Non-stomatal exchange in ammonia dry deposition models: comparison of two state-of-the-art approaches

The accurate representation of bidirectional ammonia (NH3) biosphere–atmosphere exchange is an important part of modern air quality models. However, the cuticular (or external leaf surface) pathway, as well as other non-stomatal ecosystem surfaces, still pose a major challenge to translating our knowledge into models. Dynamic mechanistic models including complex leaf surface chemistry have been able to accurately reproduce measured bidirectional fluxes in the past, but their computational expense and challenging implementation into existing air quality models call for steady-state simplifications. Here we qualitatively compare two semi-empirical state-of-the-art parameterizations of a unidirectional non-stomatal resistance (Rw) model after Massad et al. (2010), and a quasi-bidirectional non-stomatal compensation-point (χw) model after Wichink Kruit et al. (2010), with NH3 flux measurements from five European sites. In addition, we tested the feasibility of using backward-looking moving averages of air NH3 concentrations as a proxy for prior NH3 uptake and as a driver of an alternative parameterization of non-stomatal emission potentials (Γw) for bidirectional non-stomatal exchange models. Results indicate that the Rw-only model has a tendency to underestimate fluxes, while the χw model mainly overestimates fluxes, although systematic underestimations can occur under certain conditions, depending on temperature and ambient NH3 concentrations at the site. The proposed Γw parameterization revealed a clear functional relationship between backward-looking moving averages of air NH3 concentrations and non-stomatal emission potentials, but further reduction of uncertainty is needed for it to be useful across different sites. As an interim solution for improving flux predictions, we recommend reducing the minimum allowed Rw and the temperature response parameter in the unidirectional model and revisiting the temperature-dependent Γw parameterization of the bidirectional model.

Non-stomatal exchange in ammonia dry deposition models: Comparison of two state-of-the-art approaches

The accurate representation of bidirectional ammonia (NH3) biosphere-atmosphere exchange is an important part of modern air quality models. However, the cuticular (or external leaf surface) pathway, as well as other non-stomatal ecosystem surfaces, still pose a major challenge of translating our knowledge into models. Dynamic mechanistic models including complex leaf surface chemistry have been able to accurately reproduce measured bidirectional fluxes in the past, but their computational expense and challenging implementation into existing air quality models call for steady-state simplifications. We here qualitatively compare two semi-empirical state-of-the-art parameterizations of a unidirectional non-stomatal resistance (Rw) model after Massad et al. (2010), and a quasi-bidirectional non-stomatal compensation point (χw) model after Wichink Kruit et al. (2010), with NH3 flux measurements from five European sites. In addition, we tested the feasibility of using backward-looking moving averages of air NH3 concentrations as a proxy for prior NH3 uptake and driver of an alternative parameterization of non-stomatal emission potentials (Γw) for bidirectional non-stomatal exchange models. Results indicate that the Rw-only model has a tendency to underestimate fluxes, while the χw model mainly overestimates fluxes, although systematic underestimations can occur under certain conditions, depending on temperature and ambient NH3 concentrations at the site. The proposed Γw parameterization appears to have potential for improvement, but cannot be recommended for use in large scale simulations in its present state due to large uncertainties. As an interim solution for improving flux predictions, we recommend to reduce the minimum allowed Rw and the temperature response parameter in the unidirectional model and to revisit the temperature dependent Γw parameterization of the bidirectional model.

Modelling the dynamic chemical interactions of atmospheric ammonia with leaf surface wetness in a managed grassland canopy

Ammonia exchange fluxes between grassland and the atmosphere were modelled on the basis of stomatal compensation points and leaf surface chemistry, and compared with measured fluxes during the GRAMINAE intensive measurement campaign in spring 2000 near Braunschweig, Germany. Leaf wetness and dew chemistry in grassland were measured together with ammonia fluxes and apoplastic NH4+ and H+ concentration, and the data were used to apply, validate and further develop an existing model of leaf surface chemistry and ammonia exchange. Foliar leaf wetness which is known to affect ammonia fluxes may be persistent after the end of rainfall, or sustained by recondensation of water vapour originating from the ground or leaf transpiration, so measured leaf wetness values were included in the model. pH and ammonium concentrations of dew samples collected from grass were compared to modelled values. The measurement period was divided into three phases: a relatively wet phase followed by a dry phase in the first week before the grass was cut, and a second drier week after the cut. While the first two phases were mainly characterised by ammonia deposition and occasional short emission events, regular events of strong ammonia emissions were observed during the post-cut period. A single-layer resistance model including dynamic cuticular and stomatal exchange could describe the fluxes well before the cut, but after the cut the stomatal compensation points needed to numerically match measured fluxes were much higher than the ones measured by bioassays, suggesting another source of ammonia fluxes. Considerably better agreement both in the direction and the size range of fluxes were obtained when a second layer was introduced into the model, to account for the large additional ammonia source inherent in the leaf litter at the bottom of the grass canopy. Therefore, this was found to be a useful extension of the mechanistic dynamic chemistry model by keeping the advantage of requiring relatively little site-specific information.

Modeling the dynamic chemical interactions of atmospheric ammonia and other trace gases with measured leaf surface wetness in a managed grassland canopy

Ammonia exchange fluxes between grassland and the atmosphere were modeled on the basis of stomatal compensation points and leaf surface chemistry, and compared with measured fluxes during the GRAMINAE intensive measurement campaign in spring 2000 near Braunschweig, Germany. Leaf wetness and dew chemistry in grassland were measured together with ammonia fluxes and apoplastic NH4+ and H+ concentration, and the data were used to apply, validate and further develop an existing model of leaf surface chemistry and ammonia exchange. The leaf surface water storage was calculated from measured leaf wetness data using an exponential parameterisation. The measurement period was divided into three phases: a relatively wet phase followed by a dry phase in the first week before the grass was cut, and a second drier week after the cut. While the first two phases were mainly characterised by ammonia deposition and occasional short emission events, regular events of strong ammonia emissions were observed during the post-cut period. A single-layer resistance model including dynamic cuticular and stomatal exchange could describe the fluxes well before and after the cut, but unrealistically high stomatal compensation points were needed after the cut in order to match measured fluxes. Significant improvements were obtained when a second layer was introduced into the model, to account for the large additional ammonia source inherent in the leaf litter at the bottom of the grass canopy.

Aphid Resistance and Leaf Surface Chemistry of Sugar Ester Producing Tobaccos1

Sugar ester producing tobacco lines were evaluated for aphid resistance and other surface chemicals. The cembrenoid and labdenoid diterpenes, α- and β-4,8,13-duvatrien-1-ols, α- and β-4,8,13-duvatriene-1,3-diols, (12Z)-labda-12,14-diene-8α-ol (cis-abienol), (13E)-labda-13-ene-8α,15-diol (labdenediol), docosanol, and sugar esters were quantified using high pressure liquid chromatography and compared with aphid infestation ratings. Regression analysis of aphid [Myzus persicae (Sulzer)] infestation rating and leaf surface chemistry was statistically significant and showed that surface chemicals were important in explaining the observed variation in the aphid infestation ratings. A significant negative correlation was found between aphid ratings and sugar ester levels among the 62 entries evaluated (r = −0.2758, P = 0.0301). α and β monols (α- and β-4,8,13-duvatrien-1-ols) were also significantly correlated with aphid infestations in this study (r = −0.2743, P = 0.0310 and r = −0.2797, P= 0.0109, respectively). None of the other surface chemicals were statistically correlated with aphid resistance. Although high sugar ester levels were correlated with aphid resistance, not all tobacco entries with high levels of sugar esters, such as Tl 1568 were resistant. This would suggest that there may be different types of sugar esters present in these tobaccos, and total sugar ester levels alone could not be used to predict aphid resistance. Also, some tobacco lines, like Tl 1674 and Tl 59 with lower sugar ester levels, were resistant in this study because of high monol levels. The ten tobacco entries with the highest levels of sugar esters in this study were Tl 698, Tl 675, Tl 704, Tl 998, Tl 193, JA 389, Tl 722R, Tl 1092, Tl 711, and Tl 1007. All of these lines exhibited high levels of aphid resistance, but some also had low-to-moderate levels of monols that may have elevated the aphid resistance level. A number of these tobaccos could be used for production of natural sugar ester biorationals or used in a breeding program for development of aphid resistant cultivars.