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.

Abstract. Alkyl and multifunctional organic nitrates, molecules of the chemical form RONO 2 , are products of chain terminating reactions in the tropospheric HO x and NO x catalytic cycles and thereby impact ozone formation locally. Many of the molecules in the class have lifetimes that are long enough that they can be transported over large distances. If the RONO 2 then decompose to deliver NO x to remote regions they affect ozone production rates in locations distant from the original NO x source. While measurements of total RONO 2 ( ANs) and small straight chain alkyl nitrates are routine, measurements of the specific multifunctional RONO 2 molecules that are believed to dominate the total have rarely been reported and never reported in coincidence with ambient ANs measurements. Here we describe observations obtained during the BEARPEX 2009 experiment including ANs and a suite of multifunctional nitrates including isoprene derived hydroxynitrates, oxidation products of those nitrates, 2-methyl-3-buten-2-ol (MBO) derived hydroxynitrates, and monoterpene nitrates. At the BEARPEX field site, the sum of the individual biogenically derived nitrates account for two-thirds of the ANs, confirming predictions of the importance of biogenic nitrates to the NO y budget. Isoprene derived nitrates, transported to the site, are a much larger fraction of the ANs at the site than the nitrates derived from the locally emitted MBO. Evidence for additional nitrates, possibly from nocturnal chemistry of isoprene and α-pinene, is presented.

Abstract. Using measurements from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, we show that methyl peroxy nitrate (CH 3 O 2 NO 2 ) is present in concentrations of ∼5-15 pptv in the springtime arctic upper troposphere. We investigate the regional and global effects of CH 3 O 2 NO 2 by including its chemistry in the GEOS-Chem 3-D global chemical transport model. We find that at temperatures below 240 K inclusion of CH 3 O 2 NO 2 chemistry results in decreases of up to ∼20 % in NO x , ∼20 % in N 2 O 5 , ∼5 % in HNO 3 , ∼2 % in ozone, and increases in methyl hydrogen peroxide of up to ∼14 %. Larger changes are observed in biomass burning plumes lofted to high altitude. Additionally, by sequestering NO x at low temperatures, CH 3 O 2 NO 2 decreases the cycling of HO 2 to OH, resulting in a larger upper tropospheric HO 2 to OH ratio. These results may impact some estimates of lightning NO x sources as well as help explain differences between models and measurements of upper tropospheric composition.

[1] A NASA DC-8 research aircraft penetrated tropospheric gas and aerosol plumes sourced from active volcanoes in Ecuador and Colombia during the Tropical Composition, Cloud and Climate Coupling (TC 4 ) mission in July-August 2007. The likely source volcanoes were Tungurahua (Ecuador) and Nevado del Huila (Colombia). The TC 4 data provide rare insight into the chemistry of volcanic plumes in the tropical troposphere and permit a comparison of SO 2 column amounts measured by the Ozone Monitoring Instrument (OMI) on the Aura satellite with in situ SO 2 measurements. Elevated concentrations of SO 2 , sulfate aerosol, and particles were measured by DC-8 instrumentation in volcanic outflow at altitudes of 3-6 km. Estimated plume ages range from ∼2 h at Huila to ∼22-48 h downwind of Ecuador. The plumes contained sulfate-rich accumulation mode particles that were variably neutralized and often highly acidic. A significant fraction of supermicron volcanic ash was evident in one plume. In-plume O 3 concentrations were ∼70%-80% of ambient levels downwind of Ecuador, but data are insufficient to ascribe this to O 3 depletion via reactive halogen chemistry. The TC 4 data record rapid cloud processing of the Huila volcanic plume involving aqueous-phase oxidation of SO 2 by H 2 O 2 , but overall the data suggest average in-plume SO 2 to sulfate conversion rates of ∼1%-2% h −1 . SO 2 column amounts measured in the Tungurahua plume (∼0.1-0.2 Dobson units) are commensurate with average SO 2 columns retrieved from OMI measurements in the volcanic outflow region in July 2007. The TC 4 data set provides further evidence of the impact of volcanic emissions on tropospheric acidity and oxidizing capacity.

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 hy-Correspondence 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.Published by Copernicus Publications on behalf of the European Geosciences Union. 5824 J. Mao et al.: Chemistry of hydrogen oxide radicals (HO x ) in the Arctic spring