Justin M. Langridge

Brown carbon and internal mixing in biomass burning particles

Biomass burning (BB) contributes large amounts of black carbon (BC) and particulate organic matter (POM) to the atmosphere and contributes significantly to the earth’s radiation balance. BB particles can be a complicated optical system, with scattering and absorption contributions from BC, internal mixtures of BC and POM, and wavelength-dependent absorption of POM. Large amounts of POM can also be externally mixed. We report on the unique ability of multi-wavelength photo-acoustic measurements of dry and thermal-denuded absorption to deconstruct this complicated wavelength-dependent system of absorption and mixing. Optical measurements of BB particles from the Four Mile Canyon fire near Boulder, Colorado, showed that internal mixtures of BC and POM enhanced absorption by up to 70%. The data supports the assumption that the POM was very weakly absorbing at 532 nm. Enhanced absorption at 404 nm was in excess of 200% above BC absorption and varied as POM mass changed, indicative of absorbing POM. Absorption by internal mixing of BC and POM contributed 19( ± 8)% to total 404-nm absorption, while BC alone contributed 54( ± 16)%. Approximately 83% of POM mass was externally mixed, the absorption of which contributed 27( ± 15)% to total particle absorption (at 404 nm). The imaginary refractive index and mass absorption efficiency (MAE) of POM at 404 nm changed throughout the sampling period and were found to be 0.007 ± 0.005 and 0.82 ± 0.43 m2 g-1, respectively. Our analysis shows that the MAE of POM can be biased high by up to 50% if absorption from internal mixing of POM and BC is not included.

Aircraft Instrument for Comprehensive Characterization of Aerosol Optical Properties, Part 2: Black and Brown Carbon Absorption and Absorption Enhancement Measured with Photo Acoustic Spectroscopy

This paper describes the design and performance of a photo-acoustic aerosol absorption spectrometer (PAS) built for operation on a research aircraft platform. The PAS instrument is capable of measuring dry absorption at 659 nm, 532 nm, and 404 nm, and absorption enhancement due to coatings at 532 nm and 404 nm. The measurement accuracy for all channels is < = 10% and in flight 1 Hz sensitivities lie within the range of 0.5–1.5 Mm−1. PAS measurements of calibrated absorbing aerosol samples are shown to be consistent with measurements made by a previous generation single channel photo-acoustic instrument. Aircraft data collected during a recent field campaign in California are used to demonstrate the capabilities of the PAS. In combination with an aircraft cavity ring down aerosol extinction spectrometer described in a companion paper, the new PAS instrument provides a sensitive airborne in-situ characterization of aerosol optics. Copyright 2012 American Association for Aerosol Research

Aircraft Instrument for Comprehensive Characterization of Aerosol Optical Properties, Part I: Wavelength-Dependent Optical Extinction and Its Relative Humidity Dependence Measured Using Cavity Ringdown Spectroscopy

High-quality in situ observations of aerosol particle optical properties, namely extinction, scattering, and absorption, provide important information needed to constrain the role of aerosols in the climate system. This paper outlines the design and performance of an aircraft instrument utilizing cavity ringdown spectroscopy for the measurement of aerosol extinction. The 8-channel cavity ringdown spectrometer measures extinction at multiple wavelengths (405, 532, and 662 nm) and at multiple relative humidities (e.g., 10%, 70%, and 95%). Key performance characteristics include a 1-s detection limit better than 0.1 Mm−1, accuracy of <2% for dry aerosol measurements, and a 1-s precision better than 40% for extinction levels of >10 Mm−1. Laboratory and field data demonstrate that the 1-s precision is limited by the statistics of aerosol particles in the laser beam rather than the precision of the extinction measurement per se. The measurement precision improves with averaging to 5% at 60 s for extinction levels of >10 Mm−1. Field data collected during a recent airborne campaign in California, which involved eighteen research flights during May and June 2010, are used to demonstrate the in-flight performance of new instrument.

Impact of Fuel Quality Regulation and Speed Reductions on Shipping Emissions: Implications for Climate and Air Quality

Atmospheric emissions of gas and particulate matter from a large ocean-going container vessel were sampled as it slowed and switched from high-sulfur to low-sulfur fuel as it transited into regulated coastal waters of California. Reduction in emission factors (EFs) of sulfur dioxide (SO₂), particulate matter, particulate sulfate and cloud condensation nuclei were substantial (≥ 90%). EFs for particulate organic matter decreased by 70%. Black carbon (BC) EFs were reduced by 41%. When the measured emission reductions, brought about by compliance with the California fuel quality regulation and participation in the vessel speed reduction (VSR) program, are placed in a broader context, warming from reductions in the indirect effect of SO₄ would dominate any radiative changes due to the emissions changes. Within regulated waters absolute emission reductions exceed 88% for almost all measured gas and particle phase species. The analysis presented provides direct estimations of the emissions reductions that can be realized by California fuel quality regulation and VSR program, in addition to providing new information relevant to potential health and climate impact of reduced fuel sulfur content, fuel quality and vessel speed reductions.

Air quality implications of the Deepwater Horizon oil spill

During the Deepwater Horizon (DWH) oil spill, a wide range of gas and aerosol species were measured from an aircraft around, downwind, and away from the DWH site. Additional hydrocarbon measurements were made from ships in the vicinity. Aerosol particles of respirable sizes were on occasions a significant air quality issue for populated areas along the Gulf Coast. Yields of organic aerosol particles and emission factors for other atmospheric pollutants were derived for the sources from the spill, recovery, and cleanup efforts. Evaporation and subsequent secondary chemistry produced organic particulate matter with a mass yield of 8 ± 4% of the oil mixture reaching the water surface. Approximately 4% by mass of oil burned on the surface was emitted as soot particles. These yields can be used to estimate the effects on air quality for similar events as well as for this spill at other times without these data. Whereas emission of soot from burning surface oil was large during the episodic burns, the mass flux of secondary organic aerosol to the atmosphere was substantially larger overall. We use a regional air quality model to show that some observed enhancements in organic aerosol concentration along the Gulf Coast were likely due to the DWH spill. In the presence of evaporating hydrocarbons from the oil, NOx emissions from the recovery and cleanup operations produced ozone.

CCN Spectra, Hygroscopicity, and Droplet Activation Kinetics of Secondary Organic Aerosol Resulting from the 2010 Deepwater Horizon Oil Spill

Secondary organic aerosol (SOA) resulting from the oxidation of organic species emitted by the Deepwater Horizon oil spill were sampled during two survey flights conducted by a National Oceanic and Atmospheric Administration WP-3D aircraft in June 2010. A new technique for fast measurements of cloud condensation nuclei (CCN) supersaturation spectra called Scanning Flow CCN Analysis was deployed for the first time on an airborne platform. Retrieved CCN spectra show that most particles act as CCN above (0.3 ± 0.05)% supersaturation, which increased to (0.4 ± 0.1)% supersaturation for the most organic-rich aerosol sampled. The aerosol hygroscopicity parameter, κ, was inferred from both measurements of CCN activity and from humidified-particle light extinction, and varied from 0.05 to 0.10 within the emissions plumes. However, κ values were lower than expected from chemical composition measurements, indicating a degree of external mixing or size-dependent chemistry, which was reconciled assuming bimodal, size-dependent composition. The CCN droplet effective water uptake coefficient, γ(cond), was inferred from the data using a comprehensive instrument model, and no significant delay in droplet activation kinetics from the presence of organics was observed, despite a large fraction of hydrocarbon-like SOA present in the aerosol.

Limitations of the Photoacoustic Technique for Aerosol Absorption Measurement at High Relative Humidity

Laboratory experiments were conducted to assess the suitability of photoacoustic spectroscopy (PAS) for aerosol absorption measurement at high relative humidity (RH). Initial experiments characterized the PAS sensitivity that increased strongly by ∼1.25 between dry conditions and 90% RH. Correction procedures were validated by measuring RH-independent absorption for hydrophobic absorbing particles. Absorption measured by PAS for a range of hygroscopic particles, including different morphologies, hygroscopicities, and absorbing entities, showed strong low biases at high RH (down to 0.4 at 90% RH). The bias was due to water evaporation impacting the PAS signal. Cooling samples to lower absolute humidity while maintaining a constant RH did not significantly reduce the bias magnitude within the temperature range 11–25°C. The magnitude and RH dependence of the bias were not reproduced well using a model of PAS response incorporating coupled heat and mass transfer in the transition regime. This was attributed, in part, to uncertainty related to changes in the water mass accommodation coefficient with RH. Given our inability to correct for evaporation-induced bias effects accurately, or reduce their magnitude experimentally, we conclude that PAS is not a technique well suited to the measurement of absorption at high RH. In order to minimize RH-related errors in PAS measurements made under notionally “dry” conditions, we recommend operation in the RH range 10–30%. Copyright 2013 American Association for Aerosol Research