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.

DC3 brought together simultaneous measurements of storm kinematics, structure, electrical activity, and chemistry to improve our knowledge of how thunderstorms affect the chemical composition of the troposphere.

Abstract. We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO x ≡H+OH+peroxy radicals) and their reservoirs (HO y ≡HO x +peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO 2 and H 2 O 2 concentrations. Computation of HO x and HO y gasphase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hyCorrespondence to: J. Mao (mao@fas.harvard.edu) pothesize that this could reflect HO 2 uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H 2 O 2 . We implemented such an uptake of HO 2 by aerosol in the model using a standard reactive uptake coefficient parameterization with γ (HO 2 ) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO x species and HO y reservoirs. HO 2 uptake by aerosol is then a major HO x and HO y sink, decreasing mean OH and HO 2 concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for HO 2 uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.

[1] Inorganic bromine plays a critical role in ozone and mercury depletions events (ODEs and MDEs) in the Arctic marine boundary layer. Direct observations of bromine species other than bromine oxide (BrO) during ODEs are very limited. Here we report the first direct measurements of hypobromous acid (HOBr) as well as observations of BrO and molecular bromine (Br 2 ) by chemical ionization mass spectrometry at Barrow, Alaska in spring 2009 during the Ocean-Atmospheric-Sea Ice-Snowpack (OASIS) campaign. Diurnal profiles of HOBr with maximum concentrations near local noon and no significant concentrations at night were observed. The measured average daytime HOBr mixing ratio was 10 pptv with a maximum value of 26 pptv. The observed HOBr was reasonably well correlated (R 2 = 0.57) with predictions from a simple steady state photochemical model constrained to observed BrO and HO 2 at wind speeds <6 m s À1 . However, predicted HOBr levels were considerably higher than observations at higher wind speeds. This may be due to enhanced heterogeneous loss of HOBr on blowing snow coincident with higher wind speeds. BrO levels were also found to be higher at elevated wind speeds. Br 2 was observed in significant mixing ratios (maximum = 46 pptv; average = 13 pptv) at night and was strongly anti-correlated with ozone. The diurnal speciation of observed gas phase inorganic bromine species can be predicted by a time-dependent box model that includes efficient heterogeneous recycling of HOBr, hydrogen bromide (HBr), and bromine nitrate (BrONO 2 ) back to more reactive forms of bromine.