Edward R. Lovejoy

Aerosol Absorption Measurement using Photoacoustic Spectroscopy: Sensitivity, Calibration, and Uncertainty Developments

Light absorption by aerosols is one of the most uncertain parameters associated with the direct and indirect aerosol effects on climate and is one of the most difficult quantities to measure. This article describes the development of a sensitive method of measuring aerosol absorption at 532 nm with excellent time response (detection limit: 0.08 Mm−1, 60 second average) using photoacoustic absorption spectroscopy. An accurate calibration method (accuracy of 1–2%) at atmospherically relevant absorption levels and independent validation of the photoacoustic technique is presented. An upper limit to the instrument precision for aerosol absorption measurement is ∼6% (2σ, 30 sec) while instrument accuracy is calculated to be ∼5%. A standard for aerosol absorption measurement techniques using well characterized absorbing aerosol is also proposed.

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.

Design and Application of a Pulsed Cavity Ring-Down Aerosol Extinction Spectrometer for Field Measurements

This paper describes the design and application of a pulsed cavity ring-down aerosol extinction spectrometer (CRD-AES) for in-situ atmospheric measurement of the aerosol extinction coefficient and its relative humidity dependence. This CRD-AES measures the aerosol extinction coefficient (σ ep) at 355 nm, 532 nm, 683 nm, and 1064 nm with a minimal size dependent bias for particles with diameter less than 10 μm. The σ ep at 532 nm is measured with an accuracy of 1% when extinction is ≥ 10 Mm −1 . The precision is limited by statistical fluctuations within the small optical volume and the time resolution of extinction at 2% uncertainty for various air mass types is evaluated. The CRD-AES is configured with two separate cavity ring-down cells for measurement of the extinction coefficient at 532 nm. This allows the determination of the RH dependence of extinction at 532 nm through independent RH control of the sample for each measurement. Gas phase absorption and minimization of potential interferences is also considered.

Light absorbing carbon emissions from commercial shipping

Extensive measurements of the emission of light absorbing carbon aerosol (LAC) from commercial shipping are presented. Vessel emissions were sampled using a photoacoustic spectrometer in the Gulf of Mexico region. The highest emitters (per unit fuel burnt) are tug boats, thus making significant contributions to local air quality in ports. Emission of LAC from cargo and non cargo vessels in this study appears to be independent of engine load. Shipping fuel consumption data (2001) was used to calculate a global LAC contribution of 133(±27) Ggyr−1, or ∼1.7% of global LAC. This small fraction could have disproportionate effects on both air quality near port areas and climate in the Arctic if direct emissions of LAC occur in that region due to opening Arctic sea routes. The global contribution of this LAC burden was investigated using the MOZART model. Increases of 20–50 ng m−3 LAC (relative increases up to 40%) due to shipping occur in the tropical Atlantic, Indonesia, central America and the southern regions of South America and Africa.

Parameterization for the relative humidity dependence of light extinction: Organic‐ammonium sulfate aerosol

[1]xa0Atmospheric particles are a complex mixture of inorganic and organic compounds. This study uses laboratory generated particles to examine the connection between aerosol light extinction, chemical composition, and hygroscopicity for particles composed of internal mixtures of ammonium sulfate and water-soluble organic compounds. The extinction coefficient (σep) at 532 nm was measured for size-selected particles at <10% RH and 80% RH. The ratio of the extinction coefficients at 80% RH to <10% RH is reported as fRHext(80% RH, dry). The fRHext(80% RH, dry) values were similar for particles composed of various water-soluble organic compounds and different functional groups. In addition, fRHext(80% RH, dry) values were relatively insensitive to the composition of the organic fraction for internal mixtures of ammonium sulfate with sugars, dicarboxylic acids and complex mixtures of water-soluble organic compounds. Finally, fRHext(80% RH, dry) was found to vary linearly with the organic/inorganic content, allowing for simple incorporation of organic properties into atmospheric models. We derived a generalization of fRHext(80% RH, dry) = 2.90 – 0.015(wt% organic species) for a particle size distribution with a dry mean optical diameter of 0.35 μm. This parameterization for ammonium sulfate/water-soluble organic aerosol is applicable to the fine particle mode fraction of atmospheric aerosol. Information necessary to incorporate the variation in the size distribution is also included. This work suggests that neglecting the water uptake by the organic fraction of atmospheric particles could lead to significant underestimation of the cooling at the Earths surface due to light scattering by aerosol.

Key factors influencing the relative humidity dependence of aerosol light scattering

[1]xa0We have measured the relative humidity dependence of aerosol light extinction (fσ(ep)(80%RH, Dry)) at 532 nm for non-absorbing surrogate atmospheric aerosols to determine the influence of particle size, composition (inorganic vs. organic), and mixing state (internal vs. external) on aerosol light scattering. We present results for mixtures of NaCl and (NH4)2SO4 with a few dicarboxylic acids. For atmospheric conditions the variability in the RH dependence of aerosol light scattering (fσ(sp)(RH, Dry)) is most sensitive to aerosol composition and size. The influence of the mixing state on fσ(sp)(RH, Dry) is small. These laboratory results imply that fσ(sp)(RH, Dry) can be reasonably estimated from the aerosol size distribution and composition (inorganic/organic) using the mass-weighted average of fσ(sp)(RH, Dry) for the individual components.

Uncertainty in Light Scattering Measurements by TSI Nephelometer: Results from Laboratory Studies and Implications for Ambient Measurements

ABSTRACT We performed a series of laboratory studies to assess uncertainties in aerosol optical property measurements. We used a pulsed cavity ring-down spectrometer (CRD) to measure the aerosol light extinction coefficient, a photo-acoustic absorption spectrometer (PAS) for light absorption, and a commercial integrating nephelometer (TSI, model 3563) for the scattering coefficient. The test aerosols generated for the study consisted of both non-absorbing and slightly-to-highly absorbing polydisperse submicron particles, delivered to the systems as pure compounds or mixtures. For aerosols with single scattering albedo (ω) > 0.7, we found that the ω values calculated by combining the three instruments agreed within expected uncertainty. For aerosols with ω < 0.7, the discrepancy was outside of the boundaries of existing uncertainties. Mie theory calculations of the nephelometer correction factors indicate that the current literature expressions for nephelometer truncation correction are not adequate at ω < 0.7 due to the large real part of the refractive index (n) of absorbing particles. New corrections are given for the case when n is known. Based on our instrumental capabilities we conclude that the lowest uncertainty in ω is obtained by combining CRD and PAS data. The implications for the uncertainty of aerosol optical properties when using nephelometer-based scattering coefficient in the case of, e.g., highly absorbing aerosols, coarse mode particles, and measurements at high relative humidity are discussed.