Franz Conen

Experimental assessment of N2O background fluxes in grassland systems

In the absence of, or between, fertilization events in agricultural systems, soils are generally assumed to emit N2O at a small rate, often described as the ‘background’ flux. In contrast, net uptake of N2O by soil has been observed in many field studies, but has not gained much attention. Observations of net uptake of N2O form a large fraction (about half) of all individual flux measurements in a long-term time series at our temperate fertilized grassland site. Individual uptake fluxes from chamber measurements are often not statistically significant but mean values integrated over longer time periods from days to weeks do show a clear uptake. An analysis of semi-continuous chamber flux data in conjunction with continuous measurements of the N2O concentration in the soil profile and eddy covariance measurements suggests that gross production and gross consumption of N2O are of the same order, and as consequence only a minor fraction of N2O molecules produced in the soil reaches the atmosphere.

Latitudinal distribution of radon-222 flux from continents

Abstract Global atmospheric transport models are frequently tested by using 222Rn as a tracer. Generally this tracer is assumed to be emitted at a uniform rate (1 atom cm−2 s−1) from all ice-free land surfaces. The analysis of published data suggests a strong decrease from 30°N northwards to 0.2 atom cm−2 s−1 at 70°N. This could be a result of increasing water tables and proportions of organic soils as indicated by larger proportions of wetlands in northern latitudes.

Analysis of long-term aerosol size distribution data from Jungfraujoch with emphasis on free tropospheric conditions, cloud influence, and air mass transport

Six years of aerosol size distribution measurements between 20 and 600 nm diameters and total aerosol concentration above 10 nm from March 2008 to February 2014 at the high-alpine site Jungfraujoch are presented. The size distribution was found to be typically bimodal with mode diameters and widths relatively stable throughout the year and the observation period. New particle formation was observed on 14.5% of all days without a seasonal preference. Particles typically grew only into the Aitken mode and did not reach cloud condensation nucleus (CCN) sizes on the time scale of several days. Growth of preexisting particles in the Aitken mode, on average, contributed very few CCN. We concluded that the dominant fraction of CCN at Jungfraujoch originated in the boundary layer. A number of approaches were used to distinguish free tropospheric (FT) conditions and episodes with planetary boundary layer (PBL) influence. In the absence of PBL injections, the concentration of particles larger than 90 nm (N90, roughly corresponding to the CCN concentration) reached a value ~40 cm−3 while PBL influence caused N90 concentrations of several hundred or even 1000 cm−3. Comparing three criteria for free tropospheric conditions, we found FT prevalence for 39% of the time with over 60% during winter and below 20% during summer. It is noteworthy that a simple criterion based on standard trace gas measurements appeared to outperform alternative approaches.

Atmospheric ice nuclei at the high-altitude observatory Jungfraujoch, Switzerland

Abstract The state of a slightly supercooled ephemeral cloud can be changed by the presence of a few particles capable of catalysing freezing, and potentially result in precipitation. We investigated the atmospheric abundance of particles active as ice nuclei at −8°C (IN−8) over the course of a year at the high-alpine station Jungfraujoch (3580 m.a.s.l., Switzerland) through the use of immersion freezing assays of particles collected on quartz micro-fibre filters. In addition, we determined IN−8 on a hill in the planetary boundary layer 95 km northwest of Jungfraujoch and in the dust laden Saharan Air Layer reaching Tenerife. Results indicate a strong seasonality of IN−8 at Jungfraujoch. Values were largest during summer (between 1 and 10 m−3) and about two orders of magnitude smaller during winter. Sahara dust events had a negligible influence on IN−8 at Jungfraujoch. Seasonality in the boundary layer was not observed in the upper, but in the lower bound of IN−8 values. Values<1 m−3 were only found on cold winter days, when IN−8 were more likely to have already been activated and deposited than on warmer days. A good correlation between IN−8 and maximum daily temperature at Jungfraujoch (R2=0.54) suggests IN−8 abundance at Jungfraujoch may be limited most of the year by microphysical processing related to IN activation in approaching air masses.

Ice nucleation active particles are efficiently removed by precipitating clouds

Ice nucleation in cold clouds is a decisive step in the formation of rain and snow. Observations and modelling suggest that variations in the concentrations of ice nucleating particles (INPs) affect timing, location and amount of precipitation. A quantitative description of the abundance and variability of INPs is crucial to assess and predict their influence on precipitation. Here we used the hydrological indicator δ18O to derive the fraction of water vapour lost from precipitating clouds and correlated it with the abundance of INPs in freshly fallen snow. Results show that the number of INPs active at temperatures ≥ −10 °C (INPs−10) halves for every 10% of vapour lost through precipitation. Particles of similar size (>0.5 μm) halve in number for only every 20% of vapour lost, suggesting effective microphysical processing of INPs during precipitation. We show that INPs active at moderate supercooling are rapidly depleted by precipitating clouds, limiting their impact on subsequent rainfall development in time and space.

Seasonal variations in stable carbon and hydrogen isotope ratios in methane from rice fields

[l] During two successive growing seasons, methane emissions from rice fields in Italy were measured. High-precision measurements of the methane 13 C/ 12 C and D/H ratios were carried out by mass spectrometry and tunable diode laser absorption spectrometry. Significant seasonal variations were found for both δ 13 C and δD. The results confirm earlier observations by Bergamaschi [1997] in finding a seasonal cycle with isotopically depleted methane in the main growing season and higher values at the beginning and the end of the season during drainage of the field. The measured δ 13 C diurnal cycles showed a strong correlation with the methane emission rate. The isotopic composition of methane, which depended on the season, can be explained by variations of the different pathways for methane production, oxidation, and release into the atmosphere. A model based on these parameters was able to reproduce the field measurements and indicate the principal causes of observed fluctuations in the isotopic methane composition.

Radon as a tracer of atmospheric influences on traffic-related air pollution in a small inland city

One year of radon, benzene and carbon monoxide (CO) concentrations were analysed to characterise the combined influences of variations in traffic density and meteorological conditions on urban air quality in Bern, Switzerland. A recently developed radon-based stability categorisation technique was adapted to account for seasonal changes in day length and reduction in the local radon flux due to snow/ice cover and high soil moisture. Diurnal pollutant cycles were shown to result from an interplay between variations in surface emissions (traffic density), the depth of the nocturnal atmospheric mixing layer (dilution) and local horizontal advection of cleaner air from outside the central urban/industrial area of this small compact inland city. Substantial seasonal differences in the timing and duration of peak pollutant concentrations in the diurnal cycle were attributable to changes in day length and the switching to/from daylight-savings time in relation to traffic patterns. In summer, average peak benzene concentrations (0.62 ppb) occurred in the morning and remained above 0.5 ppb for 2 hours, whereas in winter average peak concentrations (0.85 ppb) occurred in the evening and remained above 0.5 ppb for 9 hours. Under stable conditions in winter, average peak benzene concentrations (1.1 ppb) were 120% higher than for well-mixed conditions (0.5 ppb). By comparison, summertime peak benzene concentrations increased by 53% from well-mixed (0.45 ppb) to stable nocturnal conditions (0.7 ppb). An idealised box model incorporating a simple advection term was used to derive a nocturnal mixing length scale based on radon, and then inverted to simulate diurnal benzene and CO emission variations at the city centre. This method effectively removes the influences of local horizontal advection and stability-related vertical dilution from the emissions signal, enabling a direct comparison with hourly traffic density. With the advection term calibrated appropriately, excellent results were obtained, with high regression coefficients in spring and summer for both benzene (r2 ~0.90–0.96) and CO (r2 ~0.88–0.98) in the two highest stability categories. Weaker regressions in winter likely indicate additional contributions from combustion sources unrelated to vehicular emissions. Average vehicular emissions during daylight hours were estimated to be around 0.503 (542) kg km−2 h−1 for benzene (CO) in the Bern city centre.

Predicting abundance and variability of ice nucleating particles in precipitation at the high-altitude observatory Jungfraujoch

Abstract. Nucleation of ice affects the properties of clouds and the formation of precipitation. Quantitative data on how ice nucleating particles (INPs) determine the distribution, occurrence and intensity of precipitation are still scarce. INPs active at −8 °C (INPs−8) were observed for 2 years in precipitation samples at the High-Altitude Research Station Jungfraujoch (Switzerland) at 3580 m a.s.l. Several environmental parameters were scanned for their capability to predict the observed abundance and variability of INPs−8. Those singularly presenting the best correlations with observed number of INPs−8 (residual fraction of water vapour, wind speed, air temperature, number of particles with diameter larger than 0.5 µm, season, and source region of particles) were implemented as potential predictor variables in statistical multiple linear regression models. These models were calibrated with 84 precipitation samples collected during the first year of observations; their predictive power was successively validated on the set of 15 precipitation samples collected during the second year. The model performing best in calibration and validation explains more than 75 % of the whole variability of INPs−8 in precipitation and indicates that a high abundance of INPs−8 is to be expected whenever high wind speed coincides with air masses having experienced little or no precipitation prior to sampling. Such conditions occur during frontal passages, often accompanied by precipitation. Therefore, the circumstances when INPs−8 could be sufficiently abundant to initiate the ice phase in clouds may frequently coincide with meteorological conditions favourable to the onset of precipitation events.

Net CO2 surface emissions at Bern, Switzerland inferred from ambient observations of CO2, δ(O2/N2), and 222Rn using a customized radon tracer inversion

The 222Radon tracer method is a powerful tool to estimate local and regional surface emissions of, e.g., greenhouse gases. In this paper we demonstrate that in practice, the method as it is commonly used, produces inaccurate results in case of nonhomogeneously spread emission sources, and we propose a different approach to account for this. We have applied the new methodology to ambient observations of CO2 and 222Radon to estimate CO2 surface emissions for the city of Bern, Switzerland. Furthermore, by utilizing combined measurements of CO2 and δ(O2/N2) we obtain valuable information about the spatial and temporal variability of the main emission sources. Mean net CO2 emissions based on 2 years of observations are estimated at (11.2 ± 2.9) kt km−2 a−1. Oxidative ratios indicate a significant influence from the regional biosphere in summer/spring and fossil fuel combustion processes in winter/autumn. Our data indicate that the emissions from fossil fuels are, to a large degree, related to the combustion of natural gas which is used for heating purposes.

Test of a northwards‐decreasing 222Rn source term by comparison of modelled and observed atmospheric 222Rn concentrations

Model-predicted atmospheric concentrations of 222Rn based on two different 222Rn source terms have been compared with observations in the lower troposphere. One simulation used a globally uniform 222Rn source term from ice-free land surfaces of 1 atom cm−2 s−1; the other assumed a northwards-decreasing source term (linear decrease from 1 atom cm−2 s−1 at 30°N to 0.2 atom cm−2 s−1at 70°N). Zero emissions were assigned to oceans. The northwards-decreasing source term improved predictions at four out of six stations north of 50°N, reducing the mean prediction/observation ratio from 2.8 to 0.87. In the latitudinal band between 30°N and 50°N, the northwards-decreasing source term resulted in systematic under-prediction of atmospheric 222Rn, whereas the uniform source term provided predictions close to observations. Predictions based on the northwards-decreasing source term were significantly (p < 0.01) better than those based on the uniform source term for an averaged vertical 222Rn profile around 44°N, but were not for one around 38°N. The results indicate that a northwards-decreasing source term could be a more realistic representation of actual 222Rn emissions than a uniform 1 atom cm−2 s−1 source term. However, the decrease in 222Rn source strength with increasing latitude might not begin at 30°N but somewhat further north. This hypothesis should be investigated through model-independent means.