Particle fluxes above forests: observations, methodological considerations and method comparisons

This paper reports a study designed to test, evaluate and compare micro-meteorological methods for determining the particle number flux above forest canopies. Half-hour average particle number fluxes above a representative broad-leaved forest in Denmark derived using eddy covariance range from -7x10(7) m(-2) s(-1) (1st percentile) to 5x10(7) m(-2) s(-1) (99th percentile), and have a median value of -1.6x10(6) m(-2) s(-1). The statistical uncertainties associated with the particle number flux estimates are larger than those for momentum fluxes and imply that in this data set approximately half of the particle number fluxes are not statistically different to zero. Particle number fluxes from relaxed eddy accumulation (REA) and eddy covariance are highly correlated and of almost identical magnitude. Flux estimates from the co-spectral and dissipation methods are also correlated with those from eddy covariance but exhibit higher absolute magnitude of fluxes.     

Carbon neutral Biggar: calculating the community carbon footprint and renewable energy options for footprint reduction

The objective of this research was to develop a community carbon footprint model that could be used to assess the size and major components of a community’s carbon dioxide (CO₂) emissions. The town of Biggar aims to become Scotland’s first carbon neutral town. As expected for this rural community, car transport accounted for nearly half of the CO₂ emissions, with natural gas and electricity consumption resulting in a further 24% and 12% of total emissions, respectively, and air travel being the last major component at 10% of emissions. An assessment was also made of the wind and solar resources of the town. One large wind turbine would provide the town’s electricity, while three to four turbines would be needed to offset all CO₂ emissions. In contrast, offsetting by tree planting would require in the region of 2,000 ha of trees.

HNO3 fluxes to a deciduous forest derived using gradient and REA methods

Summertime nitric acid concentrations over a deciduous forest in the midwestern United States are reported, which range between 0.36 and 3.3 μg m⁻³. Fluxes to the forest are computed using the relaxed eddy accumulation technique and gradient methods. In accord with previous studies, the results indicate substantial uncertainties in the gradient-based calculations. The relaxed eddy accumulation (REA) derived fluxes are physically reasonable and are shown to be of similar magnitude to dry deposition estimates from gradient sampling. The REA derived mean deposition velocity is approximately 3 cm s⁻¹, which is also comparable to growing season estimates derived by Meyers et al. for a similar deciduous forest. Occasional inverted concentration gradients and fluxes are observed but most are not statistically significant. Data are also presented that indicate substantial through canopy penetration of nitric acid.     

Implications of Heterogeneous Chemistry of Nitric Acid for Nitrogen Deposition to Marine Ecosystems: Observations and Modeling

Size resolved particle composition and nitric acid (HNO₃)measurements from the ASEPS'98 experiment conducted in the BalticSea are used to provide observational evidence of substantialgas-particle transfer of oxidized nitrogen (N) compounds in themarine boundary layer. We then focus on the importance ofHNO₃ reactions on sea salt particles in determining spatio-temporal patterns of N dry deposition to marine ecosystems.Modelling results obtained assuming no kinetic or chemical limiton HNO₃ uptake and horizontally homogeneous conditions withnear-neutral stability, indicate that for wind speeds 3.5 – 10 ms^-1 transfer of HNO₃ to the particle phase to formparticle nitrate (NO₃ ^-) may decrease the N depositionvelocity by 50%. We extend this research using the CHEM-COASTmodel to demonstrate that, in a sulphur poor environment undermoderate wind speeds with HNO₃ concentrations representativeof those found in the marine boundary layer, inclusion ofheterogeneous reactions on sea spray significantly reducesmodelled NO₃ ^- deposition in the near coastal zone.     

New particle formation in the Midwestern USA: Event characteristics,meteorological context and vertical profiles

Two years of continuous particle size distribution measurements at three heights over and in a deciduous forest in southern Indiana exhibit evidence of particle nucleation on 46% of classifiable days, and clear nucleation with consistent subsequent growth on 14% of days. Events characterized by high ultra-fine particle production and clear growth are most frequent in spring. These events most frequently follow cold front passages and exhibit a higher frequency of northerly back-trajectories. Local meteorological conditions, as described using the Nucleation Parameter (NP) of Boy and Kulmala (2002) and stability indices, indicate a significant association with ultra-fine particle concentrations, and specifically that nucleation was frequently preceded by destabilization of the atmosphere consistent with entrainment of elevated pollution layers. There appears to be little or no dependence of nucleation frequency or intensity on the in situ condensational sink particularly during spring, which is consistent with very high regional emissions of the precursors of ternary nucleation. Vertical gradients of particle size distributions exhibit evidence for nucleation being focused above canopy and accordingly mean ratios of above and below canopy concentrations of ultra-fine particles (6–30 nm) just subsequent to nucleation are 1.8 and 1.7 respectively during leaf-on and leaf-off periods. The difference in vertical concentration ratios during leaf-on and leaf-off are used to infer that of the order of 25% of ultra-fine particles are captured by foliar elements in the canopy during turbulent mixing through the canopy.