Size Resolved Aerosol Nuclei and Physico-Chemistry

for PEM-Tropics B Aboard the P-3B


Antony D. Clarke

Vladimir Kapustin

Department of Oceanography

University of Hawaii

Rodney J. Weber

Environmental Chemistry Division
Brookhaven National Laboratory


We will make aerosol measurements over the size range from 0.003 um up to 50 um with the intent of characterizing the full aerosol size distribution in order to study aerosol nucleation, evolution and transport in the tropics. We will focus on measurements that will contribute to understanding the nucleation process, remote marine sulfur cycle, cloud related processes and the long range transport of pollution aerosol. These will employ instruments and techniques to establish the size-resolved concentration, surface area and mass for the entire size range. Our aerosol instrumentation proposed has evolved over several years of sampling remote aerosol (Clarke, 1991; Clarke, 1993a; Clarke et al., 1996; Clarke et al., 1997).  These are described below and are designed to provide information on the size and concentration of the atmospheric aerosol and to measure the size dependent properties of the aerosol that we have found to be related to its chemistry. Thermal analysis will be also used in conjunction with sizing instrumentation in order to infer aerosol physicochemistry in near-real time and to distinguish air masses and aerosol with continental vs. "clean" characteristics. For many remote regions, the submicron aerosol mass is usually dominated by mix of sulfuric acid or sulfate. We have demonstrated that the difference in thermal volatility of sulfuric acid, ammonium sulfate and other refractory constituents such as soot, salt, and dust can be used as means to identify the presence and concentration of these species (Clarke et al., 1991; Clarke, 1993a; Clarke et al., 1997) in near real-time.

Instrumentation

1) Condensation Nuclei - heated and unheated

Two condensation nuclei (CN) counters (TSI 3010) have been modified for aircraft use and count all particles between 0.02-3.0 um simultaneously at temperatures of 30 and 300C.  In this fashion we obtain total CN, refractory CN (those remaining at 300C after sulfate is removed) and volatile CN (by difference) as a continuous readout.  The ratio of refractory to total CN can be used as a rapid (1Hz) indicator for air mass variability since it is not directly dependent on aerosol concentration (Clarke, 1993a). We have observed that polluted and continental aerosol often have ratios near about 0.8 whereas more pristine regions tend to be far more volatile with low ratios often approaching zero (Clarke et al., 1993; Clarke et al., 1997). Variability in the refractory component also appears to reflect sources of continental combustion (soot) or dust aerosol whereas in "clean" marine regions this fraction is often low. This rapid measurement (ca. 1Hz) is a valuable real-time indicator of continental and oceanic sources of the aerosol that can be used as a diagnostic in flight and for stratifying data during later analysis (see below).

2) Ultrafine Condensation Nuclei Counter

We successfully deployed the TSI 3025 Ultrafine Condensation Particle Counter (UCPC-Thermal Systems Inc.) on PEMT-A after making minor modifications for aircraft use. This instrument counts particles with diameters as small as 0.003 um. Hence, it is able to detect the particles most recently formed as a result of gas to particle conversion. When this ultrafine condensation nucleus (UCPC) counter is operated in conjunction with the above mentioned CN counter we obtain the concentration of particles in the 0.003 to 0.02 um range by difference at a sample rate of about 1 Hz. This instrument has been found to be essential for identifying regions with recent in-situ nucleation of new particles from the gas phase (Clarke, 1993; Clarke et al., 1997; Clarke et al., submitted to Science).

For PEMT-B we propose to use a modified UCPC ( Weber, 1998b) that both counts and sizes these nanometer particles using pulse height analysis, PHA. The sampling rate for pulse height distributions is considerably lower at ~60 s/measurement but both the integral difference and size resolved measurements are valuable. The 1 Hz CNC differencing approach provides high spatial resolution, but at lower sensitivity and with no size-segregated information. However, the measurement is rapid and readily interpreted, and thus can be observed near-real-time and input to the aircraft data acquisition system and used for in-flight mission planning decisions. The PHA measurement is most valuable for direct links to aerosol nucleation processes but must be analyzed off-line. The full inversion [Weber et al., 1998b] is complex and cannot be done in the field. Finally, an added benefit of combining these two nanoparticle sampling strategies is that the quality of the nanoparticle size distributions from pulse height inversions can be assessed by comparison with nanoparticle concentrations measured by the 1 Hz CNC differencing technique.

3) A size-resolved thermo-optic aerosol discriminator

In order to characterize the aerosol size distribution between 0.3 and 7.0 um, where most aerosol mass and surface area resides, we will use a modified 256 channel Laser Optical Particle Counter (OPC) spectrometer similar to that used on PEMT-A but upgraded for improved aerosol sizing, faster data management and Windows capability on a PC. As on PEMT-B, a computer controlled thermal conditioning system will be used upstream of the OPC to result in a system that operates as a Thermo-Optic Aerosol Discriminator (TOAD).  The thermal conditioning by the TOAD is used to characterize aerosol components volatile at temperatures associated with sulfuric acid (150C), ammonium sulfate/bisulfate (300C) and refractory aerosol at 300C (sea salt, dust and soot). A dryer unit will maintain instrument RH below 30% in order to have it measure "dry" aerosol sizes only. This is necessary if the resulting size distributions are used to make semi-quantitative inferences of fast aerosol chemistry. Application of calibration standards for various molar ratios of NH4+/SO4= to the observed volume ratios of volatile components at these temperatures can yield size-resolved information of the aerosol molar ratio and allows inference of the total mass of SO4(Clarke, 1991).

4) Differential Mobility Analyzer (DMA) with thermal conditioning

We will employ our radial differential mobility analyzer (RDMA) with thermal analysis (see discussion for OPC above) that provides size information (mass, surface area, number distributions) over the 0.008 to 0.3 um size range (Clarke et al., 1995, Clarke et al., 1998b). It is this size range that reveals much of the dynamic evolution of the aerosol in response to coagulation, growth and cloud processing. These data can also be used to model those aerosol likely to be activated as cloud condensation nuclei. For point comparisons to other instruments we will utilize our custom Lagged Aerosol Grab (LAG) sampler, successfully employed on PEMT-A, to obtain a sample over 10-20 sec that can be subsequently analyzed over a period of 1-4 minutes depending upon whether or not thermal analysis is carried out.

5) Forward Scattering Spectrometer Probe (FSSP-300)

The FSSP a new addition to our PEMT-B package in order to improve our assessment of large aerosol surface area including cloud droplets and to clearly identify regions of cloud rain or drizzle. The FSSP mounts on the wing and sizes particles in-situ at about 20Hz for ambient "wet" sizes between 0.3 and 20 um or 0.5 and 50 um (range selectable). In-situ characterization of larger particles is an important addition to our capabilities because as the size of ambient aerosol increases above a few micrometers they become increasingly difficult to bring into the aircraft through our aspirated sampling system. Though few in number, these larger particles are important since they often contain a large fraction of the total mass and surface area. It is also essential that the presence of cloud or drizzle be identified in studies of in-situ nucleation because the nucleation of new particles appears favored near evaporating clouds but clouds can also be a source of spurious apparent nucleation due to cloud/rain droplet shatter on sampling probes (Clarke et al., 1997; Weber, personal correspondence). Hence, the measurement of coarse aerosol is crucial for the nucleation, cloud processing and sulfur cycle issues being addressed in PEMT-B. This is particularly true for sea-salt in the MBL where the P3-B will focus its efforts in investigation the sulfur cycle. Heterogeneous gas to particle conversion in cloud droplets is also an important means of transferring mass to existing aerosol that can be assessed if the cloud droplet surface area is known.

6) Nephelometer and Aethaelometer(Black Carbon)

A nephelometer provides a continuous measurement of the light scattering coefficient of the aerosol at up to 1Hz sample rates. This provides a rapid index to look at the variability in larger aerosol that often dominates aerosol surface area. This is a valuable addition to the more intermittent size distribution information particularly during vertical profiles, near clouds, aerosol plumes (eg. biomass) and any other location where rapid variability in aerosol concentrations may occur. This measurement of the size segregated optical extinction is also important to the assessment of the role in sulfate and sea-salt in contributing to the optical properties of the MBL. Two Radiance Research mini-nephelometers will be used concurrently instead of the heavier cycled TSI Nephelometer used on PEMT-A. An impactor having a size cut at 1um diameter will be upstream of one nephelometer in order to separate the contributions from these two aerosol components.

The aethaelometer provides a measure of the aerosol light absorption often related to the presence of Black Carbon from combustion. This is usually low in remote regions unless we encounter plumes from long range transport that contain light absorbing soot (biomass burning, continental pollution). This measurement can help identify these events as well as contribute to establishing the aerosol single scatter albedo (ratio of the scattering coefficient to the sum of scattering and absorption coefficients) of significance to climate models.

References

Clarke, A. D. "A Thermo-optic technique for in-situ analysis of size-resolved aerosol physicochemistry", Atmos. Env., 25A, 3/4, 635-644, 1991

Clarke, A. D., "A Global Survey of Atmospheric Nuclei in the remote Mid-Troposphere: Their Nature, Concentration and Evolution, Jour. Geophys. Res. Atmospheres, 98, D11, 20,633-20,647, 1993.

Clarke, A.D., T. Uehara and J.N. Porter, Lagrangian evolution of an Aerosol Column during ASTEX, JGR. November 1997.

Clarke, A.D. and J.N. Porter, Pacific Marine Aerosol Part II: Equatorial Gradients, Ammonium and Chlorophyll During SAGA3, Jour. Geophys. Res., 98, D9, 16,997-17,010, 1993.

Clarke, A.D., Z. Li and M. Litchy, Aerosol Dynamics in the Pacific Marine Boundary Layer: Microphysics, Diurnal Cycles and Entrainment: Geophys. Res. Lett., 23, pg 733-736, 1996.

Clarke, A.D., J. L. Varner, F. Eisele, R. Tanner, L. Mauldin and M. Litchy, Particle production in the remote marine atmsophere: Cloud outflow and subsidence during ACE-1, in press JGR 6/97.

Clarke, A.D., F. Eisele, K. Moore, R. Tanner, L. Mauldin* and M. Litchy, Nucleation in the Equatorial Troposphere: Three Environments Encountered on PEM-Tropics, in press Jour. Geophys. Res-a.

Clarke, A.D., D. Davis, V. Kapustin, F. Eisele, I. Paluch, D. Lenschow, A. R. Bandy, D. Thornton, G. Chen, K. Moore, L. Mauldin, R. Tanner , M.A. Carroll, J. Collins and M. Litchy , Particle Nucleation in the Marine Boundary Layer: A Case Study Involving Marine Sulfur Sources, submitted to SCIENCE-b.

Weber, R. J., P. H. McMurry, L. Mauldin, D. Tanner, F. Eisele, F. Brechtel, S. Kreidenweis, G. Kok, R. Schillawski, D. Baumgardner, A study of new particle formation and growth involving biogenic and trace gas species measured during ACE-1, J. Geophys. Res., in press, 1998a.

Weber, R. J., M. Stolzenburg, S. Pandis, P. H. McMurry, Inversion of UCPC pulse height distributions to obtain ultrafine (~3 to 10 nm) particle size distributions, J. Aerosol Sci., in press, 1998b.