D. J. Coffman

Modification, Calibration and a Field Test of an Instrument for Measuring Light Absorption by Particles

A filter-based single-wavelength photometer (Particle Soot Absorption Photometer, PSAP) for measuring light absorption by aerosols was modified to measure at three wavelengths, 467 nm, 530 nm, and 660 nm. The modified and an unmodified photometer were calibrated during the Reno Aerosol Optics Study (RAOS) 2002 against two absorption standards: a photoacoustic instrument and the difference between the extinction and scattering coefficient. This filter-based absorption method has to be corrected for scattering aerosol and transmission changes. A simple function for this was derived from the calibration experiment as a function of transmission and single-scattering albedo. For an unmodified PSAP at typical atmospheric absorption coefficients the algorithm yields about 5–7% lower absorption coefficients than does the usually used method. The three-wavelength PSAP was used for atmospheric measurements both during RAOS and during the New England Air Quality Study (NEAQS).

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.

Airborne measurements of particle and gas emissions from the 1990 volcanic eruptions of Mount Redoubt

Airborne in situ and remote sensing (lidar and correlation spectrometer) measurements are described for the volcanic emissions from Mount Redoubt, Alaska, in January and June 1990. The lidar provided excellent real-time information on the distribution of the volcanic effluents. In postanalysis the lidar observations were used to determine cross-sectional areas of the plumes of emissions which, together with the airborne in situ measurements, were used to derive the fluxes of particles and gases from the volcano. For the intraeruptive emissions the ranges of the derived fluxes were for water vapor, ∼160–9440 kg s−1; for CO2, ∼30–1710 kg s−1; for SO2, ∼1–140 kg s−1; for particles (<48 μm diameter), ∼1–6 kg s−1; for SO4=, <0.1–2 kg s−1; for HCl, <0.01–2 kg s−1; and for NOx, <0.1–2 kg s−1;. Independent measurements of SO2 from a correlation spectrometer during the period of active dome growth between late March and early June 1990 gave fluxes from 12 to 75 kg s−1;. The particles in the intraeruptive emissions consisted primarily of silicate rock and mineral fragments devoid of any sulfuric acid coating. Very little of the SO2 (∼0.1%) was oxidized to sulfate in the cold, dark conditions of the Arctic atmosphere. During a large eruption of Mount Redoubt on January 8, 1990, the particle (<48 μm diameter) emission flux averaged ∼104 kg s−1. During posteruptive emissions on June 11, 1990, the fluxes of both particles and gases were either close to or less than our lower detection limits (except for water vapor, which had a flux of ∼6×103 kg s−1).

A preliminary study of the effect of ammonia on particle nucleation in the marine boundary layer

A ternary nucleation model for the H2SO4-NH3-H2O system is presented in an effort to examine the effect of NH3 on heteromolecular homogeneous nucleation in the marine boundary layer (MBL). The results from this nucleation model suggest that ammonia could, in fact, enhance the nucleation rate over that of the binary system, H2SO4-H2O. The magnitude of this enhancement is introduced as an enhancement ratio, which, in principle, is applicable to any binary nucleation rate for H2SO4-H2O. Also presented are preliminary results from a simple aerosol model using this enhancement ratio. These results suggest that under conditions typical of the marine environment it may be possible to produce enough particles to balance the various particle sinks characteristic of the MBL.

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.

Particulate emissions from commercial shipping: Chemical, physical, and optical properties

provide chemical and physical characteristics including sulfate (SO4� ) mass, organic matter (OM) mass, black carbon (BC) mass, particulate matter (PM) mass, number concentrations (condensation nuclei (CN) > 5 nm), and cloud condensation nuclei (CCN). Optical characterization included multiple wavelength visible light absorption and extinction, extinction relative humidity dependence, and single scatter albedo (SSA). The global contribution of shipping PM was calculated to be 0.90 Tg a � 1 , in good agreement with previous inventories (0.91 and 1.13 Tg a � 1 from Eyring et al. (2005a) and Wang et al. [2008]). Observed PM composition was 46% SO4� , 39% OM, and 15% BC and differs from inventories that used 81%, 14%, and 5% and 31%, 63%, and 6% SO4� , OM, and BC, respectively. SO4� and OM mass were found to be dependent on fuel sulfur content as were SSA, hygroscopicity, and CCN concentrations. BC mass was dependent on engine type and combustion efficiency. A plume evolution study conducted on one vessel showed conservation of particle light absorption, decrease in CN > 5 nm, increase in particle hygroscopicity, and an increase in average particle size with distance from emission. These results suggest emission of small nucleation mode particles that subsequently coagulate/condense onto larger BC and OM. This work contributes to an improved understanding of the impacts of ship emissions on climate and air quality and will also assist in determining potential effects of altering fuel standards.

Aerosol optical properties measured on board the Ronald H. Brown during ACE-Asia as a function of aerosol chemical composition and source region

except aerosol optical depth and the vertical profiles of aerosol extinction, are reported at a relative humidity of 55 ± 5%. An overdetermined data set was collected so that measured and calculated aerosol properties could be compared, internal consistency in the data set could be assessed, and sourcesof uncertainty could beidentified. Byadjusting the measured size distribution to take into account nonsphericity of the dust aerosol, calculated and measured aerosol mass and scattering coefficients agreed within overall experimental uncertainties. Differences between measured and calculated aerosol absorption coefficients were not within reasonable uncertainty limits, however, and may indicate the inability of Mie theory and the assumption of internally mixed homogeneous spheres to predict absorption by the ACE-Asia aerosol. Mass scattering efficiencies of non-sea-salt sulfate aerosol, sea salt, submicron particulate organic matter, and dust found for the ACE-Asia aerosol are comparable to values estimated for ACE 1, Aerosols99, and the Indian Ocean Experiment (INDOEX). Unique to the ACE-Asia aerosol were the large mass fractions of dust, the dominance of dust in controlling the aerosol optical properties, and the interaction of dust with soot aerosol. INDEXTERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: aerosol optical properties, aerosol chemical composition, ACE-Asia

Impact of particulate organic matter on the relative humidity dependence of light scattering: A simplified parameterization

[1] Measurementsduringrecentfieldcampaignsdownwind of the Indian subcontinent, Asia, and the northeastern United States reveal a substantial decrease in the relative humidity dependence of light scattering, fssp(RH), with increasing mass fraction of particulate organic matter (POM) for submicrometer aerosol. Using data from INDOEX (INDian Ocean EXperiment), ACE Asia (Aerosol Characterization Experiment – Asia), and ICARTT (International Consortium for Atmospheric Research on Transport and Transformation), we have identified, within measurement limitations, the impact of POM on the fssp(RH) of accumulation mode sulfate-POM mixtures. The result is a parameterization that quantifies the POM mass fraction - fssp(RH) relationship for use in radiative transfer and air quality models either as input or as validation. The parameterization is valid where the aerosol consists of an internally mixed sulfatecarbonaceous accumulation mode and other externally mixed components (e.g. sea salt, dust) and is applicable on both global and regional scales. Citation: Quinn, P. K., et al. (2005), Impact of particulate organic matter on the relative humidity dependence of light scattering: A simplified parameterization, Geophys. Res. Lett., 32, L22809, doi:10.1029/ 2005GL024322.

A comparison of aerosol chemical and optical properties from the 1st and 2nd Aerosol Characterization Experiments

Shipboard measurements of aerosol chemical composition and optical properties were made during both ACE-1 and ACE-2. ACE-1 focused on remote marine aerosol minimally perturbed by continental sources. ACE-2 studied the outflow of European aerosol into the NE Atlantic atmosphere. A variety of air masses were sampled during ACE-2 including Atlantic, polar, Iberian Peninsula, Mediterranean, and Western European. Reported here are mass size distributions of non-sea salt (nss) sulfate, sea salt, and methanesulfonate and submicron and supermicron concentrations of black and organic carbon. Optical parameters include submicron and supermicron aerosol scattering and backscattering coefficients at 550 nm, the absorption coefficient at 550±20 nm, the Ångström exponent for the 550 and 700 nm wavelength pair, and single scattering albedo at 550 nm. All data are reported at the measurement relative humidity of 55%. Measured concentrations of nss sulfate aerosol indicate that, relative to ACE-1, ACE-2 aerosol during both marine and continental flow was impacted by continental sources. Thus, while sea salt controlled the aerosol chemical composition and optical properties of both the submicron and supermicron aerosol during ACE-1, it played a relatively smaller role in ACE-2. This is confirmed by the larger average Ångström exponent for ACE-2 continental aerosol of 1.2±0.26 compared to the ACE-1 average of -0.03±0.38. The depletion of chloride from sea salt aerosol in ACE-2 continental air masses averaged 55±25% over all particle sizes. This compares to the ACE-2 marine average of 4.8±18% and indicates the enhanced interaction of anthropogenic acids with sea salt as continental air masses are transported into the marine atmosphere. Single scattering albedos averaged 0.95±0.03 for ACE-2 continental air masses. Averages for ACE-2 and ACE-1 marine air masses were 0.98±0.01 and 0.99±0.01, respectively.

Regional marine boundary layer aerosol size distributions in the Indian, Atlantic, and Pacific Oceans: A comparison of INDOEX measurements with ACE‐1, ACE‐2, and Aerosols99

[1] Aerosol number size distributions were measured aboard the R/V Ronald H. Brown during the Indian Ocean Experiment (INDOEX) 1999 Intensive Field Phase (IFP) using a differential mobility particle sizer (DMPS) and an aerodynamic particle sizer (APS), covering a size range from 0.02 to 7 mm geometric diameter at 55% relative humidity (RH). The Indian Ocean marine boundary layer (MBL) aerosol number size distributions measured during the 1999 IFP were categorized into eight air mass source regions based on air mass back trajectories. The number and volume size distributions in these eight regions were distinctly different as a result of the different aerosol sources, meteorological conditions during transport, and time spent in the MBL. The aerosol sampling and data reduction during INDOEX were similar to that used during the Aerosol Characterization Experiment (ACE)-1 (Mid-Pacific Ocean 37� Nt o 32� S and Southern Ocean south of Tasmania, Australia), ACE-2 (North Atlantic Ocean west of Portugal and North Africa), and Aerosols99 (Atlantic Ocean transit from Norfolk, USA to Cape Town, South Africa) thus facilitating comparisons of the number and volume size distributions from these different experiments. The combined data set, summarized in this paper, provides regional aerosol parameters for comparison with global climate models and satellite retrieval algorithms. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: aerosol, INDOEX, size distributions, ACE