K. Moore

Size distributions and mixtures of dust and black carbon aerosol in Asian outflow: Physiochemistry and optical properties

[1] During Transport and Chemical Evolution over the Pacific (TRACE-P) and Asian Aerosol Characterization Experiment (ACE-Asia) we measured the dry size distribution of Asian aerosols, their state of mixing, and the optical properties of dust, black carbon (BC) and other aerosol constituents in combustion and/or dust plumes. Optical particle sizing in association with thermal heating extracted volatile components and resolved sizes for dust and refractory soot that usually dominated light absorption. BC was internally mixed with volatile aerosol in � 85% of accumulation mode particles and constituted � 5–15% of their mass. These optically effective sizes constrained the soot and dust size distributions and the imaginary part of the dust refractive index, k, to 0.0006 ± 0.0001. This implies a single-scatter albedo, v (550 nm), for dust ranging from 0.99+ for Dp <1 m mt o� 0.90 at Dp =1 0mm and a size-integrated campaign average near 0.97 ± 0.01. The typical mass scattering efficiency for the dust was � 0.3 m 2 g � 1 , and the mass absorption efficiency (MAE) was 0.009 m 2 g � 1 . Less dust south of 25� N and stronger biomass burning signatures resulted in lower values for v of � 0.82 in plumes aloft. Chemically inferred elemental carbon was moderately correlated with BC light absorption (R 2 = 0.40), while refractory soot volume between 0.1 and 0.5 mm was highly correlated (R 2 = 0.79) with absorption. However, both approaches yield an MAE for BC mixtures of � 7±2m 2 g � 1 and higher than calculated MAE values for BC of 5 m 2 g � 1 . The increase in the mass fraction of soot and BC in pollution aerosol in the presence of elevated dust appears to be due to uptake of the volatile components onto the coarse dust. This predictably lowered v for the accumulation mode from 0.84 in typical pollution to � 0.74 in high-dust events. A chemical transport model revealed good agreement between model and observed BC absorption for most of SE Asia and in biomass plumes but underestimated BC for combustion sources north of 25� N by a factor of � 3. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0350 Atmospheric Composition and Structure: Pressure, density, and temperature; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere— constituent transport and chemistry; KEYWORDS: dust, black carbon, absorption, single scatter albedo

Nucleation in the equatorial free troposphere: Favorable environments during PEM-Tropics

A combination of aerosol and gas phase instrumentation was employed aboard the NASA-P3B as part of the Pacific Exploratory Mission-Tropics (PEM-T) in the eastern equatorial Pacific during August-October 1996. Recent particle production was found in cloud-processed air over extended regions aloft (6–8 km). These were clearly associated with clean marine air lofted by deep convection and scavenged of most aerosol mass in the Intertropical Convergence Zone (ITCZ) and in more aged cloud-scavenged air influenced by a distant continental combustion near the South Pacific Convergence Zone (SPCZ). Recent particle production was evident in regions where sulfuric acid concentrations were about 0.5 to 1 × l07 molecules cm−3, when surface areas were near or below 5 µm2 cm−3, and when relative humidity (RH) was elevated over adjacent regions. In regions of recent particle production, the calculated critical sulfuric acid concentrations, based upon classical binary nucleation theory and corrected for in situ conditions near cloud, were generally consistent with nearby observed sulfuric acid concentrations. This indicates that classical binary nucleation theory and natural sources of sulfuric acid can account for nucleation in the near-cloud environment. Data from six equatorial flights between 20°N and 20°S demonstrate that this process populates extensive regions of the equatorial free troposphere with new particles. Vertical profiles suggest that nucleation, subsidence, and mixing into the MBL can supply the MBL with new aerosol.

Dust and pollution transport on global scales: Aerosol measurements and model predictions

Vertical profiles of aerosol and gas phase species were measured on flights near Hawaii on April 9 and 10, 1999, during NASAs Pacific Exploratory Mission (PEM) Tropics B program. These measurements characterized aerosol microphysics, inferred chemistry, optical properties, and gases in several extensive dust and pollution plumes, also detected by satellites, which had 10,000-km trajectories back to sources in Asia. Size-resolved measurements indicative of aerosol sulfate, black carbon, dust, light scattering, and absorption allowed determination of their concentrations and contributions to column aerosol optical depth. A new Chemical Transport Model (CTM) that includes aerosol, meteorological fields, dynamics, gas and particle source emissions, a chemistry component (MATCH), and assimilated satellite data was used to predict aerosol and gas concentrations and the aerosol optical effects along our flight path. Flight measurements confirmed the “river-like” plume structures predicted by the CTM and showed close agreement with the predicted contributions of dust and sulfate to aerosol concentrations and optical properties for this global-scale transport path. Consistency between satellite, model and in situ assessment of aerosol optical depth was found, with noted exceptions, within ∼25%. Both observations and model results confirmed that this aerosol was being entrained into the marine boundary layer between Hawaii and California where it can be expected to modify the type and concentration of cloud condensation nuclei in ways that may alter properties of low-level clouds. These observations document the significance and complexity of long-range aerosol transport and highlight the potential of emerging CTM models to extend observational data and address related issues on global scales.

Sea-Salt Size Distributions from Breaking Waves: Implications for Marine Aerosol Production and Optical Extinction Measurements during SEAS*

The authors’ participation in the Shoreline Environment Aerosol Study (SEAS) involved measurements focused on the coastal aerosol size distribution and related optical measurements, including aerosol light scattering, visibility, and remote sensing of aerosol using lidar backscatter. Aerosol production from shoreline breaking waves and the more distant reef (;1 km) was characterized for dry sizes between 0.01 and 10 mm for both their contribution to the marine aerosol population and their influence on near-surface lidar extinction. Thermal volatility was used to extract the refractory sea-salt particles from the other constituents volatile at 360 8C. At 7 m ASL and 20 m inland from the water’s edge the production of sea-salt nuclei number was often in the range of 50‐100 cm23 above the open-ocean value of ;250 cm23. This number peak was near 0.03-mm dry diameter, while light scattering was dominated by a few particles larger than 1 mm. This indicates that production of sea salt from breaking waves contributes not only to aerosol mass and optical effects but also to nuclei mode particle number in remote regions. Separate studies of optical closure quantified links between the size distribution and optical scattering measurements, visibility, and extinction values for both nearshore breaking waves and openocean conditions. These data confirmed that extinction derived from coastal lidar measurements at 0.530 mm was accurate to better than the 25% uncertainty claimed for the lidar inversion.

A Study of the Extinction-to-Backscatter Ratio of Marine Aerosol during the Shoreline Environment Aerosol Study*

Abstract Ground-based aerosol optical measurements made at near-ambient relative humidity (RH) under clean marine sampling conditions are presented and compared to 1) almost identical optical measurements made at a polluted continental site and 2) optical properties calculated from measured size distributions and Mie theory. The use of Mie theory (which assumes homogeneous spheres) is justified based on the fact that the sea-salt aerosol was measured in a hydrated state. This study focuses on the extinction-to-backscatter ratio S, an optical property required to interpret remote measurements by elastic backscatter lidar. For clean marine conditions, S is found to be 25.4 ± 3.5 sr at 532 nm (central value ± 95% confidence uncertainty). Other optical properties reported include single-scattering albedo, wavelength dependence of scattering, fraction of scattering due to submicrometer particles, and hemispheric-backscatter fraction, as well as the extensive properties (e.g., scattering coefficient) upon which...

Nucleation in the equatorial Pacific during PEM-Tropics B: Enhanced boundary layer H2SO4 with no particle production

During the second phase of the NASA Pacific Exploratory Mission to the Pacific Tropics (PEM-Tropics B), regions of unusually high concentrations of sulfuric acid vapor ranging from 107 to 108 molecules cm−3 were detected near the ocean surface in equatorial regions between Hawaii and Tahiti. No 3–4 nm diameter nanoparticles were observed near the ocean surface where acid concentrations were highest; however, 3–4 nm particles were detected at higher elevations in regions near clouds. Calculations show that in some regions of high acid concentrations newly formed particles would be readily detected by our instruments and thus the lack of nanoparticles suggests that there was no nucleation. In contrast, in the previous PEM-Tropics A mission Clarke et al. [1998a] observed a large nucleation event in the equatorial marine boundary layer under similar temperatures, relative humidity, and sulfuric acid concentrations. Comparison between these two studies further demonstrates that some additional species or unknown process is necessary to significantly enhance nucleation in the remote marine troposphere and that this component is not always present at levels sufficient to sustain nucleation throughout the region, even at a low continuous rate. We speculate that if ammonia is one example of a critical nucleation precursor, tropospheric ternary nucleation (sulfuric acid/ammonia/water) under some conditions requires ammonia concentrations to be greater than typical background concentrations for particle production via this mechanism.

A Miniature Optical Particle Counter for In Situ Aircraft Aerosol Research

Abstract Modification of a commercial Met One 237A optical sensor to accept custom electronics consisting of a single logarithmic amplifier providing 256 size bins over the 0.3–14-μm diameter range is described. Configuration of the optical particle spectrometer for airborne aerosol measurements is found to be effective for both miniature remote control aircraft and large research aircraft (NASA P-3B). The instrument is rugged, of low cost, uses low power, and is easily integrated into various platforms. The high size resolution and the 1.6 l min–1 sample rate provide excellent count statistics and high sensitivity for ambient out-of-cloud aircraft measurements and for other diverse applications. It can be readily configured for isokinetic or subisokinetic aircraft sampling. Initial comparison with other optical particle counters over the Sea of Japan reveals it to be an effective instrument for in situ aircraft measurements.