Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
1680-7367
1680-7375
4
2
2004
10.5194/acpd-4-1567-2004
http://www.atmos-chem-phys-discuss.net/4/1567/2004/
http://www.atmos-chem-phys-discuss.net/4/1567/2004/acpd-4-1567-2004.html
http://www.atmos-chem-phys-discuss.net/4/1567/2004/acpd-4-1567-2004.pdf
1567
1621
2004-03-15
Gas-particle interactions above a Dutch heathland: III. Modelling the influence of the NH<sub>3</sub>-HNO<sub>3</sub>-NH<sub>4</sub>NO<sub>3</sub> equilibrium on size-segregated particle fluxes
E. Nemitz
en@ceh.ac.uk
M. A. Sutton
Atmospheric Sciences, Centre for Ecology and Hydrology (CEH), Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, EH26 0QB, United Kingdom
Micrometeorological measurements of size-segregated particle number fluxes above Dutch heathlands and forests have
repeatedly shown simultaneous apparent emission of particles with a diameter (<i>D</i><sub>p</sub>) < 0.18 µm
and deposition of larger particles when measured with optical particle counters. In order to assess whether this
observation may be explained by the equilibrium reaction of ammonia (NH<sub>3</sub>), nitric acid (HNO<sub>3</sub>) and ammonium (NH<sub>4</sub><sup>+</sup>), a new numerical model is developed to predict the vertical concentration and flux profiles of the different species as
modified by the interaction of equilibration and surface/atmosphere exchange processes. In addition to former
studies, the new approach explicitly models the height-dependence of the NH<sub>4</sub><sup>+</sup> and total
aerosol size-distribution. Using this model, it is demonstrated that both gas-to-particle conversion (gtpc) and aerosol
evaporation can significantly alter the apparent surface exchange fluxes, and evoke the observed bi-directional particle fluxes under certain conditions. Thus, in general, the NH<sub>3</sub>-HNO<sub>3</sub>-NH<sub>4</sub>NO<sub>3</sub> equilibrium needs to
be considered when interpreting eddy-covariance particle fluxes. Applied to an extensive dataset of simultaneous flux measurements of particles and gases at Elspeet, NL, the model reproduces the diurnal pattern of the bi-directional exchange well. In agreement with the observation of fast NH<sub>4</sub><sup>+</sup> deposition,
slow nitric acid deposition (both as measured by the aerodynamic gradient
method) and small concentration products of NH<sub>3</sub>×HNO<sub>3</sub> at this site, this study suggests that NH<sub>4</sub><sup>+</sup> evaporation at this site significantly alters surface exchange fluxes.