Abstract

Abstract. This study addresses and attempts to mitigate persistent uncertainty and scatter among existing approaches for determining the rate of sea spray aerosol production by breaking waves in the open ocean. The new approach proposed here utilizes passive microwave emissions from the ocean surface, which are known to be sensitive to surface roughness and foam. Direct, simultaneous, and collocated measurements of the aerosol production and microwave emissions were collected aboard the FLoating Instrument Platform (FLIP) in deep water ~ 150 km off the coast of California over a period of ~ 4 days. Vertical profiles of coarse-mode aerosol (0.25–23.5 μm) concentrations were measured with a forward-scattering spectrometer and converted to surface flux using dry deposition and vertical gradient methods. Back-trajectory analysis of eastern North Pacific meteorology verified the clean marine origin of the sampled air mass over at least 5 days prior to measurements. Vertical and horizontal polarization surface brightness temperature were measured with a microwave radiometer at 10.7 GHz frequency. Data analysis revealed a strong sensitivity of the brightness temperature polarization difference to the rate of aerosol production. An existing model of microwave emission from the ocean surface was used to determine the empirical relationship and to attribute its underlying physical basis to microwave emissions from surface roughness and foam within active and passive phases of breaking waves. A possibility of and initial steps towards satellite retrievals of the sea spray aerosol production are briefly discussed in concluding remarks.

Highlights

  • As waves grow under the forcing of near-surface wind, some of their energy dissipates through whitecap-generating wave breaking

  • This study suggests that one possibility is the brightness temperature of the ocean surface, which can be measured by microwave radiometers on ships, aircrafts, and satellites

  • The main objective of this study is to investigate the relationship between aerosol production flux and microwave brightness temperature of the sea surface

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Summary

Introduction

As waves grow under the forcing of near-surface wind, some of their energy dissipates through whitecap-generating wave breaking These whitecaps are progenitors of sea spray droplets ejected into the air when whitecap bubbles burst. The focus of this paper is on measurement and parameterization of the production rate of these sea spray aerosol (SSA) particles in the open ocean. Lewis and Schwartz (2004) and de Leeuw et al (2011) review a variety of methods for measuring and estimating production fluxes of SSA. The dry deposition method requires only sizedependent measurements of aerosol concentration, N (r), at some height, and as a result it produces the total surface flux, F (r), at a desired reference height. To convert F (r) from a measurement height, H , to the desired reference height of 10 m above the water level, aerosol concentration can be extrapolated using a logarithmic profile (Hoppel et al, 2002): Vg

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