Gas-particle interactions above a Dutch heathland: II. Concentrations and surface exchange fluxes of atmospheric particles

Abstract

Abstract. Size-dependent particle number fluxes measured by eddy-covariance (EC) and continuous fluxes of ammonium (NH -N dry deposition amounts to 20% of the dry input of NH 3 -N over the measurement period. These surface exchange fluxes are analyzed together with simultaneous gas-phase flux measurements for indications of gas-particle interactions. On warm afternoons the apparent fluxes of acids and aerosol above the heathland showed several coinciding anomalies, all of which are consistent with NH + 4 evaporation during deposition: (i) canopy resistances for HNO 3 and HCl of up to 100 s m −1 , (ii) simultaneous particle emission of small particles (D p <0.18 µm) and deposition of larger particles (D p >0.18 µm), (iii) NH + 4 deposition faster than derived from size-distributions and size-segregated EC particle fluxes. These observations coincide with the observations of (i) surface concentration products of NH 3 and HNO 3 well below the thermodynamic equilibrium value and (ii) Damköhler numbers that indicate chemical conversion to be sufficiently fast to modify exchange fluxes. The measurements imply a removal rate of volatile NH + 4 of 3−30×10 −6 s −1 averaged over the 1 km boundary-layer, while NH 3 deposition is underestimated by typically 20 ng m −2 s −1 (28%) and flux reversal may occur.

Exchange of reactive nitrogen compounds: concentrations and fluxes of total ammonium and total nitrate above a spruce canopy

“…Nemitz et al, 2004a;Brost et al, 1988;Huebert et al, 1988). However, as long as the characteristic time scale of chemical transformation is large in comparison to the turbulent timescale, fluxes of compounds that underlie rapid chemical transformation may be determined with sufficient accuracy when treating them as not-reactive (De Arellano and Duynkerke, 1992;Nemitz et al, 2004b). Photochemical reactions involving NH 3 and HNO 3 are slow compared to turbulence; however, timescales of phase changes within the NH 3 -HNO 3 -NH 4 NO 3 triad (see Reaction R1), may be comparable to characteristic times of turbulent transport (Trebs et al, 2006).…”

“…For particles with mean diameters of 0.25, 0.71 and 2 µm, v d at the encountered wind speeds would theoretically range between 0.01-0.6, 0.01-1.08, and 0.06-3.5 cm s −1 , respectively (Peters and Eiden, 1992). According to Gallagher et al (1997) 4 and those derived from particle number flux measurements have been reported (Nemitz et al, 2004b;Thomas, 2007). These were explained by changes in the thermodynamic equilibrium towards the more rapidly depositing gaseous species between the measurement height and the vegetated surface (Fowler et al, 2009).…”

Governing processes for reactive nitrogen compounds in the European atmosphere

“…In addition, NH 3 deposition depends on the N status of the receiving surface, with fertilised vegetation and vegetation receiving high atmospheric N deposition inputs acting as a less efficient sink or even, in the case of fertilised vegetation, a net source of NH 3 Walker et al, 2012a). In wet regions, leaf cuticles frequently provide an efficient sink for water soluble gases (NH 3 and HNO 3 ), although the cuticular sinks for both gases can become saturated under very dry conditions, increasing the importance of stomata as an uptake pathway (Nemitz et al, 2004b). Challenges in modelling NH 3 fluxes over vegetation have been addressed in a number of papers Wu et al, 2009).…”

“…surface resistance is negligible. This is probably a reasonable approximation for most situations, but emission gradients or reduced uptake rates of HNO 3 have been observed, probably due to non-zero HNO 3 surface concentrations in equilibrium with NH 4 NO 3 deposited to leaf surfaces (Neftel et al, 1996;Nemitz et al, 2004b;Zhang et al, 1994). For trace gases with negligible surface resistance, the deposition velocity is sensitive to the atmospheric resistances (R a and R b ), which over rough surfaces are usually small (5-10 s m −1 ).…”

Technical note: Comparison and interconversion of pH based on different standard states for aerosol acidity characterization

“…Moreover, aerosol acidity plays a key role in the gas-particle partitioning of species such as HCl/Cl -, HNO3/NO3 -and NH3/NH4 + , and is therefore vital for predicting lifetimes of gaseous compounds such as NH3 and HNO3 in the atmosphere (Nemitz et al, 2004;Oss et al, 1998). Further, aerosol acidity is known to affect the 5 formation of secondary organic aerosols (SOA), e.g.…”