Experiments were conducted to investigate light absorption of organic aerosol (OA) in fresh and photo-chemically aged biomass-burning emissions. The experiments considered residential hardwood fuel ...(oak) and fuels commonly consumed in wild-land and prescribed fires in the United States (pocosin pine and gallberry). Photo-chemical aging was performed in an environmental chamber. We constrained the effective light-absorption properties of the OA using conservative limiting assumptions, and found that both primary organic aerosol (POA) in the fresh emissions and secondary organic aerosol (SOA) produced by photo-chemical aging contain brown carbon, and absorb light to a significant extent. This work presents the first direct evidence that SOA produced in aged biomass-burning emissions is absorptive. For the investigated fuels, SOA is less absorptive than POA in the long visible, but exhibits stronger wavelength-dependence and is more absorptive in the short visible and near-UV. Light absorption by SOA in biomass-burning emissions might be an important contributor to the global radiative forcing budget.
Atmospheric black carbon (BC) is a leading climate warming agent, yet uncertainties on the global direct radiative forcing (DRF) remain large. Here we expand a global model simulation (GEOS-Chem) of ...BC to include the absorption enhancement associated with BC coating and separately treat both the aging and physical properties of fossil-fuel and biomass-burning BC. In addition we develop a global simulation of brown carbon (BrC) from both secondary (aromatic) and primary (biomass burning and biofuel) sources. The global mean lifetime of BC in this simulation (4.4 days) is substantially lower compared to the AeroCom I model means (7.3 days), and as a result, this model captures both the mass concentrations measured in near-source airborne field campaigns (ARCTAS, EUCAARI) and surface sites within 30%, and in remote regions (HIPPO) within a factor of 2. We show that the new BC optical properties together with the inclusion of BrC reduces the model bias in absorption aerosol optical depth (AAOD) at multiple wavelengths by more than 50% at AERONET sites worldwide. However our improved model still underestimates AAOD by a factor of 1.4 to 2.8 regionally, with the largest underestimates in regions influenced by fire. Using the RRTMG model integrated with GEOS-Chem we estimate that the all-sky top-of-atmosphere DRF of BC is +0.13 Wm−2 (0.08 Wm−2 from anthropogenic sources and 0.05 Wm−2 from biomass burning). If we scale our model to match AERONET AAOD observations we estimate the DRF of BC is +0.21 Wm−2, with an additional +0.11 Wm−2 of warming from BrC. Uncertainties in size, optical properties, observations, and emissions suggest an overall uncertainty in BC DRF of −80%/+140%. Our estimates are at the lower end of the 0.2–1.0 Wm−2 range from previous studies, and substantially less than the +0.6 Wm−2 DRF estimated in the IPCC 5th Assessment Report. We suggest that the DRF of BC has previously been overestimated due to the overestimation of the BC lifetime (including the effect on the vertical profile) and the incorrect attribution of BrC absorption to BC.
The wet removal of black carbon aerosol (BC) in the atmosphere is a crucial factor in determining its atmospheric lifetime and thereby the vertical and horizontal distributions, dispersion on local ...and regional scales, and the direct, semi-direct and indirect radiative forcing effects. The in-cloud scavenging and wet deposition rate of freshly emitted hydrophobic BC will be increased on acquisition of more-hydrophilic components by coagulation or coating processes. The lifetime of BC is still subject to considerable uncertainty for most of the model inputs, which is largely due to the insufficient constraints on the BC hydrophobic-to-hydrophilic conversion process from observational field data. This study was conducted at a site along UK North Norfolk coastline, where the BC particles were transported from different regions within Western Europe. A hygroscopicity tandem differential mobility analyser (HTDMA) was coupled with a single particle soot photometer (SP2) to measure the hygroscopic properties of BC particles and associated mixing state in real time. In addition, a Soot Particle AMS (SP-AMS) measured the chemical compositions of additional material associated with BC particles. The ensemble of BC particles persistently contained a less-hygroscopic mode at a growth factor (gf) of around 1.05 at 90% RH (dry diameter 163 nm). Importantly, a more-hygroscopic mode of BC particles was observed throughout the experiment, the gf of these BC particles extended up to ~1.4–1.6 with the minimum between this and the less hygroscopic mode at a gf ~1.25, or equivalent effective hygroscopicity parameter κ ~0.1. The gf of BC particles (gfBC) was highly influenced by the composition of associated soluble material: increases of gfBC were associated with secondary inorganic components, and these increases were more pronounced when ammonium nitrate was in the BC particles; however the presence of secondary organic matter suppressed the gfBC below that of pure inorganics. The Zdanovskii-Stokes-Robinson (ZSR) mixing rule captures the hygroscopicity contributions from different compositions within ±30% compared to the measured results, however is subject to uncertainty due to the complex morphology of BC component and potential artefacts associated with semivolatile particles measured with the HTDMA. This study provides detailed insights on BC hygroscopicity associated with its mixing state, and the results will importantly constrain the microphysical mixing schemes of BC as used by a variety of high level models. In particular, this provides direct evidence to highlight the need to consider ammonium nitrate ageing of BC particles because this will result in particles becoming hydrophilic on much shorter timescales than for sulphate formation, which is often the only mechanism considered.
Biomass burning (BB) is a major global source of trace gases and particles. Accurately representing the production and evolution of these emissions is an important goal for atmospheric chemical ...transport models. We measured a suite of gases and aerosols emitted from an 81 hectare prescribed fire in chaparral fuels on the central coast of California, US on 17 November 2009. We also measured physical and chemical changes that occurred in the isolated downwind plume in the first ~4 h after emission. The measurements were carried out onboard a Twin Otter aircraft outfitted with an airborne Fourier transform infrared spectrometer (AFTIR), aerosol mass spectrometer (AMS), single particle soot photometer (SP2), nephelometer, LiCor CO2 analyzer, a chemiluminescence ozone instrument, and a wing-mounted meteorological probe. Our measurements included: CO2; CO; NOx; NH3; non-methane organic compounds; organic aerosol (OA); inorganic aerosol (nitrate, ammonium, sulfate, and chloride); aerosol light scattering; refractory black carbon (rBC); and ambient temperature, relative humidity, barometric pressure, and three-dimensional wind velocity. The molar ratio of excess O3 to excess CO in the plume (ΔO3/ΔCO) increased from −5.13 (±1.13) × 10−3 to 10.2 (±2.16) × 10−2 in ~4.5 h following smoke emission. Excess acetic and formic acid (normalized to excess CO) increased by factors of 1.73 ± 0.43 and 7.34 ± 3.03 (respectively) over the same time since emission. Based on the rapid decay of C2H4 we infer an in-plume average OH concentration of 5.27 (±0.97) × 106 molec cm−3, consistent with previous studies showing elevated OH concentrations in biomass burning plumes. Ammonium, nitrate, and sulfate all increased over the course of 4 h. The observed ammonium increase was a factor of 3.90 ± 2.93 in about 4 h, but accounted for just ~36% of the gaseous ammonia lost on a molar basis. Some of the gas phase NH3 loss may have been due to condensation on, or formation of, particles below the AMS detection range. NOx was converted to PAN and particle nitrate with PAN production being about two times greater than production of observable nitrate in the first ~4 h following emission. The excess aerosol light scattering in the plume (normalized to excess CO2) increased by a factor of 2.50 ± 0.74 over 4 h. The increase in light scattering was similar to that observed in an earlier study of a biomass burning plume in Mexico where significant secondary formation of OA closely tracked the increase in scattering. In the California plume, however, ΔOA/ΔCO2 decreased sharply for the first hour and then increased slowly with a net decrease of ~20% over 4 h. The fraction of thickly coated rBC particles increased up to ~85% over the 4 h aging period. Decreasing OA accompanied by increased scattering/particle coating in initial aging may be due to a combination of particle coagulation and evaporation processes. Recondensation of species initially evaporated from the particles may have contributed to the subsequent slow rise in OA. We compare our results to observations from other plume aging studies and suggest that differences in environmental factors such as smoke concentration, oxidant concentration, actinic flux, and RH contribute significantly to the variation in plume evolution observations.
Black carbon aerosols (BC) at a London urban site were characterised in both winter- and summertime 2012 during the Clean Air for London (ClearfLo) project. Positive matrix factorisation (PMF) ...factors of organic aerosol mass spectra measured by a high-resolution aerosol mass spectrometer (HR-AMS) showed traffic-dominant sources in summer but in winter the influence of additional non-traffic sources became more important, mainly from solid fuel sources (SF). Measurements using a single particle soot photometer (SP2, DMT), showed the traffic-dominant BC exhibited an almost uniform BC core size (Dc) distribution with very thin coating thickness throughout the detectable range of Dc. However, the size distribution of Dc (project average mass median Dc = 149 ± 22 nm in winter, and 120 ± 6 nm in summer) and BC coating thickness varied significantly in winter. A novel methodology was developed to attribute the BC number concentrations and mass abundances from traffic (BCtr) and from SF (BCsf), by using a 2-D histogram of the particle optical properties as a function of BC core size, as measured by the SP2. The BCtr and BCsf showed distinctly different Dc distributions and coating thicknesses, with BCsf displaying larger Dc and larger coating thickness compared to BCtr. BC particles from different sources were also apportioned by applying a multiple linear regression between the total BC mass and each AMS-PMF factor (BC–AMS–PMF method), and also attributed by applying the absorption spectral dependence of carbonaceous aerosols to 7-wavelength Aethalometer measurements (Aethalometer method). Air masses that originated from westerly (W), southeasterly (SE), and easterly (E) sectors showed BCsf fractions that ranged from low to high, and whose mass median Dc values were 137 ± 10 nm, 143 ± 11 nm and 169 ± 29 nm, respectively. The corresponding bulk relative coating thickness of BC (coated particle size/BC core – Dp/Dc) for these same sectors was 1.28 ± 0.07, 1.45 ± 0.16 and 1.65 ± 0.19. For W, SE and E air masses, the number fraction of BCsf ranged from 6 ± 2% to 11 ± 5% to 18 ± 10%, respectively, but importantly the larger BC core sizes lead to an increased fraction of BCsf in terms of mass than number (for W, SE and E air masses, the BCsf mass fractions ranged from 16 ± 6%, 24 ± 10% and 39 ± 14%, respectively). An increased fraction of non-BC particles (particles that did not contain a BC core) was also observed when SF sources were more significant. The BC mass attribution by the SP2 method agreed well with the BC–AMS–PMF multiple linear regression method (BC–AMS–PMF : SP2 ratio = 1.05, r2 = 0.80) over the entire experimental period. Good agreement was found between BCsf attributed with the Aethalometer model and the SP2. However, the assumed absorption Ångström exponent (αwb) had to be changed according to the different air mass sectors to yield the best comparison with the SP2. This could be due to influences of fuel type or burn phase.
The molecular basis of hybrid vigor (heterosis) has remained unknown despite the importance of this phenomenon in evolution and in practical breeding programs. To formulate a molecular basis of ...heterosis, an understanding of gene expression in inbred and hybrid states is needed. In this study, we examined the amount of various transcripts in hybrid and inbred individuals (B73 and Mo17) to determine whether the quantities of specific messenger RNAs were additive or nonadditive in the hybrids. Further, we examined the levels of the same transcripts in hybrid triploid individuals that had received unequal genomic contributions, one haploid genome from one parent and two from the other. If allelic expression were merely the additive value in hybrids from the two parents, the midparent values would be observed. Our study revealed that a substantial number of genes do not exhibit the midparent value of expression in hybrids. Instead, transcript levels in the diploid hybrids correlate negatively with the levels in diploid inbreds. Although transcript levels were clearly nonadditive, transcript levels in triploid hybrids were affected by genomic dosage.
This paper investigates the physical and chemical characteristics of biomass burning aerosol over West Africa using data from the UK Facility for Airborne Atmospheric Measurements aircraft. ...Measurements of biomass burning aerosol were made during the Dust and Biomass‐burning Experiment (DABEX) and Dust Outflow and Deposition to the Ocean (DODO) field experiments in January and February 2006. Layers of aged biomass burning aerosols were observed throughout the region, and fresh biomass burning aerosols were encountered during the penetration of smoke plumes at low altitudes. Vertical profiles of aerosol properties across the region are shown. Measurements from an Aerodyne Quadrupole Aerosol Mass Spectrometer (Q‐AMS) show changes in chemical composition between fresh and aged biomass burning aerosols, over a region spanning thousands of kilometers. These data represent the first time that continental‐scale variability in biomass burning aerosol composition has been observed. However, an almost linear relationship between organic aerosol mass concentration and carbon monoxide concentration was observed across the region. A net carbon loss occurs over the aerosol lifetime in the region owing to a combination of chemical processing and repartitioning of organic mass to the gas phase. Evolution of the number size distribution was observed, with coagulation concluded to be the dominant process involved, a finding supported by coagulation box modeling. Regional‐scale emission ratios for organic (0.041) and black carbon (0.0072) with respect to CO have been estimated over West Africa, one of the largest sources of biomass burning globally. Biomass burning emissions from the West African Sahel are poorly represented in the literature, and these results represent important continental‐scale emissions. They are in good agreement with literature values from other regions.
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.
Key Points
Laboratory experiments represent aircraft measurements reasonably wellBlack carbon emissions in inventories may require upward revision
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 Ångström exponent or AAE = 1–2) and others producing weakly light‐absorbing particles (SSA at 781 nm ~1) with strong wavelength dependence of absorption (AAE ~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.
Key Points
At 781 nm, SSA ~1 corresponded to the largest absorption Angstrom exponents, ~7Nonrefractory material enhanced visible light absorption at least 20–50%A portion of the absorption enhancement was attributed to brown carbon
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