[1] We characterized the gas-and speciated aerosol-phase emissions from the open combustion of 33 different plant species during a series of 255 controlled laboratory burns during the Fire Laboratory at Missoula Experiments (FLAME). The plant species we tested were chosen to improve the existing database for U.S. domestic fuels: laboratory-based emission factors have not previously been reported for many commonly burned species that are frequently consumed by fires near populated regions and protected scenic areas. The plants we tested included the chaparral species chamise, manzanita, and ceanothus, and species common to the southeastern United States (common reed, hickory, kudzu, needlegrass rush, rhododendron, cord grass, sawgrass, titi, and wax myrtle). Fire-integrated emission factors for gas-phase CO 2 , CO, CH 4 , C 2 -4 hydrocarbons, NH 3 , SO 2 , NO, NO 2 , HNO 3 , and particle-phase organic carbon (OC), elemental carbon (EC), SO 4 2À , NO 3 À , Cl À , Na + , K + , and NH 4 + generally varied with both fuel type and with the fire-integrated modified combustion efficiency (MCE), a measure of the relative importance of flaming-and smoldering-phase combustion to the total emissions during the burn. Chaparral fuels tended to emit less particulate OC per unit mass of dry fuel than did other fuel types, whereas southeastern species had some of the largest observed emission factors for total fine particulate matter. Our measurements spanned a larger range of MCE than prior studies, and thus help to improve estimates of the variation of emissions with combustion conditions for individual fuels.

[1] The Ron Brown cruise during ACE-Asia (March-April 2001) encountered complex aerosol that at times was dominated by marine, polluted, volcanic, and dust aerosols. Average total light scattering coefficients (s sp for D p < 10 mm, relative humidity (RH) = 19%, and l = 550 nm) ranged from 23 (marine) to 181 Mm À1 (dust). Aerosol hygroscopicity ranged from deliquescent with hysteresis (marine frequently and polluted variably) to hygroscopic without hysteresis (volcanic) to nearly hygrophobic (dustdominated). Average deliquescence and crystallization RH were 77 ± 2% and 42 ± 3%, respectively. The ambient aerosol was typically on the upper branch of the hysteresis loop for marine and polluted air masses and the lower branch for dust-dominated aerosols. Average f (RH = ambient), defined as s sp (RH = ambient)/s sp (RH = 19%), ranged from 1.25 (dust) to 2.88 (volcanic). Average h(RH $60%), defined as f (RH) upper branch / f (RH) lower branch , were 1.6, 1.3, 1, and 1.25 for marine, polluted, volcanic, and dust, demonstrating an importance of hysteresis to optical properties. Hemispheric backscatter fraction (b) at ambient RH ranged from 0.077 (marine) to 0.111 (dust), while single scattering albedo (w) at ambient RH ranged from 0.94 (dust and polluted) to 0.99 (marine).

[1] We examine the hygroscopic properties of particles freshly emitted from laboratory biomass burning experiments conducted during the second Fire Lab At Missoula Experiment (FLAME-II). Values of the hygroscopicity parameter, kappa, were derived from both hygroscopic growth measurements and size-resolved (30-300 nm in diameter) cloud condensation nuclei (CCN) measurements for smokes emitted by the open combustion of 24 biomass fuels from the United States and Asia. To analyze the complex cloud condensation nuclei response curves we propose a new inversion scheme that corrects for multiple charge effects without the necessity of prior assumptions about the chemical composition and mixing state of the particles. Kappa varied between 0.02 (weakly hygroscopic) and 0.8 (highly hygroscopic). For individual smokes, kappa was a function of particle size, with 250 nm particles being generally weakly hygroscopic and sub-100 nm particles being more hygroscopic. At any given size the emissions were often externally mixed, showing more and less hygroscopic growth modes and bimodal CCN activation spectra. Comparisons between growth factor-derived and CCN-derived hygroscopicities were consistent when taking this heterogeneity into account. A conceptual model of biomass burning emissions suggests that most particles are CCN active at the point of emission and do not require conversion in the atmosphere to more hygroscopic compositions before they can participate in cloud formation and undergo wet deposition.

Hygroscopicity and cloud condensation nucleus (CCN) activity were measured for three mineral dust samples: one from the Canary Islands, representing North African dust transported across the Atlantic; one from outside Cairo, representing North African dust transported to the eastern Mediterranean; and Arizona Test Dust, representing dust in the southwestern United States. To reaerosolize bulk samples, dust samples were either suspended in high purity water and particles generated by atomization, or samples were resuspended in dry air using a fluidized bed. Only the Canary Island sample generated from aqueous suspension showed appreciable hygroscopic growth at subsaturated conditions; all other samples exhibited diameter growth factors of less than 1.1 for relative humidities ≤90%. Despite their low hygroscopicities at subsaturated conditions, all samples activated as cloud droplets at supersaturations lower than required for insoluble particles. We suggest that the CCN activity of these mineral dusts are well‐represented using the hygroscopicity parameter 0.01 ≤ κ ≤ 0.08.

Abstract. Secondary Organic Aerosols (SOA) studied in previous laboratory experiments generally showed only slight hygroscopic growth, but a much better activity as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud Interaction Simulator), using a portable generator that produced SOA particles from the ozonolysis of α-pinene, and adding butanol or butanol and water vapor during some of the experiments. The light scattering signal of dry SOA-particles was measured by the LACIS optical particle spectrometer and was used to derive a refractive index for SOA of 1.45. LACIS also measured the hygroscopic growth of SOA particles up to 99.6% relative humidity (RH), and a CCN counter was used to measure the particle activation. SOA-particles were CCN active with critical diameters of e.g. 100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively. But only slight hygroscopic growth with hygroscopic growth factors ≤1.05 was observed at RH<98% RH. At RH>98%, the hygroscopic growth increased stronger than would be expected if a constant hygroscopicity parameter for the particle/droplet solution was assumed. An increase of the hygroscopicity parameter by a factor of 4-6 was observed in the RH-range from below 90% to 99.6%, and this increase continued for increasingly diluted particle solutions for activating particles. This explains an observation already made in the past: that the relation between critical super-saturation and dry diameter for activaCorrespondence to: H. Wex (wex@tropos.de) tion is steeper than what would be expected for a constant value of the hygroscopicity. Combining measurements of hygroscopic growth and activation, it was found that the surface tension that has to be assumed to interpret the measurements consistently is greater than 55 mN/m, possibly close to that of pure water, depending on the different SOA-types produced, and therefore only in part accounts for the discrepancy between hygroscopic growth and CCN activity observed for SOA particles in the past.

Biomass burning is a significant source of carbonaceous aerosol in many regions of the world. When present, biomass burning particles may affect the microphysical properties of clouds through their ability to function as cloud condensation nuclei or ice nuclei. We report on measurements of the ice nucleation ability of biomass burning particles performed on laboratory‐generated aerosols at the second Fire Lab at Missoula Experiment. During the experiment we generated smoke through controlled burns of 21 biomass fuels from the United States and Asia. Using a Colorado State University continuous flow diffusion chamber, we measured the condensation/immersion freezing potential at temperatures relevant to cold cumulus clouds (−30°C). Smokes from 9 of the 21 fuels acted as ice nuclei at fractions of 1:10,000 to 1:100 particles in at least one burn of each fuel; emissions from the remaining fuels were below the ice nuclei detection limit for all burns of each fuel. Using a bottom‐up emission model, we estimate that smokes that emit ice nuclei fractions exceeding 1:10,000 particles can perturb ice nuclei concentrations on a regional scale.

Cloud condensation nuclei (CCN) activity and ice nucleation behavior (for temperatures

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