3.1.1. SCIENTIFIC BACKGROUND

Aerosol particles affect the radiative balance of earth both "directly," by scattering and absorbing solar and terrestrial radiation, and "indirectly," through their action as cloud condensation nuclei (CCN) with subsequent effects on the microphysical and optical properties of clouds. Evaluation of the climate forcing by aerosols, defined here as the perturbation of the earth's radiation budget induced by the presence of airborne particles, requires knowledge of the spatial distribution of the particles, their optical and cloud-nucleating properties, and suitable models of radiative transfer and cloud physics. Obtaining a predictive relationship between the aerosol forcing and the physical and chemical sources of the particles additionally requires regional and global-scale chemical process, and physical transformation and transport models for calculating the spatial distributions of the major chemical species that control the optical and cloud-nucleating properties of the particles. Developing and validating these various models requires a diverse suite of in situ and remote observations of the aerosol particles on a wide range of spatial and temporal scales.

Aerosol measurements began at the CMDL baseline observatories in the mid-1970s as part of the Geophysical Monitoring for Climate Change (GMCC) program. The 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 residence times of tropospheric aerosols are generally less than 1 week and 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; National Research Council, 1996], CMDL expanded its aerosol research program to include regional aerosol monitoring stations. 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.

A primary hypothesis to be tested by NOAA's aerosol research program is that the climate forcing by anthropogenic sulfate will change in response to future changes in sulfur emissions. The forcing is expected to decrease in and downwind of the United States as a result of emission controls mandated by the Clean Air Act, while continued economic development in China and other developing countries is expected to lead to an increased forcing in and downwind of those areas. Testing this hypothesis will require a coordinated research program involving modeling, monitoring, process, and closure studies. This report describes the observations that CMDL is conducting towards this goal.

CMDL's measurements provide ground truth for satellite measurements and global models, as well as key aerosol parameters for global-scale models (e.g., scattering efficiency of sulfate particles, hemispheric backscattering fraction). An important aspect of this strategy is that the 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.

3.1.2. EXPERIMENTAL METHODS

Extensive aerosol properties monitored by CMDL include the total particle number concentration (Ntot), aerosol optical depth (), and components of the aerosol extinction coefficient at one or more wavelengths (total scattering sp, backwards hemispheric scattering bsp, and absorption 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. All size-selective sampling, as well as the measurements of the components of the aerosol extinction coefficient at the regional stations, is performed at a low, controlled 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.

Prior to 1995, data from the baseline stations were manually edited to remove spikes from local contamination. For the 1995 baseline data an automatic editing algorithm was applied in addition to manual editing of local contamination spikes. For the baseline stations (Barrow (BRW), Mauna Loa (MLO), South Pole (SPO), and Samoa (SMO)), as well as Sable Island, data are automatically removed when the wind direction is from local sources of pollution (such as generators and buildings) as well as when the wind speed is less than a threshold value (0.5-1 ms). In addition, Mauna Loa data for upslope conditions (1800-1000 UTC) are excluded since the airmasses do not represent "background" free tropospheric air for this case. A summary of the data editing criteria is given in Table 3.1.

TABLE 3.1. Data Editing Summary for NOAA Baseline and Regional Stations

Station	Editing	Clean Sector
South Pole	a,b,c	0 < WD < 110, 330 < WD < 360
Samoa	a,b,c	0 < WD < 165, 285 < WD < 360
Mauna Loa	a,b,c,d	90 < WD < 270
Barrow	a,b,c	0 < WD < 130
Sable Island	a,b,c	0 < WD < 35, 85 < WD < 360
Niwot Ridge	a
Bondville	a

WD; Wind direction
a-Manual removal of local contamination spikes
b-Automatic removal of data not in clean sector

c-Automatic removal of data for low wind speeds

d-Removal of data for upslope wind conditions

Integrating nephelometers are used to determine the light scattering coefficient of the aerosol. These instruments operate by illuminating a fixed sample volume from the side and observing the amount of light that is scattered by particles and gas molecules in the direction of a photomultiplier tube. The instrument integrates over scattering angles of 7-170°. Depending on the station, measurements are performed at one, three, or four wave-lengths in the visible and near-infrared. Newer instruments allow determination of the hemispheric backscattering coefficient by using a shutter to prevent illumination of the portion of the instrument that yields scattering angles less than 90°. A particle filter is inserted periodically into the sample stream to measure the light scattered by gas molecules, which is subtracted from the total scattered signal to determine the contribution from the particles alone. The instruments are calibrated by filling the sample volume with a gas of known scattering coefficient; carbon dioxide (CO) is used for high sensitivity instruments, while dichlorodifluoromethane (CFC-12) is used for the few single-wavelength, lower sensitivity instruments still in use.

The aerosol light absorption coefficient is determined with a continuous light absorption photometer. This instrument continuously measures the amount of light transmitted through the filter. The rate of decrease of transmissivity, divided by the sample flow rate, is directly proportional to the light absorption coefficient of the particles. Newer instruments have been calibrated in terms of the difference of light extinction and scattering in a long-path extinction cell for laboratory test aerosols. Instruments at the baseline stations ("aethalometers") have been calibrated by the manufacturer in terms of the equivalent amount of black carbon, from which the light absorption coefficient is calculated assuming a mass absorption efficiency of the calibration aerosols of 10 m g.

Particle number concentration is determined with a CN counter that exposes the particles to a high supersaturation of butanol vapor. This causes the particles to grow to a size where they can be detected optically and counted. The instruments in use have lower particle-size detection limits of 10-20 nm diameter.

Summaries of the extensive measurements obtained at each site are given in Tables 3.2 and 3.3. Table 3.4 lists the intensive aerosol properties that can be determined from the directly-measured extensive properties. 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.

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 World Wide Web server at http://www.cmdl.noaa.gov supplies online plots of recently processed aerosol data and hypertext links to various related documents (including this one).

TABLE 3.2. CMDL Baseline Aerosol Monitoring Stations (Status as of January 1996)

		Baseline Tropical	Baseline	Baseline
Category	Baseline Arctic	Free Troposphere	Marine	Antarctic
Location	Point Barrow	Mauna Loa	American Samoa	South Pole
Status	Operational 1976	Operational 1974	Operational 1977	Operational 1974
Sample RH	Uncontrolled	Uncontrolled	Uncontrolled	Uncontrolled
Sample size fractions	Uncontrolled	Uncontrolled	Uncontrolled	Uncontrolled
Optical measurements	sp(4), ap(1)	sp(4), sp(3), ap(1), (6l)	sp(4)	sp(4)
Microphysical  measurements	Ntot	Ntot	Ntot	Ntot
Chemical  measurements	none	none	none	none

 

TABLE 3.3. CMDL Regional Aerosol Monitoring Sites (Status as of January 1996)

	Perturbed	Perturbed	Perturbed	Clean	Clean
Category	Marine	Continental	Continental	Continental	Marine
Location	Sable Island,  Nova Scotia	Bondville, Illinois	K'puszta,  Keszcemet, Hungary	Niwot Ridge, Colorado	Cheeka Peak,  Washington
Designator	WSA	BND	KPO	NWR	CPO
Latitude	+43.933	+40.053	+46.967	+40.036	+48.30
Longitude	+060.007	+088.372	-019.550	+105.534	+124.62
Elevation (m)	5	230	180	3020	480
Responsible  institute	NOAA/CMDL	NOAA/CMDL	U. Veszprem,  Hungary	NOAA/CMDL	U. Washington
Collaborating institute	Atmospheric Environment  Service, Canada,  NOAA/PMEL	University of Illinois; Illinois State Water Survey	NOAA/CMDL	U. Colorado, Boulder; Colorado State U.,  Fort Collins	NOAA/CMDL
Status	Operational  August 1992	Operational July 1994	Operational  September 1994	Site feasibility  measurements began November 1993	Operational,  May 1993
Sample RH	RH < 40%	RH < 40%	RH < 40%	Uncontrolled	RH < 40%
Sample Size	D < 1 µm	D < 1 µm	D < 1 µm	Uncontrolled	D < 1 µm
Fractions	1 < D < 10 µm	1 < D < 10 µm			1 < D <10 µm
Optical  measurements	sp(3), ap(1),  bsp(3), (4)	sp(1)	sp(1), ap(1),  (4)	sp(1)	sp(3), bsp(3),  ap(1)
Microphysical  measurements	Ntot	Ntot	Ntot	Ntot, n(D)	Ntot, n(D)
Chemical  measurements	Major ions, mass	Major ions, mass	Major ions	None	Major ions, mass

 

TABLE 3.4. Intensive Aerosol Properties Derived From the CMDL Network

Properties	Description
å	The angstrom exponent, defined by the power-law sp  ­å, describes the wavelength-dependence of scattered light. In the figures below, å is calculated from measurements at 550 and 700 nm wavelength. Situations where the scattering is dominated by submicrometer particles typically have values around 2, while values close to 0 occur when the scattering is dominated by particles larger than a few microns in diameter.
o	The aerosol single-scattering albedo, defined as sp/(ap + sp), describes the relative contributions of scattering and absorption to the total light extinction. Purely scattering aerosols (e.g., sulfuric acid) have values of 1, while very strong absorbers (e.g., elemental carbon) have values around 0.3.
g, b	Radiative transfer models commonly require one of two integral properties of the angular distribution of scattered light (phase function): the asymmetry factor g or the hemispheric backscatter fraction b. The asymmetry factor is the cosine-weighted average of the phase function, ranging from a value of -1 for entirely backscattered light to +1 for entirely forward-scattered light. The hemispheric backscatter fraction b is bsp/sp.
i	The mass scattering efficiency for species i, defined as the slope of the linear regression line relating sp and the mass concentration of the chemical species, is used in chemical transport models to evaluate the radiative effects of each chemical species prognosed by the model. This parameter has typical unites of m2 g-1.

3.1.3. ANNUAL CYCLES

The annual cycles of selected extensive and intensive properties are illustrated in Figure 3.1a-d. The data are presented in the form of box-whisker plots that summarize the distribution of values: the box ranges from the lower to upper quartiles, with a central bar at the median value, while the whiskers extend to the 5th and 95th percentiles. The statistics are based on hourly averages of each parameter for each month of the year, as well as for the entire year ("ANN").

Fig. 3.1a. Annual cycles of CN concentration for baseline stations at BRW, MLO, SMO, and (SPO Monthly median values are shown. Box-whisker plots illustrate the upper and lower quartiles (box), and 5th and 95th percentiles (whiskers). Values representing the entire year period, for all years, are also presented (ANN).
 

Fig. 3.1b. Annual cycles of CN concentration for regional stations at Bondville, Illinois (BND), Niwot Ridge, Colorado (NWR), and Sable Island, Nova Scotia (WSA).
 
 

Fig. 3.1c. Annual cycles of sp at 550 nm for baseline stations at BRW, MLO, SMO, and SPO and for regional stations at BND, NWR, and WSA.

Fig. 3.1d. Annual cycles of angstrom exponent (å, 550/700 nm) for baseline and regional stations.

In general, changes in long-range transport patterns dominate the annual cycles. For BRW, the highest values of _sp are observed during the spring because of the long-range transport of pollution from lower latitudes ("Arctic haze"). The BRW CN record shows a more variable semiannual cycle, with a maximum that usually coincides with the maximum in _sp and a secondary maximum in late summer or early fall. The secondary maximum in late summer is thought to be caused by local oceanic emissions of dimethyl sulfide (DMS) gas that are eventually converted to sulfate aerosol [Radke et al., 1990]. For MLO, the highest _sp values occur in the springtime, caused by the long-range transport of pollution and mineral dust from Asia. Little seasonality is seen in CN concentrations at MLO, however, indicating that the smallest particles (<0.1 mm diameter), which usually dominate the CN concentration, are not enriched during these long-range transport events. Little seasonality is seen in the results from SMO, while at SPO the high sp levels observed in the late winter are due to the long-range transport of sea salt in the upper troposphere from stormy regions near the Antarctic coast to the interior of the continent.

Previous reports describing the baseline aerosol data sets include BRW: Bodhaine [1989] and Quakenbush and Bodhaine [1986]; MLO: Massey et al. [1987]; SMO: Bodhaine and DeLuisi [1985]; and SPO: Bodhaine et al. [1986, 1987] and Bodhaine and Shanahan [1990].

Based on only 2-3 years of measurements, the annual cycles for the regional stations are very uncertain, therefore, it is premature to discuss the causes of the observed variability. The proximity of the regional sites to North American pollution sources is apparent in the results, however, with monthly median values that in some cases are over 2 orders of magnitude higher compared to values from the baseline stations.

3.1.4. LONG-TERM TRENDS

Long-term trends in CN concentration, sp, and angstrom exponent are plotted in Figure 3.2a-c for the baseline stations. The trends are plotted for the annual geometric average as well as for the geometric averages for the months with the lowest and highest median values observed in the annual cycle plots. Interpretation of the results are complicated by two changes in instrumentation: (a) replacement of the nephelometer at MLO in 1985 and (b) the replacement of the CN counters with butanol-based instruments at MLO in 1988; at SPO in 1989; at BRW in1990; and SMO in 1992. The two types of CN counters have different lower-size detection limits, which means that any change in the long-term record will depend on the presence of particles not detected by one of the counters. This is the likely cause for the fact that obvious step changes in CN concentration are seen at MLO and SPO, but not at BRW and SMO.

Fig. 3.2a. Long-term trends in CN concentration for baseline stations, showing months with the lowest and highest median values, and annual averages for each year (ANN).
 

Fig. 3.2b. Long-term trends sp at 550 nm for baseline stations, showing months with the lowest and highest median values and annual averages.
 
 

Fig. 3.2c. Long-term trends for å (550/700 nm) for baseline stations, showing months with the lowest and highest median values and annual averages.
 

As discussed in the 1988 Summary Report [Elkins and Rosson, 1989], sp values at MLO were generally higher since the installation of the new nephelometer in 1985 and have not reached the low values previously observed in winter. The increasing trend in sp at MLO is caused by higher winter values in the latter part of the record and the reason is believed to be instrumental. A modern, high-sensitivity three-wavelength nephelometer was deployed at MLO in 1994, and future comparison of the results from the two nephelometers is expected to quantify any biases introduced by the older, less-sensitive instrument. All data reported here are from the older instrument, however.

3.1.5. RESULTS FROM 1994-1995

Daily Mean Values of Aerosol Properties

Figures 3.3a-g show the daily mean values at each monitoring station for total number concentration (CN), aerosol scattering coefficient at 550 nm (sp) and the angstrom exponent for the 550/700 nm wavelength pair from January 1, 1994, to December 31, 1995. Significant day-to-day variability in CN concentration, aerosol scattering coefficient, and angstrom exponent can be seen in the figures. The daily variability of these parameters is due to several factors, including changes in local meteorology, aerosol sources, transport time from source regions, and processing of aerosols during transport. It is worthwhile to point out that the data editing procedure for 1995, as seen in the CN plots from the edited stations, results in a more stringent acceptance of data. This can be clearly seen in the Barrow CN plot that shows significantly more breaks in the 1995 data because of the rejection of a greater amount of data compared with the previous year. The more rigorous approach to data screening for 1995 and after, generally results in less day-to-day variability in the CN concentrations, which is likely because of the fact that data resulting from local pollution are more completely excluded.

Fig. 3.3a. Daily means of aerosol properties (CN concentration, sp, and å) for SPO for 1994, 1995.
 

Fig. 3.3b. Daily means of CN concentration for SMO for 1994, 1995.
 

Fig. 3.3c. Daily means of aerosol properties for MLO for 1994, 1995.

Fig. 3.3d. Daily means of aerosol properties for BRW for 1994, 1995.

Fig. 3.3e. Daily means of aerosol properties for WSA for 1994, 1995.
 

Fig. 3.3f. Daily means of aerosol properties for NWR 1994, 1995.

Fig. 3.3g. Daily means of aerosol properties for BND 1994, 1995.

Aerosol Intensive Properties

Figure 3.4 shows box/whisker plots of the variability in the daily averages of three different intensive aerosol properties measured at Sable Island: the angstrom exponent (550/700 nm wavelength pair) for submicrometer particles, the fraction of scattering caused by submicrometer particles, and the fraction of the light that is scattered into the backwards hemisphere. The data were classified into three cases: "clean" conditions when both N_tot and  (550 nm, D < 1 m) are below the lower quartile for the entire data set, "dirty" conditions in which N_tot and ssp are above the upper quartile, and all "other" periods that do not meet the previously defined criteria (for example periods with low N_tot and high  values). For comparison, the fine/total scattering fraction is plotted for Bondville (BND) (data for the other intensive properties are not available at BND prior to 1996). It can be seen that the values of the angstrom exponent increase for more polluted periods suggesting that the submicrometer aerosol shifts systematically towards smaller particles as the degree of pollution increases. This is also reflected in size-segregated measurements of light scattering, which show that a larger fraction of the total scatter is due to the submicrometer aerosol as the air becomes more polluted, reaching a median value of 84% at BND. In the cleanest cases at Sable Island, only 28% (median value) of the light scattering is caused by submicrometer particles; the remainder is presumably caused by larger sea salt particles. Submicrometer particles contribute a larger fraction to the total for each quartile at Bondville, suggesting the continental aerosol is always heavily influenced by fine aerosol pollution. Aerosol number concentrations and values of  for the submicrometer aerosol are consistently higher at Bondville than at Sable Island (Figure 3.1b,c). The backscatter fraction, on the other hand, exhibits relatively little dependence on the degree of pollution.

Fig. 3.4. Aerosol intensive properties (å, fraction of submicrometer scattering, and backscatter fraction) for WSA and BND. The data from Sable Island were separated according to clean, dirty, and other cases.

Linear regression of the angstrom exponent with the fraction of submicrometer scattering (Figure 3.5) demonstrates that the two variables are highly correlated, suggesting that most of the variance in the angstrom exponent is controlled by the relative abundance of submicrometer particles. This challenges the traditional notion that the angstrom exponent can be interpreted as the slope of a power-law aerosol size distribution and better supports a bimodal model of the size distribution where the angstrom exponent is a measure of the relative amounts of material in the two modes.

Fig. 3.5. Linear regression of å versus the fraction of submicrometer scattering for WSA.

3.1.6. AIRCRAFT OBSERVATIONS

A special version of the CMDL aerosol instrumentation package used at the regional aerosol monitoring sites was developed for use on research aircraft. This effort was undertaken to extend our measurement capability into the vertical dimension and to greatly increase geographic coverage as well. This airborne aerosol package includes a three-wavelength nephelometer with backscatter shutter, a light absorption photometer, a condensation nucleus counter, and a multifilter sampler, all interfaced to a laptop computer for instrument control and data logging. As is done on the ground, the sample air is heated as necessary to maintain a relative humidity below 40%, and multijet impactors are used to restrict the size-range of particles sampled (on the aircraft, only particles smaller than 1 m aerodynamic diameter are sampled). Additionally, wing-mounted probes permit 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.

Measurements of the optical properties of submicrometer aerosol particles were measured from the NOAA WP-3D Orion research aircraft during the summer 1995 Southern Oxidants Study. The majority of the flights were in the midwest and southeastern United States at altitudes below 5 km and provide a survey of the vertical and horizontal variability of the aerosols that dominate the direct aerosol radiative forcing of climate. Some flights were conducted over Colorado, allowing comparison of these aerosol properties between the humid East and arid West. Figure 3.6 shows the vertical profiles measured over Colorado of å, o, b, and sp (denoted by B_sp in the figure legend). The data in Figure 3.6 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. 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. The single-scattering albedo varies in the range 0.88-0.95, and the hemispheric backscattering fraction is 0.15-0.18.

Fig. 3.6. Vertical profile of aerosol properties over Colorado, June 6, 1995.

A similar vertical profile over the southeastern United States is seen in Figure 3.7 in spite of much higher values of light scattering (note the scale change for sp) than were observed over Colorado. Once again, values at the higher altitudes are much less reliable because of the low values of the scattering and absorption coefficients. In the boundary layer, the single-scattering albedo is 0.95 and the hemispheric backscattering fraction is 0.11. These values are somewhat different from the values obtained over Colorado, suggesting systematic differences in aerosol composition and size distribution in the two regions. However, the differences may also be due to day-to-day variations in the aerosol. Figure 3.8 shows the horizontal variability observed in the boundary layer on the transit flight from Colorado to Tennessee. As was the case for the vertical dimension, the derived parameters (o, b, å) are relatively constant in spite of large changes in the primary measured variables.

Fig. 3.7. Vertical profile of aerosol properties over the southeastern United States July 1, 1995.
 

Fig. 3.8. East-west transect, Colorado to Tennessee, June 19, 1995. Open symbols denote free tropospheric measurements.

Finally, Figure 3.9 shows the latitudinal variability that was observed in the boundary layer over the mid-western U.S. (Tennessee-Indiana), where the values of o and b are identical to the boundary layer values shown in Figure 3.8. Slightly more variability is seen in the single-scattering albedo (0.89-0.96), but the hemispheric backscattering fraction is once again nearly constant (0.12). In all four cases, the angstrom exponent stays in the range 2.0-2.5. Although instrumental noise is a limiting factor, the observed variability in å may be due to variations in the aerosol size distribution with the larger values of å corresponding to cases with smaller particle sizes.

Fig. 3.9. North-south transect, midwest United States July 10, 1995.
 

Taken as a whole, the results of this study yield values for the single-scattering albedo in the range 0.88-0.96, with more variation observed from day-to-day than from place-to-place (horizontally or vertically). Similar conclusions can be drawn for the hemispheric backscattering fraction (0.11-0.18) and the angstrom exponent (2.0-2.5), although b in the boundary layer was always below 0.13 except for the one vertical profile over Colorado. Although it is difficult to draw general conclusions from a 1-month study, 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 angstrom exponent representative of the dry aerosol throughout the lower troposphere.

3.1.7. LIDAR MEASUREMENTS AT MAUNA LOA

Vertical profiles of tropospheric and stratospheric aerosols are regularly determined at MLO with two different lidar systems. Section 1.1.2 (Aerosol Monitoring, page 7, this report) describes the instruments and analysis techniques, and the new Nd:YAG lidar.

The integrated aerosol backscatter (IABS) data at 694 nm in Figure 3.10 show that no volcanic eruptions injected large amounts of aerosols that were detectable in the stratosphere at MLO latitudes in 1994-1995. The decay of Mt. Pinatubo's aerosols continued, and by the end of 1995, the lowest levels of IABS in the past 16 years were in evidence. A small increase and decay in stratospheric aerosols just prior to the Mt. Pinatubo eruption may have been related to the eruption of Kelut which was observed by the Stratospheric Aerosol and Gas Experiment (SAGE) instrument. A similar small increase in the IABS data in the fall of 1994 may be observed in Figure 3.11 at 532 nm and 694 nm. The 532-nm data are from the new Nd:YAG lidar. This increase coincides with an eruption of Rabaul in New Guinea. The increase abruptly disappears in December 1994 coincident with the air mass above the observatory switching abruptly from tropical to midlatitude air.
 

Fig. 3.10. Integrated aerosol backscatter for 1980-1995 at 694 nm (ruby lidar) from 15.8 to 33 km.

Fig. 3.11. Integrated aerosol backscatter for 1994-1995 at 532 and 694 nm (ruby lidar) from 15.8 to 33 km.
 

In September 1994 the Lidar In-space Technology Experiment (LITE) was flown in the Space Shuttle. The lidar made aerosol measurements at 1064, 532, and 355 nm from the upper stratosphere into the troposphere. LITE observing times were concentrated over the Atlantic and Europe, but during two overflights of MLO, correlative measurements were made at 532 nm. For the first overflight (September 14), the profiles agree well throughout the stratosphere. On the second flight (September 16) the profiles agree (within calculated error) below 23 km. However, the MLO lidar observed higher aerosol backscatter at elevations from 23 to 33 km although the MLO lidar exhibited the same general features in the profile as the satellite instruments.

The MLO lidars, as part of the Network for the Detection of Stratospheric Change (NDSC), participated in an aerosol analysis intercomparison (August 1995) conducted within the lidar group of NDSC to validate the analysis methods used by NDSC lidars. In the study, raw signals and radiosonde data were provided to participants to be used in their respective analysis routines. Preliminary results show good agreement between MLO analysis and the benchmark data.

Atmospheric temperature profiles have been measured with the MLO lidar over altitudes from 33 to 70 km beginning in July 1994. A blind intercomparison of temperature profiles made between the NOAA lidar and three other NASA lidars during the MLO3 campaign was undertaken in August 1995; the results and analysis have not been released to date.
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