Peter Hofer

Summertime NO y speciation at the Jungfraujoch, 3580 m above sea level, Switzerland

During summer 1997, speciated reactive nitrogen (NO, NO2, peroxyacetyl nitrate (PAN), HNO3, and particulate nitrate) was measured in conjunction with total reactive nitrogen (NOy) at the high-alpine research station Jungfraujoch (JFJ), 3580 m above sea level (asl). The individually measured NOy components averaged to 82% of total NOy. PAN was the most abundant reactive nitrogen compound and composed on average 36% of NOy, followed by NOx, (22%), particulate nitrate (17%), and HNO3 (7%). The NOx/NOy ratio averaged 0.25, but significantly lower values (0.15–0.20) were observed in the presence of high NOy mixing ratios. A classification of the data by synoptic weather conditions indicated that thermally driven vertical transport has a strong impact on the mixing ratios measured at the JFJ during summer. A strong diurnal cycle with maximum mixing ratios in the late afternoon was observed for convective days with north-westerly advection at 500 hPa. In contrast, during a period of convective days with a wind speed below 7.5 m s−1 at 500 hPa, no obvious diurnal cycle was observed. Under these meteorological conditions the convective boundary layer can be significantly higher over the Alps (i.e., around 4 km asl) than over the surrounding lowlands. Subsequent advection may finally result in the export of reactive nitrogen reservoir compounds to the free troposphere and hence influence global atmospheric chemistry.

The anthropogenic contribution to isoprene concentrations in a rural atmosphere

Abstract Atmospheric hydrocarbons are continuously monitored at the rural site of Taenikon, Switzerland. As expected for a rural area, highest isoprene concentrations are found in summer. However, elevated concentrations are also measured on some occasions in winter, in particular during events with long-lasting surface inversions, temperatures constantly below 0°C and snow covering the vegetation. During such events, concentrations of isoprene are strongly correlated with those of 1,3-butadiene, a substance that is mainly due to human activities. For these periods, a molar ratio between the concentrations of isoprene and those of 1,3-butadiene of 0.42 is observed. This value, together with the concentrations of 1,3-butadiene, is used to estimate the anthropogenic fraction of the atmospheric isoprene for the whole of 1997. It is found that the fraction is close to 100% in January–February and again in November–December. On the other hand, as early as March, a considerable amount of the observed isoprene appears to be of biogenic origin, although isoprene emissions by trees are negligible. The relative anthropogenic contribution is minimal in midsummer, when biogenic emissions are highest. For this time of the year, the anthropogenic contribution is largest during the early morning hours, in agreement with the traffic peak on nearby country roads.

NOy speciation with a combined wet effluent diffusion denuder-aerosol collector coupled to ion chromatography

A wet effluent denuder - aerosol collector (WEDD/AC) system coupled to ion chromatography for the measurement of atmospheric HONO, HNO3 and particulate nitrite, nitrate and sulfate is described. Several experiments were performed to outline its performance. The main features are low detection limits and a fast response to concentration changes which enables measurements with high time resolution. In contrast to highly soluble gases, the collection efficiency of less soluble gases is shown to depend on the Henry’s law constant rather than on the uptake kinetics. To improve the collection efficiency for HONO under simultaneous presence of acidifying gases, NaHCO3 was added to the effluent solution. The system was tested in a field campaign in the suburban area of Zurich, Switzerland. Elevated concentrations of nitrous acid up to 3.2 ppb were detected during the measurement campaign. The diurnal variation of the HONO to NO2 ratio clearly points to a fast and persistent process producing HONO in the atmosphere. The correlation with NOx and black carbon suggests a heterogeneous formation of HONO, and is consistent with a reaction on soot aerosol particle surfaces postulated from previous laboratory results.

Variability of trace gases at the high-Alpine site Jungfraujoch caused by meteorological transport processes

The influence of meteorological transport processes on trace gas concentrations at the high-Alpine site Jungfraujoch (3580 m above sea level) is investigated. These processes are subdivided into two categories: thermally driven transport, which takes place on a local scale, and transport on a regional (fohn) or synoptic scale (fronts). During thermally induced processes, which are mainly observed in summer and spring, the carbon monoxide (CO) mixing ratios show a diurnal variation with a maximum concentration at ≈ 1800 local time. Processes on the regional and synoptic scale which occur during the whole season induce large variability in the trace gas observations. The CO concentrations can be twice as large as the monthly median value during severe fohn episodes. The NOx values exhibit an even larger increase. Depending on the season, the O3 observations during these periods show an increase in summer and a pronounced decrease in winter. In addition, it is found that the calculated trajectories using the output of a mesoscale weather prediction model are a useful tool to detect and explain episodes with high NOx or CO concentrations. Altogether, the present results imply that meteorological transport processes on different spatial and timescales are important for the interpretation of the trace gas observations at the high-Alpine site Jungfraujoch during the whole year.