relationship Between Continuous Aerosol Measurements and Firn Core Chemistry Over a 10-Year Period at SPO

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 (s_sp and å) at the South Pole was compared with the chemical record from a high resolution firn core (~10 samples per year) from 1981 to 1991. As shown in Figure 3.12, seasonal signals in å, associated with winter minima due to coarse mode seasalt and summer maxima due to accumulation mode sulfate aerosol, are reflected in the firn core SO₄=/Na⁺ concentration ratio. Summertime ratios of s_sp and aerosol optical depth, t, to corresponding firn core sulfur concentrations were determined and the “calibrations” were applied to sulfur concentrations in snowpits from a previous study. Results showed that s_sp estimates from snowpit sulfur concentrations were in agreement with atmospheric measurements while t estimates were significantly different, which is likely due to the lack of understanding of the processes that mix surface air with air aloft [Bergin et al., 1998a].

Fig. 3.12. Comparison of mean seasonal values of aerosol Ångstrom exponent and ice core sulfate/sodium from 1981-1991 at SPO.

Aerosol Evaporation in Heated Nephelometers During Sampling: A Laboratory and Modeling Study

Ammonium nitrate aerosol is ubiquitous to the atmosphere and volatile under typical ambient conditions and thus difficult to measure. In the field the scattering coefficient of the dry aerosol is measured with a nephelometer by heating the ambient aerosol to a low reference relative humidity (typically around 40%). The decrease in the light scattering coefficient of ammonium nitrate aerosol due to evaporation in a heated nephelometer was studied under laboratory conditions. Changes in the scattering coefficient of a laboratory-generated monodisperse ammonium nitrate aerosol were measured as a function of mean residence time and temperature within the nephelometer sample volume. At the same time, the change in the aerosol size distribution due to ammonium nitrate evaporation was directly measured with a laser particle counter. The changes in the aerosol size distribution and scattering coefficient were modeled as a function of mean residence time and temperature. Model results for the change in the aerosol scattering coefficient due to evaporation agree with measurements to within 10%. Application of the theory to conditions typical of NOAA field sites shown in Figure 3.13 suggests that due to the evaporation of ammonium nitrate, the decrease in the aerosol scattering coefficient is typically less than 20% [Bergin et al., 1997].

Fig. 3.13. Estimated ratio of scattering coefficient at T and t to initial scattering coefficient, R_T(t) versus mean residence time, t, for ammonium nitrate size distributions having a geometric number median diameter, D_p,g, of 0.4 mm and a geometric standard deviation, GSD, of 1.6.

Comparison of Aerosol Properties Measured at the Surface and Over the Entire Atmospheric Column at the SGP ARM Site in Oklahoma

The estimation of direct aerosol shortwave radiative forcing requires knowledge of aerosol radiative properties on relevant spatial and temporal scales. It is convenient to measure aerosol parameters associated with radiative forcing at the surface, although before these measurements can be used to quantitatively estimate direct climate forcing, it is important to determine the extent to which radiative properties represent the properties integrated over the entire column. In this study we compared measurements of the aerosol optical depth made at the SGP ARM site during several clear sky days with estimates of the aerosol optical depth based on two methods. First, the aerosol scattering coefficient measured at the surface (at a relative humidity ~20%) is multiplied by a mixing height determined from temperature profile measurements. This approach underestimates aerosol optical depth by ~70% using the dry aerosol measurements and by ~40% taking into account the estimated hygroscopic response of aerosols to ambient RH. The discrepancy is attributed to a lack of knowledge of the vertical profile of the aerosol scattering coefficient. Second, micropulse lidar (MPL) normalized aerosol backscatter profiles are used to scale the vertical profile of the aerosol scattering coefficient to surface measurements. As shown in Figure 3.14, the aerosol optical depths estimated using the dry aerosol scattering coefficient are ~30% less than measured values but are in close agreement (± ~15%) when hygroscopic growth is taken into account. These results suggest that aerosol radiative properties at the surface can be representative of radiative properties of the atmospheric column [Bergin et al., 1998b].

Fig. 3.14. Aerosol optical depths at 550 nm estimated for several clear sky days at 20:30 based on micropulse lidar (MPL) scaled nephelometer measurements versus radiometer measurements.

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