Sea-salt aerosols produced by breaking, ocean surface waves are enriched in organic matter by factors of 102 - 103 relative to conservative seawater tracers. When exposed to sunlight, this organic matter absorbs UV radiation and produces highly reactive transient species including OH, H2O2, and HO2. To quantify the photochemical production rates of OH radicals and H2O2, experiments were conducted using fresh marine aerosols that were generated by bubbling zero-air through 1.3 m of flowing oceanic seawater in a 20 cm diameter Pyrex and Teflon cylinder. When normalized to typical sea-salt aerosol ionic strength at 80% relative humidity, the extrapolated OH production rate in fresh aerosol solutions exposed to full tropical sun was 1.2 x10-8 M s-1. The corresponding H2O2 production rate was 1.7 x 10-8 M s-1. When normalized to ionic strength of seawater, the OH production rate was 2-3 orders of magnitude greater than reported values in surface seawater. This enhancement reflects the enrichment of organic matter in the aerosol samples (median factor of 387). Relative to fresh, unaltered samples, acidification of aerosol solutions increased the OH production rate by factors of 2 to 14. Because fresh sea-salt aerosols are rapidly acidified under most atmospheric conditions, production rates based on acidified samples are more representative of those in ambient air. Chromophoric organic matter associated with marine aerosols was photobleached by sunlight, which caused decreasing production rates of OH and H2O2 (factors of 2 and 4 respectively) after 7-hour exposures.
The Chemical and Physical Characteristics of Fresh Aerosols Produced at the Ocean Surface Word version
*Long, M Smsl3v@virginia.edu. Keene, W email@example.com. Maben, J firstname.lastname@example.org. Department of Environmental Sciences, University of Virginia, 280 Clark Hall, Charlottesville, VA 22908 US
Kieber, Ddjkieber@syr.edu. Davis, A Jajdavi02@syr.edu. Dahl, Eeedahl@loyola.edu. State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210 US Maring, Hhal.email@example.com. NASA Headquarters, Science Mission Directorate Mail Suite 3F71, Washington, DC 20546 US
Pszenny, Aalex.firstname.lastname@example.org. Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, 39 College Road, Durham, NH 03824 US and Mount Washington Observatory, PO Box 2310, North Conway, NH 03860 US Izaguirre, Mmizaguirre@rsmas.miami.edu. University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL 33149 US
Zhou, X email@example.com. Wadsworth Center, NYS Department of Health, and School of Public Health, SUNY at Albany, Empire State Plaza P.O. Box 509, Albany, NY 12201-0509 US
Fresh aerosols produced by wind stress at the ocean surface undergo rapid multiphase chemical and physical transformation. Consequently, it is impossible to unequivocally differentiate primary versus secondary constituents based on measurements of aerosols in ambient marine air. However, reliable characterization of the initial aerosol composition is essential to interpret the nature of subsequent chemical processing, associated biogeochemical cycles, and related environment influences. Fresh marine aerosols were generated artificially by bubbling zero-air through a 1.3 m column of flowing seawater within a 20-cm diameter Pyrex and Teflon chamber deployed at Bermuda. Aerosols produced by bursting bubbles at the surface were swept with zero-air hydrated to ~80% RH through isokinetic ports and subsequently sampled in bulk and with MOUDI cascade impactors for analysis of major ions, total halogens, and organic carbon (OC) and with Aerodynamic Particle Sizer (TSI Model 3310) and a Scanning Mobility Particle Sizer (TSI Model 3934L) for characterization of number size distributions (13 nm to > 15 micrometer diameters). The composition of feed seawater was measured in parallel. The median OC concentration (65 micrometer) and ratios of OC to sea-salt constituents in feed water were virtually identical to long-term averages for surface seawater offshore. Relative size distributions of conservative sea-salt aerosol constituents were similar to those over the open ocean. Ratios of major ions and of total Cl and Br in all size fractions were statistically indistinguishable from those in surface seawater. OC was highly enriched relative to seawater in all size fractions (median factor for all samples = 387); highest enrichments were associated with sub-micron size aerosols. Number size distributions were bimodal with peaks at 4 to 5 micrometer and 120 to 130 nm diameter. About 95 % of the inorganic sea-salt mass was associated with super-um size fractions; OC dominated the sub-micron size fractions. The efficient mechanical production of primary sub-micron marine aerosols has important implications for multiphase chemical processes in the marine troposphere and associated radiative effects.
Production of sub-micrometer aerosols by bubble bursting at the ocean surface: a potentially important global source of CCN Word version
*Maring, Hhal.firstname.lastname@example.org. NASA Headquarters, Science Mission Directorate Mail Suite 3F71, Washington, DC 20546 US
Keene, W email@example.com. Maben, J firstname.lastname@example.org. Department of Environmental Sciences, University of Virginia, 280 Clark Hall, Charlottesville, VA 22908 US
Izaguirre, Mmizaguirre@rsmas.miami.edu. University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL 33149 US
Dahl, Eedahl@esf.edu. Kieber, Ddjkieber@esf.edu. State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210 US
Identifying, characterizing and quantifying sources of hygroscopic sub-micrometer aerosols in the remote marine atmosphere is important because those aerosols can act as cloud condensation nuclei (CCN) which can affect cloud properties. Bubble bursting at the surface of the ocean is well known to produce super- micrometer sea salt aerosols, which could act as CCN. However, their importance is likely limited because they are relatively few in number and have sufficiently large gravitational settling velocities and short atmospheric residence times compared to sub-micrometer aerosols. To investigate the importance of bubble bursting as a source of sub-micrometer aerosols, we built a glass and Teflon model ocean where filtered air injected via frits formed bubbles and rose from the bottom of a column of constantly refreshed sea water. The bubbles burst at the surface of the sea water forming purely marine aerosols in enclosed headspace over the sea water. Air with controlled relative humidity carried those aerosols to an Aerodynamic Particle Sizer (TSI Model 3310) and a Scanning Mobility Particle Sizer (TSI Model 3934L) which measured the marine aerosol number size distribution from 13 nm to >15 µm. In addition, some of the same air carried the marine aerosols to a Micro Orifice Uniform Deposit Impactor (MSP Model 100) for subsequent analysis of size resolved major ions by ion chromatograph and total organic carbon by carbon analyzer. In addition to the expected super-micrometer sea salt aerosols, we observed relatively larger numbers of sub- micrometer aerosols. These aerosols occurred in a log-normal mode whose centroid ranged from ~80 to 145 nm for relative humidities ranging from 33 to 95%, respectively. The change in size with relative humidity suggests water constitutes at least 75% of the aerosol volume at 80% RH. The majority of the remaining sub- micrometer aerosol mass was made up of organic matter and inorganic sea salts with a mass ratio of roughly 6:1. These results provide unequivocal confirmation of bubble bursting at the surface of the ocean being an important source of sub-micrometer, hygroscopic, organic-rich aerosols. These results support the hypothesis that mechanical processes at the surface of the ocean are an important source of sub-micrometer CCN in the marine atmosphere.