Low Turbulence Inlet Assessment Working Group Report

Dave Covert
Peter McMurry
Tony Clarke
Chuck Brock
Steve Howell
Jim Anderson
Barry Huebert, Chair
Chuck Wilson, ex-officio

12 January, 2000

At the suggestion of Eric Saltzman, a small working group was convened to plan, execute, and assess the in-flight evaluation of the size-dependent passing efficiency of the low turbulence aerosol inlet (LTI, or porous diffuser) that is being developed by researchers at Denver University (Chuck Wilson, Russ Seebaugh, and Bernie Lafleur). The working group met briefly during the San Francisco AGU meeting, and produced the following recommendations.

I. Two hypotheses will be tested:

  1. The LTI has a demonstrably higher aerosol sampling or transmission efficiency than both the CAI (the NCAR C-130 community aerosol inlet) and traditional solid diffusers for particles in the 1-7 µm range.
  2. It is possible, using the LTI, to sample and characterize the number-size and surface area-size distributions of ambient dust and seasalt inside an aircraft with enough accuracy that uncertainties arising from inlet losses will contribute less than 20% to the assessment of radiative impacts.
Our focus is aerosol sampling and transmission efficiency, but our needs in this regard are determined by scientific questions regarding aerosol radiative forcing of climate. While it is desirable to collect all sizes of particles efficiently from an aircraft, the criterion for deciding whether ACE-Asia can meet its coarse-particle goals depends most strongly on sampling particles in the radiatively and chemically important 1-7 µm range. Thus, we will determine size-dependent sampling efficiencies in the 1-10 µm range.

The optical depth for any component of the aerosol, ti, depends on the integral of extinction over all radii, r. For a non-absorbing aerosol, this is a function of the mass-scattering efficiency of the substance, ai(r), and the mass of the substance, mi(r), in each size interval, as well as the pathlength containing that type of material, Hi.

If the airborne inlet does not pass all sizes efficiently, an efficiency term, e (r), must be inlcuded with the mass to compute the apparent optical depth, ti’:

The error in radiative forcing due to sampling losses depends, then, on both the transmission efficiency and the fraction of the mass and total optical depth in each size interval. For those sizes that contribute little to the optical depth, a poor transmission efficiency will cause little error in radiative forcing calcualtions. It is worth noting, however, that undersampled sizes could still cause significant errors for other issues, such as the computation of deposition fluxes and heterogeneous reaction rates.

II. Flights

Two types of flights will be needed to test the inlet. The first, flow and turbulence tests on the Electra, will be a part of the development process and will be conducted by the DU engineers. These tests will measure turbulence in the LTI under a variety of flight and flow conditions, to optimize the inlet. It would be useful to make local wind measurements using an 858 probe mounted near the LTI. The effects of yaw and pitch changes on LTI turbulence will be examined. This type of work will also need to be done during the second set of flights.

This working group will focus on the second set of flights: measuring the inlet passing efficiency vs size, in flight tests to be proposed for the C-130 in July, 2000. Since our goal is to assess radiative forcing in ACE-Asia, we need to use both dust and seasalt as test aerosols. We expect dust to generally be dry, while sea salt will be wet and will change its size due to evaporation while being brought into the aircraft. The tendency of particles to adhere to the inlet may be very different for dust and sea salt. Thus, we will need three 5 hour flights each in a dusty region and in the marine boundary layer. In both cases, we need to find a location with the highest wind speeds available (in July) to maximize the concentration of wind-generated large particles. We have begun the process of selecting operations sites, which may include the southwestern US (for dust) and the Pacific coast (for seasalt). However,a dust experiment will take place in Puerto Rico in July, 2000, so we are investigating the possibility of participating in it. The Saharan dust is quite reliable that time of year, and it might be our highest probability of finding dust to work with.

Our flight hour request is:
2 hrs Jeffco instrumentation test flights
8 hrs ferry (depending on final choice of airports)
15 hrs dust research
15 hrs sea salt research
--------------------------------
40 hours total

Since the C-130 is committed to TOPSE until mid-June 2000, our proposal is to integrate in late June and to conduct the flights during the first three weeks of July, 2000. We plan to give NSF a report and recommendation by the end of August, 2000.

III. Measurements to Test the Hypotheses

A. Hypothesis A

Hypothesis A can be tested relatively simply, since it only requires measurements inside the aircraft, on several air streams that have already been decelerated. We propose to use three matched aerodynamic particle sizers (TSI Model 3320 APS’s) to measure the physical size distribution behind our three test inlets: the LTI, the CAI, and a solid diffuser/curved tube inlet that has been used on many programs by Tony Clarke. The difference between the APS distributions will provide a direct test of Hypothesis A. The principal difficulty is the poor statistics (resulting from low number concentrations) when measuring the largest particles, which could require fairly long sampling times in the target airmasses. Nephelometers will be used to compare the scattering behind each inlet and to provide a real-time signal in flight to guide the tests. They will also provide a relevant integral measure of light scattering that is appropriate to the tests and one of the goals of ACE-Asia, radiative transfer.

We plan to use an OPC switched between inlets to examine any differences in small particle transmission through each inlet. This does not require high coarse particle concentrations. We will also use size-dependent sulfate measurements (next paragraph) in a region with few coarse particles to assess the submicron passing efficiencies of the inlets. In view of the importance of submicron particles, we need to know if there are significant differences between these inlet systems.

We will also collect filter samples for chemical analysis behind each of these inlets. In the seasalt, we will measure the total sodium concentration (IC analysis by Barry Huebert’s group). If we can arrange for impactors that can work in each of these flow regimes (Clarke’s inlet is designed for smaller flows) we will also measure chemical size distributions. In the dust regime, Jim Anderson will analyze mineral aerosols using EM (electron microscopy) to count and size the particles that have passed through each inlet. The EM will be set up to quantify large particles (without chemical analysis), so that it can get statistics on thousands of particles quite easily. The high volume of material (up to 12 m3 of air can be sampled per hour) will ensure robust statistics even in the larger size ranges. In each case, the analyses will be fast-tracked, so that the data will be available for our August report.

B. Hypothesis B

Hypothesis B is considerably more difficult to test, since it involves comparing aerosol distributions behind the LTI to those in ambient air. The crux of the problem is to measure the ambient (reference) distribution with a system that does not itself suffer from inlet or other artifacts. We discussed using tower-mounted instruments to measure the ambient distribution, but proving that measurements at one point accurately represent the average over a long aircraft track is virtually impossible, especially in heterogeneous continental regions. An airship or balloon might work, but we do not have the resources to obtain such a platform. The two devices that are the least likely to exhibit inlet losses are FSSP’s and total aerosol samplers (TASs). Virtually all other samplers and OPCs (even wing-mounted ones) derive their samples from some type of inlet, whose potential for losses would compromise the tests.

The FSSP seems to offer the best hope for characterizing the ambient size distribution, even though its response depends strongly on assumptions about particle refractive index. We would like to fly two or three FSSP’s, which would be set up identically and intercalibrated in the laboratory to minimize differences. At least two would be flown on the wings, to assess the best agreement that is possible. The third would be set up for use behind the LTI, where the airspeeds will be much lower. As long as all three are set up identically, using the same index of refraction, they should be able to identify differences caused by the LTI. Of course, if the choice of refractive index is wrong, this would cause errors in the apparent sizes at which efficiencies are calculated. Chemical data will be used to evaluate the appropriateness of those choices.

One of the most defendable references is the bulk concentration of particles, as measured by the TAS designed and built by NCAR’s shop. This external sampler permits an analysis of every particle that enters the inlet tip, whether it has been deposited on the inside of the diffuser or collected on its filter. The diffuser is lined by removable cones, which are replaced with each filter sample and extracted after the flight. We have 7 liner cones for the existing TAS, so we can collect 6 samples and a blank on each flight. As long as we sample isokinetically, we can be assured that the sum of the cone extract and its filter contains every particle that entered the TAS tip. This will be used to measure the reference total sodium concentration when flying in seasalt. The FSSP can then be used to determine the peak diameter and shape of the seasalt distribution, for comparison with the Na on an impactor behind the LTI. When sampling dust, the size of mineral aerosol is preserved in the TAS extract (assuming there were no aggregates of big particles), so that the ambient and LTI size distributions can be measured directly from filters by EM.

V. Import of the experiment – What size ranges are radiatively important?
Tony Clarke and Steve Howell examined some Indoex data to see what size ranges of dust had a significant impact on the total extinction in a heavy dust layer. They had access to differential optical depth measurements to constrain the total extinction in the layer. To use their OPC size distributions from behind the CAI, they had to correct for the (largely unknown – that’s the point!) size-dependent transmission efficiency of the CAI and their plumbing.

When they used the efficiency curve inferred for sea salt particles in the CAINE-2 experiment, they computed such huge concentrations in the 5-7 µm range that it clearly exceeded the extinction constraints from the differential optical depth measurements. They therefore tried an assumption that enough dust bounces back off the walls of the inlets that the efficiency never drops below 35% for mineral particles. The result is this distribution of passing efficiency.


 
 

That estimate of passing efficiency was then used to infer the ambient volume size distribution from their OPC measurements.


 

It is not surprising that the ambient concentration of 7 µm particles must be about 3 times larger than that inside the aircraft, if the passing efficiency is 35%. However, one result of that efficiency assumption is that the mass concentration was still increasing above 7 µm. The error bands only represent the OPC counting error, not the assumptions about passing efficiency or sizing errors.

When this inferred ambient distribution is then used to compute the size-dependent scattering in the layer, the following results:

If the inlet efficiency was not considered, the super-micron scattering would be thought to be a small fraction of the total. However, with a reasonable assumption about inlet efficiency, the super-micron extinction is found to be similar to that from submicron particles! (In urban pollution or dust-free air, however, the submicron particles would certainly dominate.)

Several conclusions can be drawn from this exercise: 1) in regions with significant concentrations of supermicron particles (dust or sea salt), a large fraction of the radiative impact can be due to these particles; 2) the loss of these particles in inlet systems can cause serious errors for in situ measurements of aerosol radiative impacts; and 3) knowing the inlet efficiency vs size is critical for assessing the impact of these large particles.

IV. Instrumentation and Quality Control

Among us we believe we can supply or borrow all the instrumentation we need, so the only costs will be for integration and operations. Huebert and Covert both have 3320 APS units, and a third has been offered by Tim Bates-PMEL. Clarke and RAF both have wing-mountable FSSPs, and a third might be available from DU or elsewhere. Huebert’s group already has a TAS and two high-flow impactors (although the latter need modification to add a stage with a 3 µm cut). Clarke has 3 Radiance Research nephelometers that can be configured behind each inlet to look at supermicron and submicron scattering. These will allow real-time indications of differences in integral optical properties for the inlets that are directly related to the radiative impacts of the aerosol. Huebert and Anderson have the analytical capabilities for Na and dust in hand.

Obviously, however, the uncertainty in our conclusions will depend strongly on the comparability of the APSs and FSSPs. We recommend that the APS instruments be sent to UW for bench-top setup and calibration by Dave Covert prior to the flights. They would again be compared just before mounting at RAF, to make sure nothing lost its alignment during shipping. Wind tunnel facilities are needed to do a similar setup and intercalibration of the FSSP’s. Darrel Baumgardner has offered to do a bench-top intercalibration of the FSSP-300’s. Since their sensitivity does not depend on the speed of the air, they can provide a good comparison of internal and external volumes when set up with identical bin sizes and indices of refraction.

V. Report

Before the end of August, 2000, this LTI Assessment Working Group will report our findings to NSF-ATM.

Send questions or comments to huebert@soest.hawaii.edu.