D. M. Murphy

Measurements of the concentration and composition of nuclei for cirrus formation

This article addresses the need for new data on indirect effects of natural and anthropogenic aerosol particles on atmospheric ice clouds. Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported. Measurements support that cirrus formation occurs both by heterogeneous nucleation by insoluble particles and homogeneous (spontaneous) freezing of particles containing solutions. Heterogeneous ice nuclei concentrations in the cirrus regime depend on temperature, relative humidity, and the concentrations and physical and chemical properties of aerosol particles. The cirrus-active concentrations of heterogeneous nuclei measured in November over the western U.S. were <0.03 cm–3. Considering previous modeling studies, this result suggests a predominant potential impact of these nuclei on cirrus formed by slow, large-scale lifting or small cooling rates, including subvisual cirrus. The most common heterogeneous ice nuclei were identified as relatively pure mineral dusts and metallic particles, some of which may have origin through anthropogenic processes. Homogeneous freezing of large numbers of particles was detected above a critical relative humidity along with a simultaneous transition in nuclei composition toward that of the sulfate-dominated total aerosol population. The temperature and humidity conditions of the homogeneous nucleation transition were reasonably consistent with expectations based on previous theoretical and laboratory studies but were highly variable. The strong presence of certain organic pollutants was particularly noted to be associated with impedance of homogeneous freezing.

Observations of organic material in individual marine particles at Cape Grim during the First Aerosol Characterization Experiment (ACE 1)

During the First Aerosol Characterization Experiment (ACE 1) field campaign in November and December 1995, the particle analysis by laser mass spectrometry instrument was used to measure the composition of ambient particles in situ at Cape Grim, Tasmania, under various conditions ranging from clean marine air to moderately polluted air. Internal mixtures of sea-salt compounds and organic species were detected in over half of the negative spectra during clean marine conditions and in about 62% of the negative spectra during polluted conditions. In clean marine air masses, aerosol organics appeared to have two distinct source mechanisms depending on the extent of aerosol aging. Organic peaks had a positive trend with sodium sulfate peaks, indicating that organics and excess sulfate may accumulate in aged marine aerosol particles by similar mechanisms. When the sodium sulfate content was low, iodine had a positive trend with organics, which is consistent with organics and iodine originating from the surface-active layer of the ocean and becoming incorporated into fresh marine particles by bursting bubbles. Based on limited laboratory calibrations, the average organic mass is estimated to be of the order of 10% of the sea-salt content and is consistent with some of the missing mass fraction for Cape Grim particles (S. Howell et al., manuscript in preparation, 1997), which is the measured difference between gravimetric and ionic mass.

Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere

Reactive nitrogen (NOy) and O3 were measured simultaneously from a NASA ER-2 aircraft during 1987 through 1989. These high resolution measurements cover a broad range of latitudes in the upper troposphere and lower stratosphere. The data show a striking positive correlation between NOy and O3 in the lower stratosphere at all scales sampled. The ratio NOy/O3 is nearly independent of altitude from the tropopause to above 20 km, with ratios in the stratosphere of 0.0015–0.002 in the tropics and 0.0025–0.004 elsewhere. The ratio is much more constant than either individual species, thus providing an excellent reference point for comparing limited data sets with models. Two-dimensional models reproduce general features of the vertical profile of NOy/O3 but not the gradient in the lower stratosphere between tropics and mid-latitudes. NOy and O3 are better correlated in the lower stratosphere than in the upper troposphere. The magnitude and variability of both NOy mixing ratios and NOy/O3 ratios indicate a source of NOy in the upper troposphere. The most plausible source in the tropics is lightning production of NOx. Condensation of NOy onto aerosol particles is often possible in the tropical upper troposphere and may play a role in determining the vertical distribution of NOy. In the mid-latitude upper troposphere the data suggest long-range transport of NOy. NOy mixing ratios in the tropical upper troposphere were usually between 150 and 600 pptv, enough so that upward transport can affect the NOy abundance in the tropical lower stratosphere.

Laser Ionization Mass Spectroscopy of Single Aerosol Particles

We describe an instrument that measures the chemical composition of single aerosol particles. To facilitate detection of volatile species, particles are analyzed less than 0.5 ms after leaving ambient conditions and without touching any surfaces. Particles are introduced into a vacuum through a differentially pumped nozzle, then cross a He-Ne laser beam. The scattered light provides both size information and the trigger for an excimer laser that desorbs and ionizes molecules from the particle. Mass spectra with excellent signal to noise have been obtained from laboratory and ambient particles 0.3 to 16 μm diameter.

Organic Aerosol Formation Downwind from the Deepwater Horizon Oil Spill

Organic compounds of intermediate volatility play an important role in the formation of secondary organic aerosols. A large fraction of atmospheric aerosols are derived from organic compounds with various volatilities. A National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft made airborne measurements of the gaseous and aerosol composition of air over the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico that occurred from April to August 2010. A narrow plume of hydrocarbons was observed downwind of DWH that is attributed to the evaporation of fresh oil on the sea surface. A much wider plume with high concentrations of organic aerosol (>25 micrograms per cubic meter) was attributed to the formation of secondary organic aerosol (SOA) from unmeasured, less volatile hydrocarbons that were emitted from a wider area around DWH. These observations provide direct and compelling evidence for the importance of formation of SOA from less volatile hydrocarbons.

Polar stratospheric cloud processed air and potential voracity in the northern hemisphere lower stratosphere at mid‐latitudes during winter

Small-scale (<1000 km) features in ER-2 measurements of ClO, O3, H2O, N2O, and NOy, outside the lower stratospheric Arctic vortex of 1988–1989 are compared with features on potential vorticity maps from the European Centre for Medium-range Weather Forecasts (ECMWF). The potential vorticity maps are obtained from Tl06 analyses and forecasts. Some of the plots have been truncated to lower resolution (T63 or T42) which smooths out the finer-scale structure. Comparison of these lower resolution plots shows how much detail is lost by excessive smoothing. It is also evident that the forecast plots lose fine-scale structure due to dissipation in the model resulting mainly from horizontal diffusion. We conclude that blobs of air on the maps at latitudes between the vortex edge and 25°N having potential vorticities characteristic of the vortex, did indeed originate from the vortex, but that the real atmosphere is more sharply differentiated (inhomogeneous) than the meteorological analyses, implying that the potential vorticity maps underestimate the amount of peeled-off material. Areal budgets of the ex-vortex air are considered for ER-2 flight days, and are performed for 24-hour forecasts at T63, and analyses at T42, T63, and T106 resolution at θ = 475 K. Finally, it is concluded that the lower stratospheric Arctic vortex of 1988–1989 spread considerable amounts of air to mid-latitudes which had been processed by polar stratospheric clouds, and that this mechanism is a realistic explanation for the wintertime loss of ozone observed over northern mid-latitudes during the last decade.

Contribution of isoprene-derived organosulfates to free tropospheric aerosol mass

Recent laboratory studies have demonstrated that isoprene oxidation products can partition to atmospheric aerosols by reacting with condensed phase sulfuric acid, forming low-volatility organosulfate compounds. We have identified organosulfate compounds in free tropospheric aerosols by single particle mass spectrometry during several airborne field campaigns. One of these organosulfates is identified as the sulfate ester of IEPOX, a second generation oxidation product of isoprene. The patterns of IEPOX sulfate ester in ambient data generally followed the aerosol acidity and NOx dependence established by laboratory studies. Detection of the IEPOX sulfate ester was most sensitive using reduced ionization laser power, when it was observed in up to 80% of particles in the tropical free troposphere. Based on laboratory mass calibrations, IEPOX added > 0.4% to tropospheric aerosol mass in the remote tropics and up to 20% in regions downwind of isoprene sources. In the southeastern United States, when acidic aerosol was exposed to fresh isoprene emissions, accumulation of IEPOX increased aerosol mass by up to 3%. The IEPOX sulfate ester is therefore one of the most abundant single organic compounds measured in atmospheric aerosol. Our data show that acidity-dependent IEPOX uptake is a mechanism by which anthropogenic SO2 and marine dimethyl sulfide emissions generate secondary biogenic aerosol mass throughout the troposphere.

Deactivation of ice nuclei due to atmospherically relevant surface coatings

The ice nucleation characteristics of Arizona test dust (ATD) and illite clay, surrogates for atmospheric ice nuclei, have been determined at the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) chamber located at the Research Center Karlsruhe in Germany. The objective of this research was to determine the effect of sulfuric acid and ammonium sulfate coatings on the ability of these mineral dust surrogates to nucleate ice in an environment where particles realistically compete for water vapor. Coated ATD particles required higher saturations at all temperatures considered, from −20 to −45 ◦ C, than did identical uncoated particles. Freezing of coated particles often required saturations approaching those for the homogeneous freezing of aqueous solutions of the coating material alone. Less pronounced effects were found for illite, although the presence of a coating consistently increased the saturation or decreased the temperature required for ice formation. Analysis of ice residue at the single particle level suggests that the first coated particles to freeze had thinner or incomplete coatings when compared to particles that froze later in the expansion. This observation highlights a need to verify coating properties since an assumption of homogeneity of a group of coated aerosols may be incorrect. The increase in saturation ratio for freezing suggests that gas-phase uptake of sulfates, a large fraction of which are due to anthropogenic emissions, will reduce the ice and mixed-phase cloud formation potential of atmospheric ice nuclei.

Persistence of climate changes due to a range of greenhouse gases

Emissions of a broad range of greenhouse gases of varying lifetimes contribute to global climate change. Carbon dioxide displays exceptional persistence that renders its warming nearly irreversible for more than 1,000 y. Here we show that the warming due to non-CO2 greenhouse gases, although not irreversible, persists notably longer than the anthropogenic changes in the greenhouse gas concentrations themselves. We explore why the persistence of warming depends not just on the decay of a given greenhouse gas concentration but also on climate system behavior, particularly the timescales of heat transfer linked to the ocean. For carbon dioxide and methane, nonlinear optical absorption effects also play a smaller but significant role in prolonging the warming. In effect, dampening factors that slow temperature increase during periods of increasing concentration also slow the loss of energy from the Earth’s climate system if radiative forcing is reduced. Approaches to climate change mitigation options through reduction of greenhouse gas or aerosol emissions therefore should not be expected to decrease climate change impacts as rapidly as the gas or aerosol lifetime, even for short-lived species; such actions can have their greatest effect if undertaken soon enough to avoid transfer of heat to the deep ocean.

A comparison of particle mass spectrometers during the 1999 Atlanta Supersite Project

During the Atlanta Supersite Project, four particle mass spectrometers were operated together for the first time: NOAAs Particle Analysis by Laser Mass Spectrometer (PALMS), University of California at Riversides Aerosol Time-of-Flight Mass Spectrometer (ATOFMS), University of Delawares Rapid Single-Particle Mass Spectrometer II (RSMS-II), and Aerodynes Aerosol Mass Spectrometer (AMS). Although these mass spectrometers are generally classified as similar instruments, they clearly have different characteristics due to their unique designs. One primary difference is related to the volatilization/ionization method: PALMS, ATOFMS, and RSMS-II utilize laser desorption/ionization, whereas particles in the AMS instrument are volatilized by impaction onto a heated surface with the resulting components ionized by electron impact. Thus mass spectral data from the AMS are representative of the ensemble of particles sampled, and those from the laser-based instruments are representative of individual particles. In addition, the AMS instrument cannot analyze refractory material such as soot, sodium chloride, and crustal elements, and some sulfate or water-rich particles may not always be analyzed with every laser-based instrument. A main difference among the laser-based mass spectrometers is that the RSMS-II instrument can obtain size-resolved single particle composition information for particles with aerodynamic diameters as small as 15 nm. The minimum sizes analyzed by ATOFMS and PALMS are 0.2 and about 0.35 μm, respectively, in aerodynamic diameter. Furthermore, PALMS, ATOFMS, and RSMS-II use different laser ionization conditions. Despite these differences the laser-based instruments found similar individual particle classifications, and their relative fractions among comparable sized particles from Atlanta were broadly consistent. Finally, the AMS measurements of the nitrate/sulfate mole ratio were highly correlated with composite measurements (r^2 = 0.93). In contrast, the PALMS nitrate/sulfate ion ratios were only moderately correlated (r^2 ∼ 0.7).