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The multi-seasonal NOy budget in coastal Antarctica and its link with surface snow and ice core nitrate: results from the CHABLIS campaign

Measurements of a suite of individual NOy components were carried out at Halley station in coastal Antarctica as part of the CHABLIS campaign (Chemistry of the Antarctic Boundary Layer and the Interface with Snow). Conincident measurements cover over half a year, from austral winter 2004 through to austral summer 2005. Results show clear dominance of organic NOy compounds (PAN and MeONO2) during the winter months, with low concentrations of inorganic NOy. During summer, concentrations of inorganic NOy compounds are considerably greater, while those of organic compounds, although lower than in winter, are nonetheless significant. The relative concentrations of the alkyl nitrates, as well as their seasonality, are consistent with an oceanic source. Multi-seasonal measurements of surface snow nitrate correlate strongly with inorganic NOy species (especially HNO3) rather than organic. One case study in August suggested that, on that occasion, particulate nitrate was the dominant source of nitrate to the snowpack, but this was not the consistent picture throughout the measurement period. An analysis of NOx production rates showed that emissions of NOx from the snowpack overwhelmingly dominate over gas-phase sources. This result suggests that, for certain periods in the past, the flux of NOx into the Antarctic boundary layer can be calculated from ice core nitrate data.
Carbon monoxide (CO) and ethane (C2H6) trends from ground-based solar FTIR measurements at six European stations, comparison and sensitivity analysis with the EMEP model

Trends in the CO andC2H6 partial columns ~0–15 km) have been estimated from four European ground-based solar FTIR (Fourier Transform InfraRed) stations for the 1996–2006 time period. The CO trends from the four stations Jungfraujoch, Zugspitze, Harestua and Kiruna have been estimated to −0.45 ± 0.16% yr−1, −1.00 ± 0.24% yr−1, −0.62 ± 0.19 % yr−1 and −0.61 ± 0.16% yr−1, respectively. The corresponding trends for C2H6 are −1.51 ± 0.23% yr−1, −2.11 ± 0.30% yr−1, −1.09 ± 0.25% yr−1 and −1.14 ± 0.18% yr−1. All trends are presented with their 2-σ confidence intervals. To find possible reasons for the CO trends, the global-scale EMEP MSC-W chemical transport model has been used in a series of sensitivity scenarios. It is shown that the trends are consistent with the combination of a 20% decrease in the anthropogenic CO emissions seen in Europe and North America during the 1996–2006 period and a 20% increase in the anthropogenic CO emissions in East Asia, during the same time period. The possible impacts of CH4 and biogenic volatile organic compounds (BVOCs) are also considered. The European and global-scale EMEP models have been evaluated against the measured CO and C2H6 partial columns from Jungfraujoch, Zugspitze, Bremen, Harestua, Kiruna and Ny-Ålesund. The European model reproduces, on average the measurements at the different sites fairly well and within 10–22% deviation for CO and 14–31% deviation for C2H6. Their seasonal amplitude is captured within 6–35% and 9–124% for CO and C2H6, respectively. However, 61–98% of the CO and C2H6 partial columns in the European model are shown to arise from the boundary conditions, making the global-scale model a more suitable alternative when modeling these two species. In the evaluation of the global model the average partial columns for 2006 are shown to be within 1–9% and 37–50% of the measurements for CO and C2H6, respectively. The global model sensitivity for assumptions made in this paper is also analyzed.
A global climatology of tropospheric and stratospheric ozone derived from Aura OMI and MLS measurements

A global climatology of tropospheric and stratospheric column ozone is derived by combining six years of Aura Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) ozone measurements for the period October 2004 through December 2010. The OMI/MLS tropospheric ozone climatology exhibits large temporal and spatial variability which includes ozone accumulation zones in the tropical south Atlantic year-round and in the subtropical Mediterranean/Asia region in summer months. High levels of tropospheric ozone in the Northern Hemisphere also persist in mid-latitudes over the eastern part of the North American continent extending across the Atlantic Ocean and the eastern part of the Asian continent extending across the Pacific Ocean. For stratospheric ozone climatology from MLS, largest column abundance is in the Northern Hemisphere in the latitude range 70° N–80° N in February–April and in the Southern Hemisphere around 40° S–50° S during August–October. Largest stratospheric ozone lies in the Northern Hemisphere and extends from the eastern Asian continent eastward across the Pacific Ocean and North America. With the advent of many newly developing 3-D chemistry and transport models it is advantageous to have such a dataset for evaluating the performance of the models in relation to dynamical and photochemical processes controlling the ozone distributions in the troposphere and stratosphere. The OMI/MLS gridded ozone climatology data are made available to the science community via the NASA Goddard Space Flight Center ozone and air quality website <a href = "http://ozoneaq.gsfc.nasa.gov/"target="_blank">http://ozoneaq.gsfc.nasa.gov/</a>.
Large-Eddy Simulation of a microburst

The three-dimensional structure and evolution of an isolated and stationary microburst are simulated using a time-dependent, high resolution Large-Eddy-Simulation (LES) model. The microburst is initiated by specifying a simplified cooling source at the top of the domain around 2 km a.g.l. that leads to a strong downdraft. Surface winds of the order of 30 m s−1 were obtained over a region of 500 m radius around the central point of the impinging downdraft, with the simulated microburst lasting for a few minutes. These characteristic length and time scales are consistent with results obtained from numerical simulations of microbursts using cloud-resolving models. The simulated flow replicated some of the principal features of microbursts observed by Doppler radars: in particular, the horizontal spread of strong surface winds and a ring vortex at the leading edge of the cold outflow. In addition to the primary surface outflow, the simulation also generated a secondary surge of strong winds that appears to represent a pulsation in the microburst evolution. These results highlight the capability of LES to reproduce complex phenomena like microbursts, indicating the potential usage of LES models to represent atmospheric phenomena of time and space scales between the convective scale and the microscale. These include short-lived convectively-generated damaging winds.
Microphysical simulations of new particle formation in the upper troposphere and lower stratosphere

Using a three-dimensional general circulation model with sulfur chemistry and sectional aerosol microphysics (WACCM/CARMA), we studied aerosol formation and microphysics in the upper troposphere and lower stratosphere (UTLS) as well as the middle and upper stratosphere based on three nucleation schemes (two binary homogeneous schemes and an ion-mediated scheme related to one of the binary schemes). Simulations suggest that ion-mediated nucleation rates in the UTLS are 25 % higher than its related binary scheme, but that the rates predicted by the two binary schemes vary by two orders of magnitude. None of the nucleation schemes is superior at matching the limited observations available at the smallest sizes. However, it is found that coagulation, not nucleation, controls number concentration at sizes greater than approximately 10 nm. Therefore, based on this study, processes relevant to atmospheric chemistry and radiative forcing in the UTLS are not sensitive to the choice of nucleation schemes. The dominance of coagulation over other microphysical processes in the UTLS is consistent with other recent work using microphysical models. Simulations using all three nucleation schemes compare reasonably well to observations of size distributions, number concentration across latitude, and vertical profiles of particle mixing ratio in the UTLS. Interestingly, we find that we need to include Van der Waals forces in our coagulation scheme to match the UTLS aerosol concentrations. We conclude that this model can reasonably represent sulfate microphysical processes in the UTLS, and that the properties of particles at atmospherically relevant sizes appear to be insensitive to the details of the nucleation scheme. We also suggest that micrometeorites, which are not included in this model, dominate the aerosol properties in the upper stratosphere above about 30 km.
Trace gas fluxes of CO2, CH4 and N2O in a permanent grassland soil exposed to elevated CO2 in the Giessen FACE study

Long-term field observations showed that N2O fluxes observed shortly after N application were not significantly affected by elevated CO2 in the Giessen Free Air Carbon dioxide Enrichment (FACE) study. To further investigate this unexpected result a 15N tracer study was carried out under controlled conditions where in parallel treatments either the NH4+ pool (15NH4NO3) or the NO3− pool (NH415NO3) was enriched with 15N. Fluxes of CO2, CH4, and N2O as well as the 15N enrichment of the N2O were measured. Denitrifying Enzyme Activity (DEA), total denitrification (N2 + N2O) and N2-to-N2O ratios were quantified in separate experiments. Over the 57 day incubation, N2O fluxes averaged 0.090 ng N2O-N g−1 h−1 under ambient and 0.083 ng N2O-N g−1 h−1 under elevated CO2 (not significantly different). The N2O production processes were identified by a two-source model. Results showed that N2O must have also been produced by a third source – possibly related to organic N transformation – which was stimulated by elevated CO2. Soil CO2 fluxes were approximately 20 % higher under elevated CO2 than soil from ambient but the differences were not significant. CH4 oxidation rates were on average −1.75 ng CH4-C g−1 h−1 in the elevated and −1.17 ng CH4-C g−1 h−1 in the ambient indicating that elevated CO2 increased the CH4 oxidation by 49 % compared to ambient CO2 under controlled conditions. N fertilization increased CH4 oxidation by 3-fold in both CO2 treatments. CO2 did not have any significant effect on DEA while total denitrification and N2-to-N2O ratios increased by 36 and 33 %, respectively. The results indicate that shortly after N application elevated CO2 must have stimulated both the N2O production and reduction to N2 to explain the increased N2-to-N2O ratio and at the same time explain the non-responsiveness of the N2O emissions. Thus, the observed variation of the CO2 effect on N2O emissions throughout the year is possibly governed by the dynamics of the N2O reductase activity.
High-ozone layers in the middle and upper troposphere above Central Europe: potential import from the stratosphere along the subtropical jet stream

Specific very dry high-ozone layers, starting roughly two days after the onset of high-pressure periods during the warm season, have been reproducibly observed in the middle and upper troposphere with the ozone lidar in Garmisch-Partenkirchen (Germany). These episodes, previously not understood, were recently analysed based on extending backward simulations with the FLEXPART particle dispersion model to as many as twenty days and on jet-stream analyses including calculations with the LAGRANTO transport model. In all six cases analysed the model results indicate ozone import from the stratosphere on an extremely long path along the subtropical jet stream over the Pacific Ocean, Asia and, in part, all the way back to the Atlantic Ocean. The analysis suggests that stratospheric influence is the most important factor for the increase in ozone and is related to rather shallow transfer of air from the stratosphere into the upper- and mid-tropospheric air streams observed with the lidar. Contributions from the boundary layers of East Asia and North America are just occasionally present, in one case documented by a very dense aerosol plume from the Asian deserts. The considerable vertical and temporal extent of many of these layers and peak ozone mixing ratios between 80 and 150 ppb suggest that the observations are related to an important mechanism for stratosphere-to-troposphere transport (STT) and also confirm the model predictions of pronounced and persistent STT along the subtropical jet stream.
Source apportionment of the carbonaceous aerosol in Norway – quantitative estimates based on 14C, thermal-optical and organic tracer analysis

In the present study, source apportionment of the ambient summer and winter time particulate carbonaceous matter (PCM) in aerosol particles (PM1 and PM10) has been conducted for the Norwegian urban and rural background environment. Statistical treatment of data from thermal-optical, 14C and organic tracer analysis using Latin Hypercube Sampling has allowed for quantitative estimates of seven different sources contributing to the ambient carbonaceous aerosol. These are: elemental carbon from combustion of biomass (ECbb) and fossil fuel (ECff), primary and secondary organic carbon arising from combustion of biomass (OCbb) and fossil fuel (OCff), primary biological aerosol particles (OCPBAP, which includes plant debris, OCpbc, and fungal spores, OCpbs), and secondary organic aerosol from biogenic precursors (OCBSOA). Our results show that emissions from natural sources were particularly abundant in summer, and with a more pronounced influence at the rural compared to the urban background site. 80% of total carbon (TCp, corrected for the positive artefact) in PM10 and ca. 70% of TCpin PM1 could be attributed to natural sources at the rural background site in summer. Natural sources account for about 50% of TCp in PM10 at the urban background site as well. The natural source contribution was always dominated by OCBSOA, regardless of season, site and size fraction. During winter anthropogenic sources totally dominated the carbonaceous aerosol (80–90%). Combustion of biomass contributed slightly more than fossil-fuel sources in winter, whereas emissions from fossil-fuel sources were more abundant in summer. Mass closure calculations show that PCM made significant contributions to the mass concentration of the ambient PM regardless of size fraction, season, and site. A larger fraction of PM1 (ca. 40–60%) was accounted for by carbonaceous matter compared to PM10 (ca. 40–50%), but only by a small margin. In general, there were no pronounced differences in the relative contribution of carbonaceous matter to PM with respect to season or between the two sites.
Particle size distribution factor as an indicator for the impact of the Eyjafjallajökull ash plume at ground level in Augsburg, Germany

During the time period of the Eyjafjallajökull volcano eruption in 2010 increased mass concentration of PM10 (particulate matter, diameter <10 μm) were observed at ground level in Augsburg, Germany. In particular on 19 and 20 April 2010 the daily PM10 limit value of 50 μg m−3 was exceeded. Because ambient particles are in general a complex mixture originating from different sources, a source apportionment method (positive matrix factorization (PMF)) was applied to particle size distribution data in the size range from 3 nm to 10 μm to identify and estimate the volcanic ash contribution to the overall PM10 load in the ambient air in Augsburg. A PMF factor with relevant particle mass concentration in the size range between 1 and 4 μm (maximum at 2 μm) was associated with long range transported dust. This factor increased from background concentration to high levels simultaneously with the arrival of the volcanic ash plume in the planetary boundary layer. Hence, we assume that this factor could be used as an indicator for the impact of the Eyjafjallajökull ash plume on ground level in Augsburg. From 17 to 22 April 2010 long range transported dust factor contributed on average 30 % (12 μg m−3) to PM10. On 19 April 2010 at 20:00 UTC+1 the maximum percentage of the long range transported dust factor accounted for around 65 % (35 μg m−3) to PM10 and three hours later the maximum absolute value with around 48 μg m−3 (61 %) was observed. Additional PMF analyses for a Saharan dust event occurred in May and June 2008 suggest, that the long range transported dust factor could also be used as an indicator for Saharan dust events.
Variations in time and space of trace metal aerosol concentrations in urban areas and their surroundings

Using an unprecedentedly large geochemical database, we compare temporal and spatial variations in inhalable trace metal background concentrations in a major city (Barcelona, Spain) and at a nearby mountainous site (Montseny) affected by the urban plume. Both sites are contaminated by technogenic metals, with V, Pb, Cu, Zn, Mn, Sn, Bi, Sb and Cd all showing upper continental crust (UCC) normalised values >1 in broadly increasing order. The highest metal concentrations usually occur during winter at Barcelona and summer in Montseny. This seasonal difference was especially marked at the remote mountain site in several elements such as Ti and Rare Earth Elements, which recorded campaign maxima, exceeding PM10 concentrations seen in Barcelona. The most common metals were Zn, Ti, Cu, Mn, Pb and V. Both V and Ni show highest concentrations in summer, and preferentially fractionate into the finest PM sizes (PM1/PM10 > 0.5) especially in Barcelona, this being attributed to regionally dispersed contamination from fuel oil combustion point sources. Within the city, hourly metal concentrations are controlled either by traffic (rush hour double peak for Cu, Sb, Sn, Ba) or industrial plumes (morning peak of Ni, Mn, Cr generated outside the city overnight), whereas at Montseny metal concentrations rise during the morning to a single, prolonged afternoon peak as contaminated air transported by the sea breeze moves into the mountains. Our exceptional database, which includes hourly measurements of chemical concentrations, demonstrates in more detail than previous studies the spatial and temporal variability of urban pollution by trace metals in a given city. Technogenic metalliferous aerosols are commonly fine in size and therefore potentially bioavailable, emphasising the case for basing urban background PM characterisation not only on physical parameters such as mass but also on sample chemistry and with special emphasis on trace metal content.