Relaxed eddy accumulation measurements of ammonia, nitric acid, sulfur dioxide and particulate sulfate dry deposition near Tampa, FL, USA

Eddy covariance measurements with high-resolution time-of-flight aerosol mass spectrometry: a new approach to chemically resolved aerosol fluxes

“…HR-AMS ammonium deposition velocities for BEARPEX-2007 are consistent with previous measurements at other sites (Nemitz et al, 2004b), but an order of magnitude less than the total NH 3(g) + particulate ammonium deposition velocities observed over a spruce forest in Germany (Wolff et al, 2010). To understand the observed ammonium fluxes and mass concentrations, we use the observed balance between NR-PM 1 cations and anions, also known as the "ammonium balance".…”

“…The error considered here is the error in determining the correct local co-variance between c and w. Additional error is introduced in that the local flux detected during a 29-min period at a single position may not be statistically representative of the average flux over the surface -i.e., the assumption of horizontal homogeneity is not met. Even two "perfect" eddy-covariance flux measurement systems would therefore not derive the same flux and this error decreases with increasing turbulence (e.g., Hollinger et al, 2005;Nemitz et al, 2009b and references therein). Linear regressions are calculated with a weighted robust regression to account for uncertainties in both x and y directions, with the exception of ammonium deposition velocity (f), for which we use a weighted orthogonal distance regression.…”

“…In addition, the fluxes are derived from the aerosol mass within a certain size-range, which may not be a conserved parameter where the aerosol size changes beyond the upper or lower size cut-off during vertical transport. By integrating over the total accumulation mode mass of the chemical components, the HR-AMS is relatively insensitive to changes in size distribution within the instrument's submicron range, and we can generally consider the HR-AMS flux measurement to be insensitive to artifacts due to the small changes in submicron particle size caused by evaporation/condensational growth, which have been found to affect the measurement of size-segregated particle number fluxes within individual accumulation mode size bins or fluxes of total particle number above a specified cutoff (e.g., Nemitz et al, 2004aNemitz et al, , 2009aVong et al, 2010). Under some conditions, however, vertical gradients in particle growth in or out of the AMS-observable size range due to water uptake or changes in gas/aerosol partitioning with condensable chemical species could, however, cause an artifact.…”

“…To our knowledge, the measurements described here include the first direct eddy covariance flux measurements of particulate ammonium. Availability of instrumentation has limited past studies to indirect flux methods (Nemitz et al, 2004b;Trebs et al, 2006;Wolff et al, 2010). Ammonium is an unusually challenging aerosol component for which to interpret fluxes, as one subset of ammonium is irreversibly tied to sulphate ions, while another is in equilibrium with gas-phase species: NH 3(g) + HNO 3(g) ↔ NH 4 NO 3(particle)…”

“…Fluxes of total or size-resolved aerosol number (without chemical information) have been performed for some time (e.g., Katen et al, 1985;Sievering, 1987;Buzorius et al, 1998;Dorsey et al, 2002;Mårtensson et al, 2006;Vong et al, 2010). However, total and chemically-resolved particle mass fluxes have lagged behind because most instruments measuring mass or aerosol chemical composition are far from meeting the rigorous requirements for EC, and most chemically-resolved aerosol flux measurements have been indirect with slower time resolution approaches (e.g., Nemitz et al, 2004b;Trebs et al, 2006;Myles et al, 2007;Thomas et al, 2009;Wolff et al, 2010).…”