Discrepancies between formaldehyde measurements and methane oxidation model predictions in the Antarctic troposphere: An assessment of other possible formaldehyde sources (original) (raw)
Related papers
Atmosphere-to-snow-to-firn transfer studies of HCHO at Summit, Greenland
Geophysical Research Letters, 1999
Formaldehyde (HCHO) measurements in snow, firn, atmosphere, and air in the open pore space of the firn (firn air) at Summit, Greenland, in June 1996 show that the top snow layers are a HCHO source. HCHO concentrations in fresh snow are higher than those in equilibrium with atmospheric concentrations, resulting in HCHO degassing in the days to weeks following snowfall. Maximum HCHO concentrations in firn air were 1.5-2.2 ppbv, while the mean atmospheric HCHO concentration 1 m above the surface was 0.23 ppbv. Apparent HCHO fluxes out of the snow are a plausible explanation for the discrepancy between the 0.1 ppbv atmospheric concentration predicted by photochemical modeling and the measurements. HCHO in deeper firn is near equilibrium with the lower tropospheric HCHO concentration at the annual average temperature. Thus HCHO in ice may in fact be linearly related to multiyear average atmospheric concentrations through a temperature dependent partition coefficient.
Atmosphere - to - snow - to - firn transfer studies of HCHO at Summit
2000
Formaldehyde (HCHO) measurements in snow, firn, atmosphere, and air in the open pore space of the firn (firn air) at Summit, Greenland, in June 1996 show that the top snow layers are a HCHO source. HCHO concentrations in fresh snow are higher than those in equilibrium with atmospheric concentrations, resulting in HCHO degassing in the days to weeks following snowfall. Maximum HCHO concentrations in firn air were 1.5-2.2 ppbv, while the mean atmospheric HCHO concentration 1 m above the surface was 0.23 ppbv. Apparent HCHO fluxes out of the snow are a plausible explanation for the discrepancy between the 0.1 ppbv atmospheric concentration predicted by photochemical modeling and the measurements. HCHO in deeper firn is near equilibrium with the lower tropospheric HCHO concentration at the annual average temperature. Thus HCHO in ice may in fact be linearly related to multiyear average atmospheric concentrations through a temperature dependent partition coefficient.
HCHO in Antarctic snow: Preservation in ice cores and air-snow exchange
Geophysical Research Letters, 2002
1] Formaldehyde (HCHO) measurements in snow and shallow firn at three Antarctic sites gave concentrations around 6 ppbw in surface snow and 1 ppbw and lower below 1 -2 m depth. The variable concentration patterns in shallow snow and firn result from temperature-dependent uptake and release of HCHO in response to annual temperature cycles. Deeper concentrations are constant with depth, and apparently reflect average atmospheric concentrations. This implies that after accounting for differences in temperature and accumulation, changes in ice-core HCHO concentrations with depth should linearly reflect changes in atmospheric HCHO over time. Modeling of observed HCHO profiles in the snow implies that degassing of HCHO from surface snow likely contributes a significant fraction of the HCHO found in the boundary layer in spring and summer at all three sites. Based on modeling of air-snow exchange and atmospheric photochemistry, summer HCHO levels are estimated to be on the order of 100 -200 pptv.
Formaldehyde and hydrogen peroxide in air, snow and interstitial air at South Pole
Atmospheric Environment, 2004
Average H 2 O 2 (HCHO) mixing ratios measured above the snowpack at South Pole were 278 pptv (103 pptv) in December 2000 and between 4 to 43 times (1.4 to 2.6) the expected value based on gas-phase photostationary state model calculations. The larger difference is realized if dry deposition of both species is included in the model. H 2 O 2 and HCHO fluxes from the snowpack were independently determined from gradient measurements in the air above the snow surface, from firn air measurements and from the temporal concentration changes in near-surface snow. On average, the snowpack at South Pole was releasing on the order of 1 x 10 13 and 2 x 10 12 molecules m -2 s -1 of H 2 O 2 and HCHO, respectively, in December 2000. This is consistent with the volumetric fluxes needed for the photostationary state model to reproduce the observed atmospheric mixing ratios of both H 2 O 2 and HCHO. The highly elevated levels of both species found in firn air further support the above estimates.
Atmospheric Environment, 2002
Tower-based measurements of hydrogen peroxide (H 2 O 2 ) and formaldehyde (HCHO) exchange were performed above the snowpack of the Greenland ice sheet. H 2 O 2 and HCHO fluxes were measured continuously between 16 June and 7 July 2000, at the Summit Environmental Observatory. The fluxes were determined using coil scrubber-aqueous phase fluorometry systems together with micrometeorological techniques. Both compounds exhibit strong diel cycles in the observed concentrations as well as in the fluxes with emission from the snow during the day and the evening and deposition during the night. The averaged diel variations of the observed fluxes were in the range of +1.3 Â 10 13 molecules m À2 s À1 (deposition) and À1.6 Â 10 13 molecules m À2 s À1 (emission) for H 2 O 2 and +1.1 Â 10 12 and À4.2 Â 10 12 molecules m À2 s À1 for HCHO, while the net exchange per day for both compounds were much smaller. During the study period of 22 days on average ð0:8 þ4:6 À4:3 Þ Â 10 17 molecules m À2 of H 2 O 2 were deposited and ð7:0 þ12:6 À12:2 Þ Â 10 16 molecules m À2 of HCHO were emitted from the snow per day. A comparison with the inventory in the gas phase demonstrates that the exchange influences the diel variations in the boundary layer above snow covered areas. Flux measurements during and after the precipitation of new snow shows that o16% of the H 2 O 2 and more than 25% of the HCHO originally present in the new snow were available for fast release to the atmospheric boundary layer within hours after precipitation. This release can effectively disturb the normally observed diel variations of the exchange between the surface snow and the atmosphere, thus perturbing also the diel variations of corresponding gas-phase concentrations. r
Journal of Geophysical Research, 2001
Hydrogen peroxide (H202) contributes to the atmosphere's oxidizing capacity, which determines the lifetime of atmospheric trace species. Measured bidirectional summertime H202 fluxes from the snowpack at Summit, Greenland, in June 1996 reveal a daytime H202 release from the surface snow reservoir and a partial redeposition at night. The observations also provide the first direct evidence of a strong net summertime H202 release from the snowpack, enhancing average boundary layer H20•. concentrations approximately sevenfold and the OH and HO2 concentrations by 70% and 50%, respectively, relative to that estimated from photochemical modeling in the absence of the snowpack source. The total H202 release over a 12-day period was of the order of 5x10 •3 molecules m -2 s -• and compares well with observed concentration changes in the top snow layer. Photochemical and air-snow interaction modeling indicate that the net snowpack release is driven by temperature-induced uptake and release of H202 as deposited snow, which is supersaturated with respect to ice-.air partitioning, approaches equilibrium. The results show that the physical cycling of H202 and possibly other volatile species is a key to understanding snowpacks as complex physicalphotochelnical reactors and has far reaching implications for the interpretation of ice core records as well as for the photochemistry in polar regions •nd in the vicinity of snowpacks in general.
Investigation of the role of the snowpack on atmospheric formaldehyde chemistry at Summit, Greenland
Journal of Geophysical Research, 2002
1] Ambient gas-phase and snow-phase measurements of formaldehyde (HCHO) were conducted at Summit, Greenland, during several summers, in order to understand the role of air-snow exchange on remote tropospheric HCHO and factors that determine snowpack HCHO. To investigate the impact of the known snowpack emission of HCHO, a gas-phase model was developed that includes known chemistry relevant to Summit and that is constrained by data from the 1999 and 2000 field campaigns. This gas-phase-only model does not account for the high ambient levels of HCHO observed at Summit for several previous measurement campaigns, predicting approximately 150 ppt from predominantly CH 4 chemistry, which is $25-50% of the observed concentrations for several years. Simulations were conducted that included a snowpack flux of HCHO based on HCHO flux measurements from 2000 and 1996. Using the fluxes obtained for 2000, the snowpack does not appear to be a substantial source of gas-phase HCHO in summer. The 1996 flux estimates predict much higher HCHO concentrations, but with a strong diel cycle that does not match the observations. Thus, we conclude that, although the flux of HCHO from the surface likely has a significant impact on atmospheric HCHO above the snowpack, the time-dependent fluxes need to be better understood and quantified. It is also necessary to identify the HCHO precursors so we can better understand the nature and importance of snowpack photochemistry. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Tropospherecomposition and chemistry; 1863 Hydrology: Snow and ice (1827); 3367 Meteorolgy and Atmospheric Dynamics: Theoretical modeling Citation: Dassau, T. M., et al., Investigation of the role of the snowpack on atmospheric formaldehyde chemistry at Summit, Greenland,
Chemistry of hydrogen oxide radicals (HOx) in the Arctic troposphere in spring
Atmospheric Chemistry and Physics, 2010
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 5 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 gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO 2 uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, 10 through a mechanism that does not produce H 2 O 2 . Such a mechanism could involve HO 2 aqueous-phase reaction with sulfate (58% of the ARCTAS submicron aerosol by mass) to produce peroxymonosulfate (HSO − 5 ) that would eventually convert back to sulfate and return water. 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 rang-15 ing 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 48% and 45% respectively. Circumpolar budget analysis in the model shows that transport of peroxides from northern mid-latitudes 20 contributes 50% of the HO y source above 6 km, and cloud chemistry and deposition of H 2 O 2 account together for 40% of the HO y sink below 3 km. 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. 6957 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 25 other volatile organic compounds (VOCs) yields formaldehyde (HCHO), which photolyzes to produce additional HO x radicals and amplify the original source: 6958 Abstract 20 Chen et al., 2004). Another unique aspect of HO x chemistry in the boundary layer is the interaction with halogen radicals. These interactions include HO x production from Br+HCHO (Evans et al., 2003), additional HO y reservoirs such as HOBr (Bloss et al., 2005), and additional processes for cycling between HO 2 and OH (Simpson et al., 2007). Abstract make reference to the P-3 aerosol data. The DC-8 conducted nine flights in the North 6960 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract ACPD 10, 6955-6994, 2010 Abstract 0.07-0.2) except for concentrated H 2 SO 4 (γ(HO 2 )< 0.01). However, γ(HO 2 ) for con-6967 Abstract − 2 reaction at an assumed pH 5, producing H 2 O 2 that 6968 Abstract ACPD 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract 10, 6955-6994, 2010 Abstract References Allen, D., Pickering, K., Stenchikov, G., Thompson, A., and Kondo, Y.: A three-dimensional total odd nitrogen (NO y ) simulation during sonex using a stretched-grid chemical transport model, 25 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion data for atmospheric chemistry: Volume II -gas phase reactions of organic species, Atmos. Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, J. Geophys. Res.-Atmos., 106, 23073-23095, 2001. Bian, H. S. and Prather, M. J.: FAST-J2: Accurate simulation of stratospheric photolysis in 10 global chemical models, Impact of halogen monoxide chemistry upon boundary layer OH and HO 2 concentrations at a coastal site, Geophys.
Journal of Geophysical Research, 2011
1] The snowpack is a photochemically active medium which produces numerous key reactive species involved in the atmospheric chemistry of polar regions. Formaldehyde (HCHO) is one such reactive species produced in the snow, and which can be released to the atmospheric boundary layer. Based on atmospheric and snow measurements, this study investigates the physical processes involved in the HCHO air-snow exchanges observed during the OASIS 2009 field campaign at Barrow, Alaska. HCHO concentration changes in a fresh diamond dust layer are quantitatively explained by the equilibration of a solid solution of HCHO in ice, through solid-state diffusion of HCHO within snow crystals. Because diffusion of HCHO in ice is slow, the size of snow crystals is a major variable in the kinetics of exchange and the knowledge of the snow specific surface area is therefore crucial. Air-snow exchanges of HCHO can thus be explained without having to consider processes taking place in the quasi-liquid layer present at the surface of ice crystals. A flux of HCHO to the atmosphere was observed simultaneously with an increase of HCHO concentration in snow, indicating photochemical production in surface snow. This study also suggests that the difference in bromine chemistry between Alert (Canadian Arctic) and Barrow leads to different snow composition and post-deposition evolutions. The highly active bromine chemistry at Barrow probably leads to low HCHO concentrations at the altitude where diamond dust formed. Precipitated diamond dust was subsequently undersaturated with respect to thermodynamic equilibrium, which contrasts to what was observed elsewhere in previous studies.