Nicolas J. Cullen

Vertical fluxes of NOx, HONO, and HNO3 above the snowpack at Summit, Greenland

Abstract Vertical gradients of NO x , HONO, and HNO 3 were measured in the lower 1– 2 m above the snowpack at Summit, Greenland, during summer 2000. These measurements are used with simultaneous measurements of atmospheric turbulence using eddy covariance systems to determine the vertical fluxes of NO x , HONO, and HNO 3 . Upward fluxes of NO x and HONO were observed; these emissions were highly correlated with diurnally varying sunlight intensity, consistent with the expectation that they are the result of nitrate photolysis within the snowpack. The HNO 3 flux was smaller in magnitude and more variable than those of HONO and NO x . It was usually downward, but emission was occasionally observed during mid-day. The 24-h average NO x emission (2.52×10 12 molecules m −2 s −1 ) and HONO emission (4.64×10 11 molecules m −2 s −1 ) rates were not balanced by the average HNO 3 deposition rate (7.16×10 11 molecules m −2 s −1 ) , indicating that NO x export may slowly remove nitrogen from the system composed of the atmospheric boundary layer plus the top few cm of the surface snowpack, potentially affecting the amount of nitrate ultimately stored in glacial ice. These measurements imply that snowpack (NO x +HONO) emissions may alter NO x and (through HONO photolysis) OH levels in remote, snow-covered regions, but are small relative to other NO x sources on the global scale.

Measurements of hydrogen peroxide and formaldehyde exchange between the atmosphere and surface snow at Summit, Greenland

Tower-based measurements of hydrogen peroxide (H2O2) and formaldehyde (HCHO) exchange were performed above the snowpack of the Greenland ice sheet. H2O2 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 � 10 13 molecules m � 2 s � 1 (deposition) and � 1.6 � 10 13 molecules m � 2 s � 1 (emission) for H2O2 and +1.1 � 10 12 and � 4.2 � 10 12 molecules m � 2 s � 1 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 � 4:3 Þ� 10 17 molecules m � 2 of H 2O2 were deposited and ð7:0 þ12:6 � 12:2 Þ� 10 16 molecules m � 2 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 H2O2 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 2002 Elsevier Science Ltd. All rights reserved.

Ozone and meteorological boundary-layer conditions at Summit, Greenland, during 3-21 June 2000

The temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground. The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between B9.00 and 18.00 h local time with the formation of shallow mixing heights of B70–250 m above the surface. The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37–76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/ low stratospheric air into the lower boundary layer. A B0.1–3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime. r 2002 Elsevier Science Ltd. All rights reserved.

Investigation of the role of the snowpack on atmospheric formaldehyde chemistry at Summit, Greenland

[1]xa0Ambient 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 CH4 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.

Unstable near-surface boundary conditions in summer on top of the greenland ice sheet

The surface energy balance for the interior of the Greenland ice sheet in summer 2000 is described. The most important feature of the energy balance is that net all-wave radiation is positive. Peak global radiation values of 800 W m−2 provide energy at the snow surface that results in the turbulent fluxes of sensible and latent heat being directed into the atmosphere. Heating at the surface gives rise to unstable near-surface atmospheric conditions, with sensible heat fluxes typically between 10-20 W m−2. Daily mean values of the subsurface heat flux are small and directed downwards from the surface as the snow cover warms during the summer season. Analysis of the gradient Richardson number for the past three summers shows that unstable conditions are common, with frequencies between 11-43% for a 2.5 month period. The unstable conditions in summer appear to be important in controlling katabatic forcing over the ice sheet.