E. Nemitz

Measurements and parameterizations of small aerosol deposition velocities to grassland, arable crops, and forest: Influence of surface roughness length on deposition

New micrometeorological measurements of small (0.1-0.2 μ diameter) aerosol particle fluxes using the eddy correlation technique are presented for moorland and also for grassland vegetation, the latter measurements being made both before and after cutting of the grassland to observe the resultant change in particle deposition velocity. These data are considered together with previously reported and reanalyzed micrometeorological measurements, again using the eddy correlation technique, for a number of different surface types, including arable crops and forest. Differences in observed surface deposition velocities, vds, due to the different surface roughnesses are highlighted. It was found that the various data sets showed a wholly consistent behavior when ensemble averages over the typical range of atmospheric stability ranges are considered in order to reduce the scatter inherent in these types of measurements. A working parameterization of surface deposition velocity in terms of the surfaces roughness length, z0, is presented. This is then extended for different atmospheric stabilities, using the parameterization suggested by Lamaud et al. [1994c], to yield vds/u* = k1 + k2 (-300 z/L 2/3, where k1 = k1 = 0.001222 log(z0) + 0.003906, k2 = 0.0009, where z is the measurement height, L is the Obukhov stability length, and u* is the local friction speed. The new data are finally compared to current analytical model descriptions of the deposition process, highlighting deficiencies in our understanding of the surface collection efficiency even for these small particles. Copyright 2002 by the American Geophysical Union.

Nitrogen management is essential to prevent tropical oil palm plantations from causing ground-level ozone pollution

More than half the worlds rainforest has been lost to agriculture since the Industrial Revolution. Among the most widespread tropical crops is oil palm (Elaeis guineensis): global production now exceeds 35 million tonnes per year. In Malaysia, for example, 13% of land area is now oil palm plantation, compared with 1% in 1974. There are enormous pressures to increase palm oil production for food, domestic products, and, especially, biofuels. Greater use of palm oil for biofuel production is predicated on the assumption that palm oil is an “environmentally friendly” fuel feedstock. Here we show, using measurements and models, that oil palm plantations in Malaysia directly emit more oxides of nitrogen and volatile organic compounds than rainforest. These compounds lead to the production of ground-level ozone (O3), an air pollutant that damages human health, plants, and materials, reduces crop productivity, and has effects on the Earths climate. Our measurements show that, at present, O3 concentrations do not differ significantly over rainforest and adjacent oil palm plantation landscapes. However, our model calculations predict that if concentrations of oxides of nitrogen in Borneo are allowed to reach those currently seen over rural North America and Europe, ground-level O3 concentrations will reach 100 parts per billion (109) volume (ppbv) and exceed levels known to be harmful to human health. Our study provides an early warning of the urgent need to develop policies that manage nitrogen emissions if the detrimental effects of palm oil production on air quality and climate are to be avoided.

Towards a climate-dependent paradigm of ammonia emission and deposition

Existing descriptions of bi-directional ammonia (NH3) land–atmosphere exchange incorporate temperature and moisture controls, and are beginning to be used in regional chemical transport models. However, such models have typically applied simpler emission factors to upscale the main NH3 emission terms. While this approach has successfully simulated the main spatial patterns on local to global scales, it fails to address the environment- and climate-dependence of emissions. To handle these issues, we outline the basis for a new modelling paradigm where both NH3 emissions and deposition are calculated online according to diurnal, seasonal and spatial differences in meteorology. We show how measurements reveal a strong, but complex pattern of climatic dependence, which is increasingly being characterized using ground-based NH3 monitoring and satellite observations, while advances in process-based modelling are illustrated for agricultural and natural sources, including a global application for seabird colonies. A future architecture for NH3 emission–deposition modelling is proposed that integrates the spatio-temporal interactions, and provides the necessary foundation to assess the consequences of climate change. Based on available measurements, a first empirical estimate suggests that 5°C warming would increase emissions by 42 per cent (28–67%). Together with increased anthropogenic activity, global NH3 emissions may increase from 65 (45–85) Tg N in 2008 to reach 132 (89–179) Tg by 2100.

A review of measurement and modelling results of particle atmosphere–surface exchange

Atmosphere–surface exchange represents one mechanism by which atmospheric particle mass and number size distributions are modified. Deposition velocities (vd) exhibit a pronounced dependence on surface type, due in part to turbulence structure (as manifest in friction velocity), with minima of approximately 0.01 and 0.2 cm s-1 over grasslands and 0.1–1 cm s-1 over forests. However, as noted over 20 yr ago, observations over forests generally do not support the pronounced minimum of deposition velocity (vd) for particle diameters of 0.1–2 μm as manifest in theoretical predictions. Closer agreement between models and observations is found over less-rough surfaces though those data also imply substantially higher surface collection efficiencies than were originally proposed and are manifest in current models. We review theorized dependencies for particle fluxes, describe and critique model approaches and innovations in experimental approaches, and synthesize common conclusions of experimental and modelling studies. We end by proposing a number of research avenues that should be pursued in to facilitate further insights and development of improved numerical models of atmospheric particles.

Resistance modelling of ammonia exchange over oilseed rape

Ammonia (NH3) surface/atmosphere exchange is bi-directional and as such resistance models must include canopy concentrations. An existing single layer model that describes the exchange in terms of adsorption to leaf cuticles and bi-directional transport through leaf stomata, which is governed by a stomatal compensation point ( s), is applied here to NH3 exchange over oilseed rape and compared with measured fluxes. For the first time the model is tested using values of s based on the apoplastic ratio [NH4 C ]=pH ( s) measured directly in the field. Strong NH 3 emission from decomposing leaf litter at the ground and the likelihood of high [NH4 C ] in the siliques complicate the exchange pattern with oilseed rape and limit the application of the original model. This is therefore extended by: (a) the inclusion of a litter layer (2-layer model), with an emission potential ( l), (b) additionally dividing the plant canopy into a foliage- and a silique-layer (3-layer model) and (c) considering the relative humidity (h) dependency of l. The 2-layer model is able to predict night-time emission, but daytime emission is estimated to originate from the litter layer, which is in contradiction to the NH 3 sources and sinks derived for this canopy. The 3-layer model using a constant value of l requires an emission potential for the siliques of about 1300, which is consistent with bioassay estimates. Together with a parameterization of l that increases with h this model indicates that during daytime emission originates from the siliques, in agreement with the source/sink analysis. It is concluded that the leaf stomata were an effective NH3 sink, whereas the leaf litter dominates night-time emissions and the silique-layer (probably) daytime emissions. Although the 2-layer model reproduces the net exchange, the 3-layer model appears to be the mechanistically more accurate description.

Biotic, Abiotic, and Management Controls on the Net Ecosystem CO2 Exchange of European Mountain Grassland Ecosystems

The net ecosystem carbon dioxide (CO2) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO2 uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BMag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers.

Direct measurements and parameterisation of aerosol flux, concentration and emission velocity above a city

Articles have recently been published on aerosol size distributions and number concentrations in cities, however there have been no studies on transport of these particles. Eddy covariance measurements of vertical transport of aerosol in the size range 11 nm<Dp<3 μm are presented here. The analysis shows that typical average aerosol number fluxes in this size range vary between 9000 and 90,000 cm−2 s−1. With concentrations between 3000 and 20,000 cm−3 this leads to estimates of particle emission velocity between 20 and 75 mm s−1. The relationships between number flux and traffic activity, along with emission velocity and boundary layer stability are demonstrated and parameterised. These are used to derive an empirical parameterisation for aerosol concentration in terms of traffic activity and stability. The main processes determining urban aerosol fluxes and concentrations are discussed and quantified where possible. The difficulties in parameterising urban activity are discussed

Eddy covariance fluxes of peroxyacetyl nitrates (PANs) and NOy to a coniferous forest

up to approximately � 14 ng N m � 2 s � 1 . The average daytime flux peaked at � 6.0 ng N m � 2 s � 1 and accounted for � 20% of the daytime NOy flux. Calculations suggest minimum daytime surface resistances for PAN in the range of 70–130 s m � 1 .I t was estimated that approximately half of daytime uptake was through plant stomates. Average PAN deposition velocities, Vd(PAN), showed a daytime maximum of � 10.0 mm s � 1 ; however, deposition did not cease during nighttime periods. Vd(PAN) was highly variable at night and increased when canopy elements were wet from either precipitation or dew formation. Diel patterns of deposition velocity of MPAN and PPN were similar to that of PAN. These results suggest that deposition of PAN, at least to coniferous forest canopies, is much faster than predicted with current deposition algorithms. Although deposition of PAN is unlikely to compete with thermal dissociation during warm summer periods, it will likely play an important role in removing PAN from the atmosphere in colder regions or during winter. The fate of PAN at the surface and within the plants remains unknown, but may present a previously ignored source of nitrogen to ecosystems.

An Eddy-Covariance System for the Measurement of Surface/Atmosphere Exchange Fluxes of Submicron Aerosol Chemical Species—First Application Above an Urban Area

Until now, micrometeorological measurements of surface/ atmosphere exchange fluxes of submicron aerosol chemical components such as nitrate, sulfate or organics could only be made with gradient techniques. This article describes a novel setup to measure speciated aerosol fluxes by the more direct eddy covariance technique. The system is based on the Aerodyne quadrupole-based Aerosol Mass Spectrometer (Q-AMS), providing a quantitative measurement of aerosol constituents of environmental concern at a time resolution sufficient for eddy-covariance. The Q-AMS control software was modified to maximize duty cycle and statistics and enable fast data acquisition, synchronized with that of an ultrasonic anemometer. The detection limit of the Q-AMS based system for flux measurements ranges from 0.2 for NO3 − to 15 ng m−2 s−1 for hydrocarbon-like organic aerosol (HOA), with an estimated precision of around 6 ng m−2 s−1, depending on aerosol loading. At common ambient concentrations the system is capable of resolving deposition velocity values < 1 mm s−1, sufficient for measurements of dry deposition to vegetation. First tests of the system in the urban environment (6 to 20 June 2003) in Boulder, CO, USA, reveal clear diurnal, presumably traffic related, patterns in the emission of HOA and NO3 −, with indication of fast production of moderately oxygenated organic aerosol below the measurement height, averaging about 15% of the HOA emission. The average emission factor for HOA was 0.5 g (kg fuel)−1, similar to those found in previous studies. For NO3 − an emission factor of 0.09 g (kg fuel)−1 was estimated, implying oxidation of 0.5% of the traffic derived NOx below the measurement height of 45 m. By contrast, SO4 2− fluxes were on average downward, with deposition velocities that increase with friction velocity from 0.4 to 4 mm s−1.

Sources and sinks of ammonia within an oilseed rape canopy.

Within-canopy profiles of ammonia (NH 3) and measurements of the canopy turbulence characteristics were used to calculate the vertical source/sink density profile of NH 3 and sensible heat in a mature oilseed rape (Brassica napus) canopy. For the analysis, the inverse Lagrangian technique (ILT) based on localized near-field theory was used. Turbulence was measured with a standard ultrasonic anemometer, which, although not ideal for in-canopy work, is estimated to lead to a parameterization of the normalized standard deviation of the vertical wind component .w=u/, which is 11% accurate for heights >0.16 m during the day. The NH3 profiles in the canopy consistently show largest concentrations at the ground caused by NH 3 release from decomposing litter leaves on the ground surface with values of up to 150 ng m 2 s 1 predicted by the ILT. The inverse Lagrangian source/sink analysis performs well for both sensible heat and NH3, although it proves to be sensitive to the choice of the source/sink layers and becomes uncertain at the ground. Despite the large estimated ground level emission (26 gN H 3-N ha 1 per day), the analysis indicates that for the runs considered all NH 3 is recaptured by the lowest 0.7 m of the 1.38 m tall canopy, and that the bi-directional net exchange with the atmosphere is governed by the top 0.5 m, leading to a net emission from the canopy of 12 g NH3-N ha 1 per day. Since measurements of apoplastic [NH4 C ] and pH indicate that no significant stomatal emission from foliage should have occurred, this suggests that the siliques were a further source of NH3.