Vladimir N. Kapustin

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

New particle formation in the remote troposphere : A comparison of observations at various sites

Measurements show that new particles are formed by homogenous nucleation over a wide range of conditions in the remote troposphere. In our studies, large nucleation events are found exclusively in regions of enhanced sulfuric acid vapor (H2SO4g) concentrations, with maximum concentrations never exceeding 5×107 molecules cm−3. Although these data suggest that H2SO4g participated, comparisons between ambient conditions in regions of nucleation to conditions necessary for binary H2SO4 water (H2O) nucleation indicate that the mechanism may vary with elevation. In remote marine regions, at altitudes greater than ∼4 km above sea level, observations of nucleation in clear air along cloud perimeters are in fair agreement with current classical binary nucleation models. In these regions, the low temperatures associated with high altitudes may produce sufficiently saturated H2SO4 for the production of new H2SO4/H2O particles. However, uncertainties with current binary nucleation models limit decisive comparisons. In warmer regions, closer to the earths surface, measured H2SO4 concentrations are clearly insufficient for binary nucleation. Conditions at these sites are similar to those observed in an earlier study where there was circumstantial evidence for a ternary mechanism involving H2SO4, H2O, and ammonia (NH3) [Weber et al., 1998], suggesting that this may be a significant route for particle production at lower altitudes where surface-derived species, like NH3, are more apt to participate.

Biomass burning and pollution aerosol over North America: Organic components and their influence on spectral optical properties and humidification response

[1] Thermal analysis of aerosol size distributions provided size resolved volatility up to temperatures of 400C during extensive flights over North America (NA) for the INTEX/ICARTT experiment in summer 2004. Biomass burning and pollution plumes identified from trace gas measurements were evaluated for their aerosol physiochemical and optical signatures. Measurements of soluble ionic mass and refractory black carbon (BC) mass, inferred from light absorption, were combined with volatility to identify organic carbon at 400C (VolatileOC) and the residual or refractory organic carbon, RefractoryOC. This approach characterized distinct constituent mass fractions present in biomass burning and pollution plumes every 5–10 min. Biomass burning, pollution and dust aerosol could be stratified by their combined spectral scattering and absorption properties. The ‘‘nonplume’’ regional aerosol exhibited properties dominated by pollution characteristics near the surface and biomass burning aloft. VolatileOC included most water-soluble organic carbon. RefractoryOC dominated enhanced shortwave absorption in plumes from Alaskan and Canadian forest fires. The mass absorption efficiency of this RefractoryOC was about 0.63 m 2 g � 1 at 470 nm and 0.09 m 2 g � 1 at 530 nm. Concurrent measurements of the humidity dependence of scattering, g, revealed the OC component to be only weakly hygroscopic resulting in a general decrease in g with increasing OC mass fractions. Under ambient humidity conditions, the systematic relations between physiochemical properties and g lead to a well-constrained dependency on the absorption per unit dry mass for these plume types that may be used to challenge remotely sensed and modeled optical properties.

Aerosol optical properties in the marine boundary layer during the First Aerosol Characterization Experiment (ACE 1) and the underlying chemical and physical aerosol properties

Measurements were made onboard the NOAA R/V Discoverer during the First Aerosol Characterization Experiment (ACE 1) to understand the optical properties of a minimally perturbed natural aerosol system in terms of its chemical and physical properties. ACE 1 took place during November and December of 1995 in the Southern Ocean region south of Australia. Reported here are observations at a wavelength of 550 nm of the submicron and supermicron aerosol scattering coefficient, σsp; single scattering albedo, ω0; and the hemispheric backscattered fraction and mass scattering efficiencies of non-sea-salt sulfate, sea salt, and the total aerosol. Also presented is the Angstrom exponent, a, for the 450 and 700 nm wavelength pair. Variations in these parameters were found to be a strong function of the relative concentrations and size distributions of the dominant aerosol chemical components. Both the submicron and supermicron aerosol mass were composed primarily of water-soluble ionic species. This is in agreement with an experiment-average single scattering albedo of 0.99 (−0.4, +1.0%). Of the submicron ionic mass, 80±10% was sea salt, 16±8% was non-sea-salt sulfate, and 4±3% was methanesulfonate. Sea salt composed 99±0.7% of the supermicron ionic mass. The magnitude of scattering by both submicron and supermicron aerosol was controlled by sea salt. The backscattered fraction for the submicron aerosol averaged 0.11±0.02 and was controlled by the tailing of coarse mode sea-salt mass into the submicron size range. Calculated mass scattering efficiencies for submicron non-sea-salt sulfate ion averaged 1.5±0.74 m2 g−1 (at 30 to 45% relative humidity) with the highest values corresponding to continentally influenced air masses where sulfate aerosol surface mean diameters and surface area concentrations were the largest. Mass scattering efficiencies for submicron sea salt were higher (averaging 4.2±0.96 m2 g−1) due to the tailing of coarse mode sea salt into the particle size range most efficient for light scattering. Given the similar lifetimes of submicron non-sea-salt sulfate and sea salt in the marine boundary layer, it is evident that sea salt controls the aerosol optical properties in this Southern Ocean region.

Processes controlling the distribution of aerosol particles in the lower marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)

The goals of the International Global Atmospheric Chemistry (IGAC) Programs First Aerosol Characterization Experiment (ACE 1) are to determine and understand the properties and controlling factors of the aerosol in the remote marine atmosphere that are relevant to radiative forcing and climate. A key question in terms of this goal and the overall biogeochemical sulfur cycle is what factors control the formation, growth, and evolution of particles in the marine boundary layer (MBL). To address this question, simultaneous measurements of dimethylsulfide (DMS), sulfur dioxide (SO2), the aerosol chemical mass size distribution, and the aerosol number size distribution from 5 to 10,000 nm diameter were made on the National Oceanic and Atmospheric Administration (NOAA) ship Discoverer. From these data we conclude that the background MBL aerosol during ACE 1 often was composed of four distinct modes: an ultrafine (UF) mode (Dp = 5–20 nm), an Aitken mode (Dp = 20–80 nm), an accumulation mode (Dp = 80–300 nm), and a coarse mode (Dp > 300 nm). The presence of UF mode particles in the MBL could be explained by convective mixing between the free troposphere (FT) and the MBL associated with cloud pumping and subsidence following cold frontal passages. There was no evidence of major new particle production in the MBL. Oceanic emissions of DMS appeared to contribute to the growth of Aitken and accumulation mode particles. Coarse mode particles were comprised primarily of sea salt. Although these particles result from turbulence at the air-sea interface, the instantaneous wind speed accounted for only one third of the variance in the coarse mode number concentration in this region.

Physical properties of marine boundary layer aerosol particles of the mid-Pacific in relation to sources and meteorological transport

Aerosol measurements were made on three cruises in the mid-Pacific along longitude 140°W from 55°N to 70°S for a total of about 90 days in 1992 and 1993. The three data sets document the aerosol concentration and general features of its number-size distribution in the marine boundary layer (MBL) and their variation with latitude and meteorological conditions. Mean concentration varied from 300 cm−3 in the tropics to 500 cm−3 in the midlatitudes outside of continental air masses. Infrequent short-term spikes in concentration ranged up to 2000 cm−3. Two dominant modes were observed, the Aitken and accumulation, with mean diameters of 25 to 60 nm and 150 to 200 nm, respectively. An intermittent ultrafine mode was noted at diameters less than 25 nm. The concentration and dominance of one mode over another depended on the relative strength of the entrainment of ultrafine and Aitken particles from the free troposphere (FT) into the MBL compared to the rate of growth of Aitken mode into accumulation mode particles and removal rate of the accumulation mode. In general, aging times were shorter in the subtropics, longer in the tropics, and variable in the midlatitudes. The rate of new particle formation within the MBL itself was either low and did not contribute significantly to the observed number concentration or, if the rate was high, it occurred infrequently and was not observed in these experiments.

A Pacific Aerosol Survey. Part I: A Decade of Data on Particle Production, Transport, Evolution, and Mixing in the Troposphere*

Abstract Integration of extensive aerosol data collected during the past decade around the Pacific basin provides a preliminary assessment of aerosol microphysics for this region and cycling of aerosol in the troposphere. These include aircraft-based data collected as part of numerous field experiments supported by the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the National Oceanic and Atmospheric Administration (NOAA) [Global Backscatter Experiment (GLOBE), First Aerosol Characterization Experiment (ACE-1), Pacific Exploratory Mission (PEM)-Tropics A and B]. Although these experiments had diverse goals, most included extensive data on aerosol size distributions, optical properties (light scattering and light absorption), and chemistry. Vertical profiles of aerosol concentration, size distribution, and light scattering were used to characterize vertical structure from 70°S to 70°N. The in situ data are placed in the context of meteorological regimes ov...

Dimethylsulfide/cloud condensation nuclei/climate system - Relevant size-resolved measurements of the chemical and physical properties of atmospheric aerosol particles

Atmospheric aerosol particles resulting from the oxidation of dimethylsulfide (DMS) may have an impact on global climate if they result in an enhancement in the cloud condensation nuclei (CCN) number concentration and shortwave cloud albedo. To characterize and quantify relationships within the DMS/CCN/climate system, simultaneous measurements were made over the northeastern Pacific Ocean in April and May 1991 of particulate non-sea-salt sulfate, methanesulfonate, and ammonium mass size distributions, number size distributions of particles having diameters between 0.02 and 9.6 μm, CCN concentrations at 0.3% supersaturation, relative humidity, and temperature. Comparisons between particle mass and surface area indicate that non-sea-salt sulfate, methanesulfonate, and ammonium were not involved in new particle production on the 12- to 24-hour time scale of the measurements. Instead, high levels of available particulate surface area resulted in the condensation of the gas phase precursors onto existing aerosol. A doubling of non-sea-salt sulfate, methanesulfonate, and ammonium mass corresponded to a 40 to 50% increase in number in the accumulation mode size range. Likewise, a doubling of the non-sea-salt sulfate mass corresponded to a 40% increase in the CCN number concentration. As methanesulfonate made up a very small fraction of the soluble particulate mass, no correlations were found between methanesulfonate mass and CCN number. In a separate experiment, measurements were made of particulate non-sea-salt sulfate, methanesulfonate, and ammonium mass size distributions over the central Pacific Ocean during February 1991. The percent of methanesulfonate in the supermicrometer particle size fraction was greater in these samples than in those collected over coastal waters of the northeastern Pacific. In both regions the non-sea-salt sulfate mass size distributions were bimodal, while ammonium was found to be concentrated in larger accumulation mode particles.

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.