Otto Schrems

Measurements of NOx emissions from the Antarctic snowpack

It has been shown that NOx is produced photochemically within the snowpack of polar regions. If emitted to the atmosphere, this process could be a major source of NOx in remote snowcovered regions. We report here on measurements made at the German Antarctic station, Neumayer, during austral summer 1999, aimed at detecting and quantifying emissions of NOx from the surface snow. Gradients of NOx measured, and fluxes calculated using local meteorology measurements. On the 2 days of flux measurements, the derived fluxes showed continual release from the snow surface, varying between ∼0 and 3 × 108 molecs/cm²/s. When not subject to turbulence, the variation was coincident with the uv diurnal cycle, suggesting rapid release once photochemically produced. Scaling the diurnal average of Feb. 7th (1.3 × 108 molecs/cm²/s) suggests an annual emission over Antarctica of the order 0.0076TgN.

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.

A DOAS study on the origin of nitrous acid at urban and non-urban sites

Abstract Measurements have been performed to study the origin of the tropospheric nighttime concentrations of HNO2 The experiments were carried out at three locations with different levels of pollution by using a commercial differential optical absorption spectroscopy (DOAS) and a point sampling analyser. The DOAS observations allowed study of the concentrations of HNO2, NO2, SO2 and O3. Light intensities given by the DOAS system were used to estimate rough values for the total surface of aerosols S along the beam path. NO was measured by a chemiluminescent monitor. A HNO2 formation is clearly indicated by the nighttime increase of the HNO2/NOx ratio. Correlation studies suggest the participation of the aerosol surface in the heterogeneous formation in the less polluted areas. In Milan the measured HNO2 concentrations are considered to be influenced by the nighttime boundary layer and the heterogeneous formation on permanent surfaces like buildings.

Oxidized nitrogen chemistry and speciation in the Antarctic troposphere

Understanding the NOy budget at high latitudes is important for our knowledge of present-day clean air chemistry and essential for reliable interpretation of existing ice core nitrate data. However, measurements of NOy components at high latitudes have been limited, and no measurements have attempted to address the budget of NOy. Here we report on a campaign conducted in the austral summer of 1997 at the German Antarctic research station, Neumayer, with first Antarctic measurements for NOy in addition to light alkyl nitrates, NO, HNO3 and p−NO3−. Inorganic nitrate has generally been assumed to be the dominant component of NOy in Antarctica, although this idea has not previously been tested. However, our results show that for this coastal station, methyl nitrate was present in much higher concentration than inorganic nitrate (median CH3ONO2 = 38 pptv, HNO3 = 5 pptv). It has been suggested earlier that some alkyl nitrates might have a marine source. If this suggestion is correct, the implication arises that the oceans are an important source of NOy to the Antarctic troposphere and that their role in determining nitrate concentrations in ice must be considered.

Meridional distribution of hydroperoxides and formaldehyde in the marine boundary layer of the Atlantic (48°N‐35°S) measured during the Albatross campaign

Gas phase H 2 O 2 , organic peroxides, and formaldehyde (HCHO) have been measured in situ during October/November 1996 on board RV Polarstern in surface air over the Atlantic from 48°N-35°S with different analytical methods. The results indicate that recombination and self-reactions of peroxy radicals largely dominate over scavenging by NO. The peroxy radical chemistry was governed by the photooxidation of CH 4 and CO, as could be deduced from our failure to detect organic hydroperoxides other than CH 3 OOH (methyl hydroperoxide (MHP)). Hydroperoxide and formaldehyde mixing ratios were highest within the tropics with peak values of around 2000 parts per trillion by volume (pptv) (H 2 O 2 ). 1500 pptv (MHP), and 1000 pptv (HCHO). In the case of H 2 O 2 and MHP we observed diurnal variations of the mixing ratios in the tropical North Atlantic and derived deposition rates of around (1.8±0.6)×10 -5 s -1 for H 2 O 2 and (1.2±0.4)×10 -5 s -1 for MHP. The measured MHP/(H 2 O 2 +MHP) and MHP/HCHO ratios corresponded to 0.32±0.12 and 0.87±0.4, respectively. HCHO mixing ratios observed during the expedition were significantly higher than predicted by current photochemical theory based on the photooxidation of CH4 and CO.

Vertical ozone distribution in the marine atmosphere over the central Atlantic Ocean (56°S – 50°N)

Abstract. The vertical ozone distribution over the Atlantic Ocean has been measured in situ by shipborne ozone soundings during three RV Polarstern meridional transects in January/February 1993, October/November 1993, and May/June 1994. The low ozone column densities measured by Nimbus 7 and Meteor 3 satellites in 1993 could be confirmed by our investigations. We observed distinct differences in the vertical distribution pattern of tropospheric ozone between the northern and the southern hemisphere: The ozone mixing ratio gradients were flat in the northern hemisphere and ozone mixing ratios in the free troposphere never did exceed 80 parts per billion by volume (ppbv) up to the tropopause, while the southern hemisphere exhibited a pronounced vertical gradient. Extremely dry air masses with enhanced ozone amounts up to 120 ppbv have been found in the tropical free troposphere of the southern hemisphere between 0° and 20°S. The vertical ozone stratification in the troposphere of the southern hemisphere was dominated by this large-scale feature. Photochemical ozone production as a consequence of the emissions of natural fires or intrusions of stratospheric air masses are the most probable sources for these ozone-rich layers. On the basis of our results, a stringent differentiation between both alternatives could not be given.

Multiwavelength lidar measurements of stratospheric aerosols above Spitsbergen during winter 1992/93

Within the period of December 1992 to March 1993 lidar investigations of stratospheric aerosols were performed at Ny-Alesund, Spitsbergen (79°N, 12°E). Backscatter signals at wavelengths of 353, 532 and 1064 nm and depolarization signals at 532 nm in altitudes ranging from the tropopause up to 30 km were analyzed. Throughout the whole measurement period we observed an aerosol layer of volcanic origin in the lower stratosphere. Depolarization profiles suggest that the volcanic aerosol layer consisted mainly of liquid droplets. Comparison with model calculations indicate median particle radii between 0.1 and 0.2 µm. Surface densities exceeded approximately 40 µm²/cm³ in the lower part of the layer around 12 km. In January 1993 polar stratospheric clouds (PSCs) were frequently observed at altitudes up to 22 km. We analyzed the backscatter and depolarization data with respect to the temperature history of backward trajectories reaching Ny-Alesund. Signatures for micron sized crystalline PSC particles appear for cooling rates of −1 to −3 K day−1. Larger cooling rates of −4 to −10 K day −1 produced submicron sized aerosols, presumably supercooled droplets, characterized by enhanced backscatter ratios and reduced depolarization.

Arctic Study of Tropospheric Aerosol and Radiation (ASTAR) 2000: Arctic haze case study

The ASTAR 2000 (Arctic Study of Tropospheric Aerosol and Radiation) campaign ran from 12 March until 25 April 2000 with extensive flight operations in the vicinity of Svalbard (Norway) from Longyearbyen airport (78.25°N, 15.49°E). It was a joint Japanese (NIPR Tokyo)–German (AWI Bremerhaven/Potsdam) airborne measurement campaign using AWI aircraft POLAR 4 (Dornier 228-101). Simultaneous ground-based measurements were done at the international research site Ny-Ålesund (78.95°N, 11.93°E) in Svalbard, at the German Koldewey station, at the Japanese Rabben station and at the Scandinavian station at Zeppelin Mountain (475 m above sea level). During the campaign 19 profiles of various aerosol properties were measured. In general, the Arctic spring aerosol in the vicinity of Svalbard had significant temporal and vertical variability. A strong haze event occurred between 21 and 25 March in which the optical depth from ground-based observation was 0.18, which was significantly greater than the background value of 0.06. Airborne measurements on 23 March during this haze event showed a high aerosol layer with an extinction coefficient of 0.03 km−1 or more up to 3 km and a scattering coefficient from 0.02 in the same altitude range. From the chemical analyses of airborne measurements, sulfate, soot and sea salt particles were dominant, and there was a high mixing ratio of external soot particles in some layers during the haze event, whereas internal mixing of soot in sulfate was noticeable in some layers for the background condition. We argue that the high aerosol loading is due to direct transport from anthropogenic source regions. In this paper we focus on the course of the haze event in detail through analyses of the airborne and ground-based results.

Latitudinal variations of trace gas concentrations in the free troposphere measured by solar absorption spectroscopy during a ship cruise

The latitudinal variations of atmospheric trace gas column abundances have been measured during a ship cruise between 57°N and 45°S on the central Atlantic. The measurements were performed in October 1996 using high-resolution solar absorption spectroscopy in the infrared. The analysis method employed permits the retrieval of the total column densities of 20 different trace gases and for a few compounds the vertical mixing ratio profiles. For CH4 an interhemispheric difference of 3% was observed. The total columns of the shorter-lived trace gases CO and C2H6, analyzed between 57°N and 45°S, reveal a slight maximum in the tropics and a substantial increase north of 45°N. The total columns of C2H2 and HCN, detectable between 30°N and 30°S, reveal a maximum in the tropics of the Southern Hemisphere. For CH2O, studied between 57°N and 45°S, a well-pronounced maximum is observed in the tropics. The profile retrieval gives high mixing ratios for CO, C2H6, and O3 north of 40°N in the lower troposphere. In the tropics high concentrations are found for all three compounds in the entire troposphere, even above 12 km. The measurements have been used to estimate averaged mixing ratios of the trace gases for the free troposphere between 0 and 12 km. In the tropics the data give high values: for example, more than 200 pptv for HCN, 750 pptv for CH2O, 100 ppbv for CO and 100 pptv for C2H2. These values are comparable to or higher than what has been observed at midlatitudes, indicating the importance of biomass burning emissions on the tropospheric composition.

Wet deposition of metals to the tropical North and the South Atlantic ocean

Abstract The dominance of wet trace element deposition to the tropical North Atlantic Ocean over dry input is proven by analysis results of rain samples from shipboard sampling. Concentrations of Al, Ca, Cd, Co, Cu, Fe, Mg, Mn, Na, Ni, Pb, Tl and Zn were measured in 9 rain samples from the equatorial Atlantic Ocean and 4 rain samples from the South Atlantic Ocean. The results include some of the first data for trace metal concentrations in South Atlantic precipitation. Elemental concentrations from seawater samples taken in parallel are introduced to allow for an estimation of recycling rates of seasalt components. This revealed a net trace metal flux from the atmosphere into the ocean in the vicinity of the intertropical convergence zone (ITCZ). Recycling rates of Al, Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn ranged from a minimum of 7 × 10−4% (Co) to a maximum of 75% (Al). Higher values were found in the southwest Atlantic. Trace metal concentrations in southwestern Atlantic precipitation also seem to be connected to anthropogenic sources, as implied by enrichment factors relative to the earth crust. Data on trace metal flux by wet deposition are given on an annual scale for the ITCZ, further allowing for an estimation of elemental residence times in surface seawater.