Gavin R. McMeeking

Emissions of trace gases and aerosols during the open combustion of biomass in the laboratory

[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 CO2, CO, CH4 ,C 2–4 hydrocarbons, NH3 ,S O2, NO, NO2, HNO3, and particle-phase organic carbon (OC), elemental carbon (EC), SO4� ,N O3 ,C l � ,N a + ,K + , and NH4 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.

Sea spray aerosol as a unique source of ice nucleating particles

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using “dry” geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.

Characterizing elemental, equivalent black, and refractory black carbon aerosol particles: a review of techniques, their limitations and uncertainties

Elemental-, equivalent black- and refractory black-carbon are terms that have been defined in order to dissect the more general term, black carbon, into its component parts related to its specific chemical and optical properties and its impact on climate and health. Recent publications have attempted to clarify the meaning of these terms with respect to their environmental impact, particularly on climate. Here, we focus on the measurement aspects, reviewing the most commonly implemented techniques for the direct and indirect derivation of black carbon properties, their strengths, limitations, and uncertainties, and provide a non-exhaustive bibliography where the reader can find more detailed information. This review paper is designed as a guide for those wishing to learn about the current state of black carbon measurement instrumentation, how calibration is carried out, when one instrument may have the advantage over another, and where new techniques are needed to fill important knowledge gaps.

Deposition of reactive nitrogen during the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study.

Increases in reactive nitrogen deposition are a growing concern in the U.S. Rocky Mountain west. The Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study was designed to improve understanding of the species and pathways that contribute to nitrogen deposition in Rocky Mountain National Park (RMNP). During two 5-week field campaigns in spring and summer of 2006, the largest contributor to reactive nitrogen deposition in RMNP was found to be wet deposition of ammonium (34% spring and summer), followed by wet deposition of nitrate (24% spring, 28% summer). The third and fourth most important reactive nitrogen deposition pathways were found to be wet deposition of organic nitrogen (17%, 12%) and dry deposition of ammonia (14%, 16%), neither of which is routinely measured by air quality/deposition networks operating in the region. Total reactive nitrogen deposition during the spring campaign was determined to be 0.45 kg ha(-1) and more than doubled to 0.95 kg ha(-1) during the summer campaign.

Aerosol emissions from prescribed fires in the United States: A synthesis of laboratory and aircraft measurements

Aerosol emissions from prescribed fires can affect air quality on regional scales. Accurate representation of these emissions in models requires information regarding the amount and composition of the emitted species. We measured a suite of submicron particulate matter species in young plumes emitted from prescribed fires (chaparral and montane ecosystems in California; coastal plain ecosystem in South Carolina) and from open burning of over 15 individual plant species in the laboratory. We report emission ratios and emission factors for refractory black carbon (rBC) and submicron nonrefractory aerosol and compare field and laboratory measurements to assess the representativeness of our laboratory-measured emissions. Laboratory measurements of organic aerosol (OA) emission factors for some fires were an order of magnitude higher than those derived from any of our aircraft observations; these are likely due to higher-fuel moisture contents, lower modified combustion efficiencies, and less dilution compared to field studies. Nonrefractory inorganic aerosol emissions depended more strongly on fuel type and fuel composition than on combustion conditions. Laboratory and field measurements for rBC were in good agreement when differences in modified combustion efficiency were considered; however, rBC emission factors measured both from aircraft and in the laboratory during the present study using the Single Particle Soot Photometer were generally higher than values previously reported in the literature, which have been based largely on filter measurements. Although natural variability may account for some of these differences, an increase in the BC emission factors incorporated within emission inventories may be required, pending additional field measurements for a wider variety of fires.

Airborne observations of regional variation in fluorescent aerosol across the United States

Airborne observations of fluorescent aerosol were made aboard an airship during CloudLab, a series of flights that took place in September and October of 2013 and covered a wideband of longitude across the continental U.S. between Florida and California and between 28 and 37 N latitudes. Sampling occurred from near the surface to 1000 m above the ground. A Wideband Integrated Bioaerosol Sensor (WIBS-4) measured average concentrations of supermicron fluorescent particles aloft (1 µm to 10 µm), revealing number concentrations ranging from 2.1 ± 0.8 to 8.7 ± 2.2 × 104 particles m−3 and representing up to 24% of total supermicron particle number. We observed distinct variations in size distributions and fluorescent characteristics in different regions, and attribute these to geographically diverse bioaerosol. Fluorescent aerosol detected in the east is largely consistent with mold spores observed in a laboratory setting, while a shift to larger sizes associated with different fluorescent patterns is observed in the west. Fluorescent bioaerosol loadings in the desert west were as high as those near the Gulf of Mexico, suggesting that bioaerosol is a substantial component of supermicron aerosol both in humid and arid environments. The observations are compared to model fungal and bacterial loading predictions, and good agreement in both particle size and concentrations is observed in the east. In the west, the model underestimated observed concentrations by a factor between 2 and 4 and the prescribed particle sizes are smaller than the observed fluorescent aerosol. A classification scheme for use with WIBS data is also presented.

Impacts of nonrefractory material on light absorption by aerosols emitted from biomass burning

We present laboratory measurements of biomass-burning aerosol light-scattering and light absorption coefficients at 405, 532, and 781 nm and investigate their relationship with aerosol composition and fuel type. Aerosol composition measurements included nonrefractory components measured by a high-resolution aerosol mass spectrometer (AMS), composition of refractory black carbon-containing particles by a soot particle aerosol mass spectrometer (SP-AMS), and refractory black carbon measured by a single-particle soot photometer (SP2). All measurements were performed downstream of a thermal denuder system to probe the effects of nonrefractory material on observed optical properties. The fires studied emitted aerosol with a wide range of optical properties with some producing more strongly light-absorbing particles (single-scattering albedo or SSA at 781 nm = 0.4) with a weak wavelength dependence of absorption (absorption Angstrom exponent or AAE = 1-2) and others producing weakly light-absorbing particles (SSA at 781 nm similar to 1) with strong wavelength dependence of absorption (AAE similar to 7). Removal of nonrefractory material from the particles by the thermal denuder system led to substantial (20-80%) decreases in light absorption coefficients, particularly at shorter wavelengths, reflecting the removal of light-absorbing material that had enhanced black carbon absorption in internally mixed untreated samples. Observed enhancements of absorption by all mechanisms were at least factors of 1.2-1.5 at 532 nm and 781 nm as determined from the heated samples. A mass absorption cross-section-based approach indicated larger enhancements, particularly at shorter wavelengths.

Characteristics of atmospheric ice nucleating particles associated with biomass burning in the US: Prescribed burns and wildfires

An improved understanding of atmospheric ice nucleating particles (INP), including sources and atmospheric abundance, is needed to advance our understanding of aerosol-cloud-climate interactions. This study examines diverse biomass burning events to better constrain our understanding of how fires impact populations of INP. Sampling of prescribed burns and wildfires in Colorado and Georgia, U.S.A., revealed that biomass burning leads to the release of particles that are active as condensation/immersion freezing INP at temperatures from −32 to −12°C. During prescribed burning of wiregrass, up to 64% of INP collected during smoke-impacted periods were identified as soot particles via electron microscopy analyses. Other carbonaceous types and mineral-like particles dominated INP collected during wildfires of ponderosa pine forest in Colorado. Total measured nINP and the excess nINP associated with smoke-impacted periods were higher during two wildfires compared to the prescribed burns. Interferences from non-smoke sources of INP, including long-range transported mineral dust and local contributions of soils and plant materials lofted from the wildfires themselves, presented challenges in using the observations to develop a smoke-specific nINP parameterization. Nevertheless, these field observations suggest that biomass burning may serve as an important source of INP on a regional scale, particularly during time periods that lack other robust sources of INP such as long-range transported mineral dust.

High Relative Humidity as a Trigger for Widespread Release of Ice Nuclei

Copyright 2014 American Association for Aerosol Research

Organic aerosol emission ratios from the laboratory combustion of biomass fuels.

Organic aerosol (OA) emission ratios (ER) have been characterized for 67 burns during the second Fire Laboratory at Missoula Experiment. These fires involved 19 different species representing 6 major fuels, each of which forms an important contribution to the U.S. biomass burning inventory. Average normalized ΔOA/ΔCO ratios show a high degree of variability, both between and within different fuel types and species, typically exceeding differen-ces between separate plumes in ambient measurements. This variability is strongly influenced by highly contrasting ΔOA levels between burns and the increased partitioning of semivolatile organic compounds to the particle phase at high ΔOA concentrations. No correlation across all fires was observed between ΔOA/ΔCO and modified combustion efficiency (MCE), which acts as an indicator of the proportional contributions of flaming and smoldering combustion phases throughout each burn. However, a negative correlation exists with MCE for some coniferous species, most notably Douglas fir, for which there is also an influence from fuel moisture content. Changes in fire efficiency were also shown to dramatically alter emissions for fires with very similar initial conditions. Although the relationship with MCE is variable between species, there is greater consistency with the level of oxygenation in OA. The ratio of the m/z 44 fragment to total OA mass concentration (f44) as measured by aerosol mass spectrometer provides an indication of oxygenation as influenced by combustion processes at source, with ΔOA/ΔCO decreasing with increasing f44 for all fuel types. Inconsistencies in the magnitude of the effects associated with each potential influence on ΔOA/ΔCO emphasize the lack of a single dominant control on fire emissions, and a dependency on both fuel properties and combustion conditions.