Malte Julian Deventer

Urban eddy covariance measurements reveal significant missing NO x emissions in Central Europe

Nitrogen oxide (NOx) pollution is emerging as a primary environmental concern across Europe. While some large European metropolitan areas are already in breach of EU safety limits for NO2, this phenomenon does not seem to be only restricted to large industrialized areas anymore. Many smaller scale populated agglomerations including their surrounding rural areas are seeing frequent NO2 concentration violations. The question of a quantitative understanding of different NOx emission sources is therefore of immanent relevance for climate and air chemistry models as well as air pollution management and health. Here we report simultaneous eddy covariance flux measurements of NOx, CO2, CO and non methane volatile organic compound tracers in a city that might be considered representative for Central Europe and the greater Alpine region. Our data show that NOx fluxes are largely at variance with modelled emission projections, suggesting an appreciable underestimation of the traffic related atmospheric NOx input in Europe, comparable to the weekend-weekday effect, which locally changes ozone production rates by 40%.

One-year measurement of size-resolved particle fluxes in an urban area

Size-resolved particle flux measurements were carried out in an urban area from April 2012 to April 2013. Together with a standard eddy covariance system, two fast optical particle counters have been employed on a 65-meter-high tower in Münster, Germany. Particle number fluxes were directly calculated for particles with diameters from 0.06 to 10 µm within 16 individual size-bins. Whereas particle number concentrations show a distinct yearly pattern with maxima in winter and minima in summer, the flux time series is more multifaceted. Average daily maxima of 3.0e+07 particles m−2 s−1 occurred during winter while minima of 2.0e+06 particles m−2 s−1 were observed in fall. The size-resolved measurements revealed that during spring and summer a considerable number of accumulation mode particles deposits while a simultaneous net particle emission occurred, which is mostly driven by particles smaller than 0.12 µm. These bi-directional fluxes lead to a net mass deposition of up to 13.5 µg m−2 d−1. The tipping-point between the emission and deposition lay between 0.16 and 0.19 µm. In a comprehensive analysis of the flux and concentration time series, the degree of atmospheric stability, the seasons, and the type of source region have been identified as key influences for particle fluxes. Different responses between particle fluxes and concentrations have been found along these drivers.

Size-resolved flux measurement of sub-micrometer particles over an urban area

From April 11 th to May 27 th , 2011, the turbulent exchange of sub-micrometer particles between the urban surface and the urban boundary-layer was measured above the city area of Munster (NW Germany). The scope of the study is to examine the contributions of particles of different size classes to the total measured fluxes. Eddy-covariance measurements were performed at 65 m above ground. The particle concentrations in 99 size bins with particle diameters ranging from 55 to 1000 nm were measured with an optical particle spectrometer. For flux calculations we grouped these 99 original bins into 18 wider channels with an upper cut-off of 320 nm, and a further rather coarse channel for particles up to 1 lm. The overall results reveal that Munster is a relevant source of about 2.8 AE 10 8 particles m � 2 d � 1 on weekdays and 1.8 AE 10 8 particles m � 2 d � 1 on Sundays within the indicated size range. These emissions are predominantly driven by secondary particles of the Aitken mode, which are most likely caused by traffic. Hence traffic hotspots are a major contribution to the net fluxes. On the other hand, considering the mass fluxes, Munster is a sink of 0.53 l gm � 2 d � 1 on weekdays and 0.08 l gm � 2 d � 1 on Sundays. Here, mainly particles of the accumulation mode with diameters above 167 nm lead to deposition fluxes. Number and mass fluxes exhibit distinct daily and weekly patterns.

Canopy‐atmosphere interactions under foggy condition—Size‐resolved fog droplet fluxes and their implications

Microphysical processes of fog and their spatial and temporal pattern are a challenge to study under natural conditions. This work focuses on the development of bidirectional fluxes of fog droplets above a forest canopy in northeastern Taiwan. Bidirectional fluxes occurred regularly, start from the smallest droplet class (<2.66 µm diameter), and subsequently extend to larger droplets up to 7.41 µm diameter. The development of the bidirectional fluxes with positive (upward) fluxes of smaller droplets and downward fluxes of larger fluxes is associated with a temperature gradient and with the activation of fog droplets according to the Kohler theory. Small fog droplets develop close to the canopy as result of evapotranspiration and subsequent condensation. The rapid growth of small fog droplets and the accelerated growth of activated droplets, a process which is more likely to occur at higher levels of the fog layer, lead to a sink of small droplets and a source of larger droplets within the fog. This is in accordance with the observation that positive droplet number fluxes of small fog droplets outnumber the negative fluxes from the larger fog droplets. For liquid water, the net flux is negative.

Reactive Trace Gas and Aerosol Fluxes

Quantifying the atmosphere-surface exchange of reactive trace gases and aerosols is extremely important for a full understanding of biogeochemical cycles and their implications for air quality and climate. However, turbulent fluxes of reactive gases such as ozone and volatile organic compounds (VOC) as well as aerosol particles are still difficult to measure. Chemical reactions contribute to changes in trace gas or aerosol concentrations, and production or loss processes have to be carefully separated from turbulent transport. Also, for many trace gas measurements and for size-resolved and chemically speciated aerosol measurements, instruments are limited with respect to time resolution, sensitivity, and accuracy, which restricts their application in micrometeorological techniques. Here, we present flux measurements of reactive trace gases and aerosols above tall vegetation. We focus on ozone deposition and its implications for the NO/NO2/O3 triad, biogenic emissions of volatile organic compounds and their subsequent oxidation reactions, and finally, turbulent aerosol fluxes in a spruce forest ecosystem.