Abstract. In March 2006 two instrumented aircraft made the first detailed field measurements of biomass burning (BB) emissions in the Northern Hemisphere tropics as part of the MILAGRO project. The aircraft were the National Center for Atmospheric Research C-130 and a University of Montana/US Forest Service Twin Otter. The initial emissions of up to 49 trace gas or particle species were measured from 20 deforestation and crop residue fires on the Yucatan peninsula. This included two trace gases useful as indicators of BB (HCN and acetonitrile) and several rarely, or never before, measured species: OH, peroxyacetic acid, propanoic acid, hydrogen peroxide, methane sulfonic acid, and sulfuric acid. Crop residue fires emitted more organic acids and ammonia than deforestation fires, but the emissions from the main fire types were otherwise fairly similar. The Yucatan firesCorrespondence to: R. J. Yokelson (bob.yokelson@umontana.edu) emitted unusually high amounts of SO 2 and particle chloride, likely due to a strong marine influence on this peninsula. As smoke from one fire aged, the ratio O 3 / CO increased to ∼15% in <∼1 h similar to the fast net production of O 3 in BB plumes observed earlier in Africa. The rapid change in O 3 occurs at a finer spatial scale than is employed in global models and is also faster than predicted by microscale models. Fast increases in PAN, H 2 O 2 , and two organic acids were also observed. The amount of secondary organic acid is larger than the amount of known precursors. Rapid secondary formation of organic and inorganic aerosol was observed with the ratio PM 2.5 / CO more than doubling in ∼1.4±0.7 h. The OH measurements revealed high initial levels (>1×10 7 molecules/cm 3 ) that were likely caused in part by high initial HONO (∼10% of NO y ). Thus, more research is needed to understand critical post emission processes for the second-largest trace gas source on Earth. It is estimated that ∼44 Tg of biomass burned in the Yucatan in the spring Published by Copernicus Publications on behalf of the European Geosciences Union.

[1] During Transport and Chemical Evolution over the Pacific (TRACE-P) and Asian Aerosol Characterization Experiment (ACE-Asia) we measured the dry size distribution of Asian aerosols, their state of mixing, and the optical properties of dust, black carbon (BC) and other aerosol constituents in combustion and/or dust plumes. Optical particle sizing in association with thermal heating extracted volatile components and resolved sizes for dust and refractory soot that usually dominated light absorption. BC was internally mixed with volatile aerosol in $85% of accumulation mode particles and constituted $5-15% of their mass. These optically effective sizes constrained the soot and dust size distributions and the imaginary part of the dust refractive index, k, to 0.0006 ± 0.0001. This implies a single-scatter albedo, v (550 nm), for dust ranging from 0.99+ for D p < 1 mm to $0.90 at D p = 10 mm and a size-integrated campaign average near 0.97 ± 0.01. The typical mass scattering efficiency for the dust was $0.3 m 2 g À1 , and the mass absorption efficiency (MAE) was 0.009 m 2 g À1 . Less dust south of 25°N and stronger biomass burning signatures resulted in lower values for v of $0.82 in plumes aloft. Chemically inferred elemental carbon was moderately correlated with BC light absorption (R 2 = 0.40), while refractory soot volume between 0.1 and 0.5 mm was highly correlated (R 2 = 0.79) with absorption. However, both approaches yield an MAE for BC mixtures of $7 ± 2 m 2 g À1 and higher than calculated MAE values for BC of 5 m 2 g À1 . The increase in the mass fraction of soot and BC in pollution aerosol in the presence of elevated dust appears to be due to uptake of the volatile components onto the coarse dust. This predictably lowered v for the accumulation mode from 0.84 in typical pollution to $0.74 in high-dust events. A chemical transport model revealed good agreement between model and observed BC absorption for most of SE Asia and in biomass plumes but underestimated BC for combustion sources north of 25°N by a factor of $3.

[1] Thermal analysis of aerosol size distributions provided size resolved volatility up to temperatures of 400°C during extensive flights over North America (NA) for the INTEX/ICARTT experiment in summer 2004. Biomass burning and pollution plumes identified from trace gas measurements were evaluated for their aerosol physiochemical and optical signatures. Measurements of soluble ionic mass and refractory black carbon (BC) mass, inferred from light absorption, were combined with volatility to identify organic carbon at 400°C (VolatileOC) and the residual or refractory organic carbon, RefractoryOC. This approach characterized distinct constituent mass fractions present in biomass burning and pollution plumes every 5-10 min. Biomass burning, pollution and dust aerosol could be stratified by their combined spectral scattering and absorption properties. The ''nonplume'' regional aerosol exhibited properties dominated by pollution characteristics near the surface and biomass burning aloft. VolatileOC included most water-soluble organic carbon. RefractoryOC dominated enhanced shortwave absorption in plumes from Alaskan and Canadian forest fires. The mass absorption efficiency of this RefractoryOC was about 0.63 m 2 g À1 at 470 nm and 0.09 m 2 g À1 at 530 nm. Concurrent measurements of the humidity dependence of scattering, g, revealed the OC component to be only weakly hygroscopic resulting in a general decrease in g with increasing OC mass fractions. Under ambient humidity conditions, the systematic relations between physiochemical properties and g lead to a well-constrained dependency on the absorption per unit dry mass for these plume types that may be used to challenge remotely sensed and modeled optical properties.Citation: Clarke, A., et al. (2007), Biomass burning and pollution aerosol over North America: Organic components and their influence on spectral optical properties and humidification response,

Abstract. Measurements show that new particles are formed by homogenous nucleation over a wide range of conditions in the remote troposphere. In our studies, large nucleation events are found exclusively in regions of enhanced sulfuric acid vapor (H2SO4g) concentrations, with maximum concentrations never exceeding 5x107 molecules cm '3. Although these data suggest that H2SO4g participated, comparisons between ambient conditions in regions of nucleation to conditions necessary for binary H2SO4 water (H20) nucleation indicate that the mechanism may vary with elevation. In remote marine regions, at altitudes greater than --4 km above sea level, observations of nucleation in clear air along cloud perimeters are in fair agreement with current classical binary nucleation models. In these regions, the low temperatures associated with high altitudes may produce sufficiently saturated H2SO4 for the production of new H•SO4/H•O particles. However, uncertainties with current binary nucleation models limit decisive comparisons. In warmer regions, closer to the earth's surface, measured H:SO4 concentrations are clearly insufficient for binary nucleation.Conditions at these sites are sinfilar to those observed in an earlier study where there was circumstantial evidence for a ternary mechanism involving H2SO4, H20, and ammonia (NH3) [Weber et al., 1998], suggesting that this may be a significant route for particle production at lower altitudes where surfacederived species, like NH3, are more apt to participate.

Integration of extensive aerosol data collected during the past decade around the Pacific basin provides a preliminary assessment of aerosol microphysics for this region and cycling of aerosol in the troposphere. These include aircraft-based data collected as part of numerous field experiments supported by the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the National Oceanic and Atmospheric Administration (NOAA) [Global Backscatter Experiment (GLOBE), First Aerosol Characterization Experiment (ACE-1), Pacific Exploratory Mission (PEM)-Tropics A and B]. Although these experiments had diverse goals, most included extensive data on aerosol size distributions, optical properties (light scattering and light absorption), and chemistry. Vertical profiles of aerosol concentration, size distribution, and light scattering were used to characterize vertical structure from 70ЊS to 70ЊN. The in situ data are placed in the context of meteorological regimes over the Pacific as well as processes associated with particle formation, growth, and evolution, and include dust, pollution, sea salt, sulfates, and clean cloud-processed air. The Tropics commonly have low aerosol mass but very high number concentrations in the upper free troposphere (FT) that appear to form from sulfuric acid (nucleation) in convective regions and near cloud edges. These age and subside to become effective cloud condensation nuclei (CCN) when mixed into the marine boundary layer. Fewer number but larger aerosol are more evident in the midlatitude FT. These can often be internally mixed and with a nonvolatile core indicative of black carbon with volatile components (sulfate, organics, etc.). In the North Pacific springtime a combustion-derived aerosol is frequently found associated with the same meteorology that transports ''dust events.'' Both constituents may dominate the scattering and absorption properties of the aerosol even though the increase in large dust particles in such events generally dominates the mass. The FT in the subtropics tends to exhibit frequent and marked transitions and mixing between these clean and continental aerosol types.