Abstract
Abstract. The eddy covariance technique is the most direct of the methods that have been used to measure the flux of sea-spray aerosol between the ocean and atmosphere, but has been applied in only a handful of studies. However, unless the aerosol is dried before the eddy covariance measurements are made, the hygroscopic nature of sea-spray may combine with a relative humidity flux to result in a bias in the calculated aerosol flux. "Bulk" methods have been presented to account for this bias, however, they rely on assumptions of the shape of the aerosol spectra which may not be valid for near-surface measurements of sea-spray. Here we describe a method of correcting aerosol spectra for relative humidity induced size variations at the high frequency (10 Hz) measurement timescale, where counting statistics are poor and the spectral shape cannot be well represented by a simple power law. Such a correction allows the effects of hygroscopicity and relative humidity flux on the aerosol flux to be explicitly evaluated and compared to the bulk corrections, both in their original form and once reformulated to better represent the measured mean aerosol spectra. In general, the bulk corrections – particularly when reformulated for the measured mean aerosol spectra – perform relatively well, producing flux corrections of the right sign and approximate magnitude. However, there are times when the bulk methods either significantly over- or underestimate the required flux correction. We conclude that, where possible, relative humidity corrections should be made at the measurement frequency.
Highlights
Instrumentation Methods andSea-spray aerosol, generatDedaintatheSoypsenteomcesan through bubbeen applied in only a handful of studies
Unless the ble bursting in whitecaps, or as droplets are physically ripped aerosol is dried before the eddy covariance measurements are from the crests of waves by the wind, is the second largest made, the hygroscopic nature of sea-spray may combine with a relative humidity flux to result in a bias in the calculated aerosol flux
The use of the eddy covariance technique to measure the size-segregated flux of sea-spray aerosol in the presence of a relative humidity flux may lead to a significant systematic bias in the recorded flux
Summary
Sea-spray aerosol, generatDedaintatheSoypsenteomcesan through bubbeen applied in only a handful of studies. The effects of hygroscopicity and relative humidity flux on Calculation of a sea-spray source function requires the the aerosol flux to be explicitly evaluated and compared to the bulk corrections, both in their original form and once qspuhaenrteifi. What the eddy covariance technique measures is the net turbulent aerosol flux, it is only an adequate approximation to the true sea-spray source flux in conditions far from equilibrium. The calculation of an eddy covariance sea-spray source function requires the collocation of high-frequency aerosol and vertical wind measurements. This requires the use of a small, weatherproof aerosol spectrometer or a long sample line from the point of measurement back to the aerosol instrument. Number of deliquescent particles in a certain particle size interval is a non-conservative scalar and, size-segregated eddy correlation measurements of the number flux, N w , where N is the ambient aerosol concentration, may not be representative of the true aerosol flux, and net particle fluxes can be measured even where none are present
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