CMDL Aerosol Monitoring Program

J. Ogren (group leader)

Scientific Background

Aerosol measurements began at the CMDL baseline observatories in the mid-1970's as part of the Geophysical Monitoring for Climate Change (GMCC) program. The original objective of these "baseline" measurements was

· to detect a response, or lack of response, of atmospheric aerosols to changing conditions on a global scale.

Since the inception of the program, scientific understanding of the behavior of atmospheric aerosols has improved considerably. One lesson learned is that human activities primarily influence aerosols on regional/continental scales rather than global scales. In response to this increased understanding, and to more recent findings that anthropogenic aerosols create a significant perturbation in the Earth's radiative balance on regional scales (Charlson et al., 1992; NRC, 1996), CMDL expanded its aerosol research program starting in 1992 to include regional aerosol monitoring stations at Sable Island (Nova Scotia, Canada), Bondville (IL), Cheeka Peak (WA), and K'puszta (Kecskemet, Hungary). In 1997, CMDL took responsibility for the in-situ aerosol sampling system at the U.S. Department of Energy's ARM (Atmospheric Radiation Measurement) site in Lamont (OK), effectively adding another site to the regional network. The goals of this regional-scale monitoring program are

· to characterize means, variabilities, and trends of climate-forcing properties of different types of aerosols, and

· to understand the factors that control these properties.

CMDL's measurements also provide ground-truth for satellite measurements and global models, as well as key aerosol parameters for global-scale models. An important aspect of the sampling strategy is that chemical measurements are linked to the physical measurements through simultaneous, size-selective sampling, which allows the observed aerosol properties to be connected to the atmospheric cycles of specific chemical species.

Experimental Methods

Aerosol properties monitored by CMDL include the particle number concentration larger than ~15 nm diameter (N_tot), aerosol optical depth (d), and components of the light extinction coefficient at one or more wavelengths (total scattering s_sp, backwards hemispheric scattering s_bsp, and absorption s_ap). At the regional sites, size-resolved impactor and filter samples (submicrometer and supermicrometer size fractions) are obtained for gravimetric and chemical (ion chromatograph) analyses. At the regional stations, all size-selective sampling, as well as the measurements of the components of the aerosol extinction coefficient, is performed at reduced relative humidity (<40%) to eliminate confounding effects due to changes in ambient relative humidity. Data from the continuous sensors are screened to eliminate contamination from local pollution sources. At the regional stations, the screening algorithms use measured wind speed, direction, and total particle number concentration in real-time to prevent contamination of the chemical samples. Algorithms for the baseline stations use measured wind speed and direction to exclude data that are likely to have been locally contaminated. Additional details of the aerosol measurement systems, along with an overview of the observations, are included in the latest CMDL Summary Report (Ogren et al., 1996).

Table 1 lists the aerosol radiative properties that can be derived from the directly-measured quantities. These properties are used in chemical transport models to determine the radiative effects of the aerosol concentrations calculated by the models. Inversely, these properties are used by algorithms for interpreting satellite remote-sensing data to determine aerosol amounts based on measurements of the radiative effects of the aerosol.

Recent Accomplishments

A complete list of recent publications from the aerosol group is included in the Appendix. Highlights of results for the baseline stations, regional stations, and special field projects are given below.

Baseline stations

Bodhaine and Dutton (1993) reported a long-term decrease in Arctic haze at Barrow (AK) from 1982-1992. The observed trend was attributed to decreased anthropogenic pollution emissions in Europe and the former Soviet Union. Measurements since then (Fig. 1) show that Arctic haze levels continue to be lower than the values observed in the early 1980's, but that the reported trend has not persisted during the past five years. Research efforts supported by the NOAA Arctic Research Initiative are currently underway to better understand the observed change in Arctic haze.

Chemical signals embedded in ice cores are believed to contain a record of past climate-altering events (volcanic eruptions, El Niño episodes), as well as anthropogenic influences. Both the atmospheric and ice core records at the South Pole contain a seasonal signal associated with winter sea salt peaks and summer sulfate peaks. Summertime peaks in the ice core sulfate to sodium mass concentration ratio correspond to peaks in the aerosol Ångström exponent (Fig. 2). This suggests that ice cores at the South Pole record aerosol chemical composition information on a seasonal basis. In addition, peaks in the aerosol scattering coefficient are seen for the volcanic eruptions of El Chichón (1982) and Pinatubo (1991). These peaks correspond to elevated sulfate concentrations in the ice. Ice core peaks of methanesulfonic acid are significantly higher during El Niño periods, although there is no obvious change in the aerosol properties. Overall, the results suggest that linking CMDL's aerosol observations to ice core records may eventually allow a quantitative estimate of atmospheric aerosol properties over the past 1000 years on a seasonal basis.

Fig. 2: Seasonal variations in ice core composition at the South Pole (1981-91) are associated with systematic changes in the wavelength dependence of aerosol light scattering.

Regional stations

A primary application of the observations at the regional aerosol monitoring sites is evaluation of uncertainties in radiative forcing assessments by the Intergovernmental Panel on Climate Change (IPCC). IPCC assessments have not been based on observed variabilities of aerosol radiative properties, but rather on model predictions that use estimates of average values of these properties. The aerosol radiative forcing per unit optical depth (adapted from eqn. 3 of Haywood and Shine, 1995) is controlled by aerosol single-scattering albedo and upscatter fraction (in addition to non-aerosol properties such as surface reflectance). The IPCC (1996) detection and attribution studies considered only aerosol light scattering (not absorption) and used a constant value for this parameter of 25 W m^-2, i.e., an annual average optical depth of 0.1 would lead to a forcing of -2.5 W m^-2. Fig. 3 presents the results of about one year of observations of submicrometer aerosol light scattering and absorption at Bondville in terms of the diurnally-averaged forcing per unit optical depth for a surface reflectance of 0.15. Measurements were made at low relative humidity; the magnitude of the forcing would be higher at ambient relative humidity, but the relative variability would be similar. These results demonstrate that observed variations in aerosol upscatter fraction alone can lead to about ± 15% variations in aerosol forcing, and that aerosol absorption further increases the variability of the forcing and reduces its magnitude by about 30%.

Fig. 3: Frequency distribution of aerosol radiative forcing per unit optical depth (W m^-2) observed at Bondville (IL). The dashed curve is based on scattering measurements only, to facilitate comparison with IPCC (1996), while the solid curve is based on measurements of scattering and absorption.

Field projects

CMDL aerosol scientists have been involved in a number of field projects designed to characterize the spatial distributions of aerosol properties and to understand the processes that control aerosol forcing of climate.

Hygroscopic growth

During a 1996 field campaign at Sable Island, measurements of size-resolved aerosol chemical composition and of the humidity dependence of s_sp were made to investigate the factors controlling aerosol hygroscopic growth at this site. Air mass back trajectories were used to identify cases of clean marine and polluted continental origin. For the clean marine case, roughly 90% of the light scattering of dry submicrometer particles was from sea salt, and the ratio of the aerosol light scattering coefficient at an RH of 85% to that at 40%, termed the hygroscopic growth factor, was 2.7. For the polluted continental case, the contribution of sea salt to the light scattering coefficient of submicrometer particles was 45%, with contributions of roughly 20% each by non-seasalt sulfate, carbonaceous, and mineral aerosols. In addition, the hygroscopic growth factor for the polluted continental case was 1.6. Fig. 4 shows the measured ratio of the light scattering coefficient at a given RH to that at 40% RH, f_RH(s_sp), for the clean marine and polluted continental cases. The results suggest that for the polluted case the contribution of carbonaceous aerosol to the light scattering budget is similar to that of non-seasalt sulfate, and that carbonaceous species may be responsible for the observed suppression of hygroscopic growth.

Fig. 4: Hygroscopic growth for clean marine (¢ ) and polluted continental (n ) submicrometer aerosols.

Aircraft measurements

Measurements from aircraft have been perfomed in order to extend the measurements into the vertical dimension and to increase geographical coverage. The airborne aerosol package includes the same instruments for determining s_sp, s_bsp, s_ap, and N_tot as at the regional sites, along with a multi-filter sampler. As is done on the ground, the sample air is heated as necessary to maintain a relative humidity below 40%, and multi-jet impactors restrict the size range of particles sampled (on the aircraft, only particles smaller than 1 mm aerodynamic diameter are sampled). Additionally, a wing-mounted probe permits the determination of aerosol size distributions. These instruments constitute a comprehensive airborne aerosol measurement platform capable of determining a wide suite of aerosol chemical, optical and microphysical properties. The example in Fig. 5 shows a vertical profile measured over Colorado of å, _o, b, and s_sp. The data were obtained over a large area of the state, and some of the variations are due to horizontal inhomogeneity. Nevertheless, the results show fairly constant values of o, b, and å throughout the lower troposphere, in spite of large variations in s_sp. The increased variability above 5 km results from the very low (and hence imprecise) values of the primary measured variables, leading to large variations in parameters that are defined as ratios of the primary variables. Although it is difficult to draw general conclusions from a limited data set, the results suggest that ground-based measurements of the light scattering and absorption coefficients of submicrometer, continental particles can be used to derive values of the single-scattering albedo, hemispheric backscattering fraction, and Ångström exponent representative of the dry aerosol throughout the lower troposphere.

Fig. 5: Vertical profile of aerosol properties over Colorado, 14 June 1995

Data flow and access

Considerable effort has been devoted in the past five years to improving the quality of data in CMDL's archives and to reducing the delay between data collection and public release. Information and data from the aerosol group at CMDL are available on the Internet via FTP and World Wide Web servers. Recently processed data, file format specifications, documents summarizing data processing and flow, and clean processed data presented in hourly averaged files for all years of station operation are available via anonymous FTP to ftp.cmdl.noaa.gov, directory "aerosol". In addition to the above, the CMDL WWW server at http://www.cmdl.noaa.gov supplies online plots of recently processed aerosol data (generally from the previous day) and hypertext links to various related documents.

Personnel and Funding

The aerosol group at CMDL presently consists of four full-time Ph.D. scientists (M. Bergin, A. Jefferson, J. Ogren, P. Sheridan), a part-time, undergraduate data aide, and a shared (~0.4 FTE) electronics technician (J. Wendell). The composition of the group has changed considerably since 1991, when it consisted of one full-time Ph.D. scientist (B. Bodhaine), two undergraduate data aides, and a shared electronics technician. Fig. 6 shows the sources of salary support for the past five years, and illustrates the increased dependence on support outside of CMDL to maintain staffing levels. CMDL base funds currently provide salary support for J. Ogren and J. Wendell, laboratory and office space, secretarial and administrative support, field staff and facilities at the four baseline stations, plus an additional $50K/yr for research expenses. Fig. 7 compares the funding contributions from NOAA with contributions from other agencies. CMDL's involvement in the DOE/ARM program is the cause of the reversal of the downward trend in 1997.

Figure 6: Salary support (FTE) by fiscal year.

Figure 7: Research funding ($1,000) by fiscal year. CMDL infrastructure support is included as 80% of the salary costs. Field support at CMDL baseline stations is not included.

Future Plans and Challenges

During the past five years, the scope of CMDL's aerosol research program has expanded substantially, in spite of the loss of one CMDL-funded, Ph.D.-level FTE in FY95. Much effort has gone into establishing new monitoring stations with updated instrumentation and sampling protocols. During the next five years, more effort will be devoted to evaluation of the results from the new sites, as the data records become long enough to evaluate trends and seasonal variability. We have sufficient instruments to establish another regional monitoring site, but lack funding for deployment and operation of the system. Although we plan to pursue outside funding support and partners for using this system to characterize regional aerosols, another option for this system is to deploy it at MLO to improve our ability to characterize Asian and free tropospheric aerosols.

We hope to strengthen our ability to evaluate aerosol radiative forcing with initiatives in three areas: aerosol hygroscopic growth, carbonaceous aerosols, and aerosol vertical profiles. As summarized above, results from preliminary measurements indicate the potential for initiatives in each of these areas to reduce uncertainties in current estimates of aerosol radiative forcing.

A major challenge facing the group is finding a way to continue aerosol observations at the baseline stations in spite of erosion of CMDL base support. The 0.4 FTE for technician support is insufficient to maintain operations at all the baseline and regional sites, and the instruments at the baseline stations are now old and hard to maintain. Support from the DOE ARM program has allowed a major upgrade of the aerosol sampling system at BRW, bringing it up to the level of the new regional stations, and the possibility exists of bringing MLO up to the same level as well. Both of these sites can be used to characterize regional aerosols that may be climatically relevant. While continued measurements at SPO may be useful for interpretation of paleoclimatic observations, the current measurement program at SMO (N_tot only) is not particularly useful for climate studies. Although the decision to terminate a long-term measurement record cannot be taken lightly, the current level of base support may lead to shut down of aerosol samplers at SMO and possibly SPO.

External Collaborations

The aerosol group maintains active collaborations with the University of Washington (Anderson, Charlson, Covert), University of Illinois at Urbana-Champaign (Rood), and University of Veszprem, Hungary (Meszaros) and NOAA/PMEL (Quinn, Bates) for operation of the sampling network. Aircraft sampling programs have been conducted in collaboration with NOAA/AL (Fehsenfeld), and NOAA/GFDL (Ramaswamy) is using our results in model studies of the sensitivity of aerosol radiative forcing to observed variations in aerosol properties. Other collaborative efforts involve Brookhaven National Laboratory (Schwartz), University of New Hampshire (Dibb), and Denver University (Wilson). CMDL scientists participate in the International Global Atmospheric Chemistry project and advise the WMO Global Atmospheric Watch.

References

Bodhaine, B. A., and E. G. Dutton, A long-term decrease in Arctic Haze at Barrow, Alaska, Geophys. Res. Lett., 20, 947-950, 1993.

Charlson, R.J., S.E. Schwartz, J.M. Hales, R.D. Cess, J.A. Coakley, Jr., J.E. Hansen, and D.J. Hofmann, Climate forcing by anthropogenic aerosols, Science 255, 423-430, 1992.

Haywood, J.M., and K.P. Shine, The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget, Geophys. Res. Lett., 22, 603-606, 1995.

IPCC, Climate Change 1995 - The Science of Climate Change, 572 pp., Cambridge University Press, Cambridge, 1996.

Ogren, J.A., Anthony, S., Barnes, J., Bergin, M. Huang, W., McInnes, L., Myers, C., Sheridan, P., Thaxton, S., and Wendell, J. Aerosol monitoring. In Climate Monitoring and Diagnostics Laboratory No. 23: Summary Report 1994-95, (eds. Hofmann, D.J., Peterson, J.T., and Rosson, R.M.), U.S. Department of Commerce report available from NTIS, 1996.

National Research Council, Aerosol Radiative Forcing and Climate Change. Panel on Aerosol Radiative Forcing and Climate Change, Board on Atmospheric Sciences and Climate, Commission on Geosciences, Environment, and Resources. National Academy Press, Washington, D.C., 161 pp., 1996.