Biomass burning (BB) is the second largest source of trace gases and the largest source of primary fine carbonaceous particles in the global troposphere. Many recent BB studies have provided new emission factor (EF) measurements. This is especially true for non-methane organic compounds (NMOC), which influence secondary organic aerosol (SOA) and ozone formation. New EF should improve regional to global BB emissions estimates and therefore, the input for atmospheric models. In this work we present an up-to-date, comprehensive tabulation of EF for known pyrogenic species based on measurements made in smoke that has cooled to ambient temperature, but not yet undergone significant photochemical processing. All EFs are converted to one standard form (g compound emitted per kg dry biomass burned) using the carbon mass balance method and they are categorized into 14 fuel or vegetation types. Biomass burning terminology is defined to promote consistency. We compile a large number of measurements of biomass consumption per unit area for important fire types and summarize several recent estimates of global biomass consumption by the major types of biomass burning. Post emission processes are discussed to provide a context for the emission factor concept within overall atmospheric chemistry and also highlight the potential for rapid changes relative to the scale of some models or remote sensing products. Recent work shows that individual biomass fires emit significantly more gas-phase NMOC than previously thought and that including additional NMOC can improve photochemical model performance. A detailed global estimate suggests that BB emits at least 400 Tg yr⁻¹ of gas-phase NMOC, which is almost 3 times larger than most previous estimates. Selected recent results (e.g. measurements of HONO and the BB tracers HCN and CH₃CN) are highlighted and key areas requiring future research are briefly discussed.

The last decade has seen tremendous advances in atmospheric aerosol particle research that is often performed in the context of climate and global change science. Biomass burning, one of the largest sources of accumulation mode particles globally, has been closely studied for its radiative, geochemical, and dynamic impacts. These studies have taken many forms including laboratory burns, in situ experiments, remote sensing, and modeling. While the differing perspectives of these studies have ultimately improved our qualitative understanding of biomass-burning issues, the varied nature of the work make inter-comparisons and resolutions of some specific issues difficult. In short, the literature base has become a milieu of small pieces of the biomass-burning puzzle. This manuscript, the second part of four, examines the properties of biomass-burning particle emissions. Here we review and discuss the literature concerning the measurement of smoke particle size, chemistry, thermodynamic properties, and emission factors. Where appropriate, critiques of measurement techniques are presented. We show that very large differences in measured particle properties have appeared in the literature, in particular with regards to particle carbon budgets. We investigate emissions uncertainties using scale analyses, which shows that while emission factors for grass and brush are relatively well known, very large uncertainties still exist in emission factors of boreal, temperate and some tropical forests. Based on an uncertainty analysis of the community data set of biomass burning measurements, we present simplified models for particle size and emission factors. We close this review paper with a discussion of the community experimental data, point to lapses in the data set, and prioritize future research topics.

Because of its wide coverage over much of the globe, biomass burning has been widely studied in the context of direct radiative forcing. Such study is warranted as smoke particles scatter and at times absorb solar radiation efficiently. Further, as much of what is known about smoke transport and impacts is based on remote sensing measurements, the optical properties of smoke particles have far reaching effects into numerous aspects of biomass burning studies. Global estimates of direct forcing have been widely varying, ranging from near zero to −1 W m^-2. A significant part of this difference can be traced to varying assumptions on the optical properties of smoke. This manuscript is the third part of four examining biomass-burning emissions. Here we review and discuss the literature concerning measurement and modeling of optical properties of biomass-burning particles. These include available data from published sensitivity studies, field campaigns, and inversions from the Aerosol Robotic Network (AERONET) of Sun photometer sites. As a whole, optical properties reported in the literature are varied, reflecting both the dynamic nature of fires, variations in smoke aging processes and differences in measurement technique. We find that forward modeling or ''internal closure'' studies ultimately are of little help in resolving outstanding measurement issues due to the high degree of degeneracy in solutions when using ''reasonable'' input parameters. This is particularly notable with respect to index of refraction and the treatment of black carbon. Consequently, previous claims of column closure may in fact be more ambiguous. Differences between in situ and retrieved ω_o values have implications for estimates of mass scattering and mass absorption efficiencies. In this manuscript we review and discuss this community dataset. Strengths and lapses are pointed out, future research topics are prioritized, and best estimates and uncertainties of key smoke particle parameters are provided.

Gas and particle measurements are described for optically thick regional hazes, dominated by aged smoke from biomass burning, in the cerrado and rain forested regions of Brazil. The hazes tended to be evenly mixed from the surface to the trade wind inversion at 3–4 km in altitude. The properties of aged gases and particles in the regional hazes were significantly different from those of young smoke (<4 min old). As the smoke aged, the total amount of carbon in non‐methane hydrocarbon species (C<11) was depleted by about one third due to transformations into CO₂, CO, and reactive molecules, and removed by dry deposition and/or by conversion to particulate matter. As the smoke particles aged, their sizes increased significantly due to coagulation and mass growth by secondary species (e.g., ammonium, organic acids and sulfate). During aging, condensation and gas‐to‐particle conversion of inorganic and organic vapors increased the aerosol mass by ∼20–40%. One third to one half of this mass growth likely occurred in the first few hours of aging due to the condensation of large organic molecules. The remaining mass growth was probably associated with photochemical and cloud‐processing mechanisms operating over several days. Changes in particle sizes and compositions during aging had a large impact on the optical properties of the aerosol. Over a 2 to 4 day period, the fine particle mass‐scattering efficiency and single‐scattering albedo increased by 1 m²g⁻¹, and ∼0.06, respectively. Conversely, the Angstrom coefficient, backscatter ratio, and mass absorption efficiency decreased significantly with age.

A multitude of sensitivity studies in the literature point to the importance of proper chemical and morphological characterization of particles when the radiative impacts of airborne dusts are modeled. However, the community data set is based on heterogeneous measurement methods relying on varying aerodynamic, chemical, morphological, and optical means. During the Puerto Rico Dust Experiment, size distributions of dust particles from Africa were measured using a variety of aerodynamic, optical, and geometric means. Consistent with the literature, comparisons of these size distributions showed quite dissimilar results. “Measured” volume median diameters varied from 2.5 to 9 μm for various geometric, aerodynamic, optical, and optical inversion methods. Aerodynamic systems showed mixed performance. Column integrated size distributions inverted from AERONET Sun/sky radiance data produced somewhat reasonable results in the coarse mode when given proper constraints and taken in the proper context. The largest systematic errors were found in optical particle counters due to insensitivities to particle size in the 4–10 μm region with further complications due to dust particle morphology and index of refraction issues. As these methods can produce quite dissimilar size distributions, considerable errors in calculated radiative properties can occur if incorrectly modeled into dust parameters. None of the methods compared in this study can adequately reproduce the measured mass extinction or mass scattering efficiency of the dust using spherical geometry methods. Given all of the uncertainties in the sizing methods, we promote the use of fundamental and quantifiable descriptors of particles such as mass as a function of aerodynamic diameter.