The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60-180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climaterelevant properties.clouds | marine aerosols | biologically active | cloud condensation nuclei | ice nucleation

Brown carbon (BrC), which may include secondary organic aerosol (SOA), can be a significant climate-forcing agent via its optical absorption properties. However, the overall contribution of SOA to BrC remains poorly understood. Here, correlations between oxidation level and optical properties of SOA are examined. SOA was generated in a flow reactor in the absence of NOx by OH oxidation of gas-phase precursors used as surrogates for anthropogenic (naphthalene, tricyclo[5.2.1.0(2,6)]decane), biomass burning (guaiacol), and biogenic (α-pinene) emissions. SOA chemical composition was characterized with a time-of-flight aerosol mass spectrometer. SOA mass-specific absorption cross sections (MAC) and refractive indices were calculated from real-time cavity ring-down photoacoustic spectrometry measurements at 405 and 532 nm and from UV-vis spectrometry measurements of methanol extracts of filter-collected particles (300 to 600 nm). At 405 nm, SOA MAC values and imaginary refractive indices increased with increasing oxidation level and decreased with increasing wavelength, leading to negligible absorption at 532 nm. Real refractive indices of SOA decreased with increasing oxidation level. Comparison with literature studies suggests that under typical polluted conditions the effect of NOx on SOA absorption is small. SOA may contribute significantly to atmospheric BrC, with the magnitude dependent on both precursor type and oxidation level.

Surface-active compounds present in aerosols can increase their cloud condensation nuclei (CCN) activation efficiency by reducing the surface tension (σ ) in the growing droplets. However, the importance of this effect is poorly constrained by measurements. Here we present estimates of droplet surface tension near the point of activation derived from direct measurement of droplet diameters using a continuous flow streamwise thermal gradient chamber (CFSTGC). The experiments used sea spray aerosol (SSA) mimics composed of NaCl coated by varying amounts of (i) oleic acid, palmitic acid or myristic acid, (ii) mixtures of palmitic acid and oleic acid, and (iii) oxidized oleic acid. Significant reductions in σ relative to that for pure water were observed for these mimics at relative humidity (RH) near activation ( ∼ 99.9 %) when the coating was sufficiently thick. The calculated surface pressure (π = σ H 2 O − σ observed ) values for a given organic compound or mixture collapse onto one curve when plotted as a function of molecular area for different NaCl seed sizes and measured RH. The observed critical molecular area (A 0 ) for oleic acid determined from droplet growth was similar to that from experiments conducted using macroscopic solutions in a Langmuir trough. However, the observations presented here suggest that oleic acid in microscopic droplets may exhibit larger π values during monolayer compression. For myristic acid, the observed A 0 compared well to macroscopic experiments on a fresh subphase, for which dissolution has an important impact. A significant kinetic limitation to water uptake was observed for NaCl particles coated with pure palmitic acid, likely as a result of palmitic acid (with coating thicknesses ranging from 67 to 132 nm) being able to form a solid film. However, for binary palmitic-acid-oleic-acid mixtures there was no evidence of a kinetic limitation to water uptake. Oxidation of oleic acid had a minor impact on the magnitude of the surface tension reductions observed, potentially leading to a slight reduction in the effect compared to pure oleic acid. A CCN counter was also used to assess the impact on critical supersaturations of the substantial σ reductions observed at very high RH. For the fatty-acid-coated NaCl particles, when the organic fraction (ε org ) was > 0.90 small depressions in critical supersaturation were observed. However, when ε org < 0.90 the impact on critical supersaturation was negligible. Thus, for the fatty acids considered here, the substantial σ reductions observed at high RH values just below activation have limited impact on the ultimate critical supersaturation. A surface film model is used to establish the properties that surface-active organic molecules must have if they are to ultimately have a substantial impact on the activation efficiency of SSA. To influence activation, the average properties of surface-active marine-derived organic molecules must differ substantially from the long-chain fatty acids examined, having either smaller molecular volumes o...

A discrepancy between laboratory and field‐derived parameterizations for the dependence of sea spray aerosol (SSA) particle number concentrations (Np) and size distributions on water temperature (Tw) exists. Here we address this discrepancy by quantifying the Tw dependence of SSA production over the range −2–25 °C for laboratory‐generated particles using a marine aerosol reference tank (MART), a miniature MART (miniMART), and a plunging jet. Four water types were considered: NaCl water, reef salt (RS) water, filtered and autoclaved reef salt (FARS), and filtered but not autoclaved seawater (NASW). For NaCl, RS, and FARS water the Np exhibited a moderate, monotonic increase with Tw for all generation methods (sensitivity of 2.1–4.1%/°C). This contrasts with some previous laboratory studies but is consistent with parameterizations derived from ambient observations. The reconciliation of laboratory results with these parameterizations supports their use in global models. The Tw sensitivity also increased with particle size in the submicron size range. This indicates the fraction of primary SSA that can act as cloud condensation nuclei has a different sensitivity to Tw than the total particle number. The particle production for actual seawater (NASW) differed from the other water types, exhibiting complex, irreproducible behavior with no clear Tw dependence due most likely to biologically or physically driven temporal evolution of seawater composition. This suggests that variability in seawater composition may have as large an impact as temperature on actual SSA production. These observations provide new constraints and context for understanding the dependence of SSA production on water temperature and composition.

Sea spray aerosol (SSA) can have complex carbon speciation that is affected by biological conditions in the seawater from which it originates. Biologically derived molecules can also interact with other longer-lived organic and inorganic carbon species in the sea surface microlayer and in the process of bubble bursting. An isolated wave channel facility was used to generate sea spray aerosol during a 1 month mesocosm study. Two consecutive phytoplankton blooms occurred, and sea spray aerosol was sampled throughout. Scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) was used to determine spatially resolved carbon speciation within individual particles from 0.18 to 3.2 μm. During phytoplankton blooms, coarse-mode particles exhibited an increased abundance of carboxylic acid-rich needlelike structures. The extent of organic enrichment in fine-mode particles correlates with the occurrence of aliphatic-rich organic species, as detected by an intense C 1s → σ(C−H)* excitation. These aliphatic-rich species had a strong association with graphitic carbon, as detected by a C 1s → σ* exciton excitation. This enrichment was unique to particles collected in the aerodynamic size range 0.18−0.32 μm and corresponded with the decrease in hygroscopicity. Aliphatic organics can significantly suppress the particle hygroscopicity when they replace salt, thus influencing the effect of sea spray aerosol on light scattering and cloud formation. These results suggest that graphitic carbon is concentrated in the sea surface microlayer during phytoplankton blooms and released through wave action. These results may have implications for radiative transfer and carbon cycling in the ocean−atmosphere system.

Exposure to gas-phase isocyanic acid (HNCO) has been previously shown to be associated with the development of atherosclerosis, cataracts and rheumatoid arthritis. As such, accurate emission inventories for HNCO are critical for modeling the spatial and temporal distribution of HNCO on a regional and global scale. To date, HNCO emission rates from light duty gasoline vehicles, operated under driving conditions, have not been determined. Here, we present the first measurements of real-time emission factors of isocyanic acid from a fleet of eight light duty gasoline-powered vehicles (LDGVs) tested on a chassis dynamometer using the Unified Driving Cycle (UC) at the California Air Resources Board (CARB) Haagen-Smit test facility, all of which were equipped with three-way catalytic converters. HNCO emissions were observed from all vehicles, in contrast to the idealized laboratory measurements. We report the tested fleet averaged HNCO emission factors, which depend strongly on the phase of the drive cycle; ranging from 0.46 ± 0.13 mg kg fuel(-1) during engine start to 1.70 ± 1.77 mg kg fuel(-1) during hard acceleration after the engine and catalytic converter were warm. The tested eight-car fleet average fuel based HNCO emission factor was 0.91 ± 0.58 mg kg fuel(-1), within the range previously estimated for light duty diesel-powered vehicles (0.21-3.96 mg kg fuel(-1)). Our results suggest that HNCO emissions from LDGVs represent a significant emission source in urban areas that should be accounted for in global and regional models.

Previous laboratory measurements and field observations have suggested that the reactive uptake of N 2 O 5 to sea spray aerosol particles is a complex function of particle chemical composition and phase, where surface active organics can suppress the reactive uptake by up to a factor of 60. To date, there are no direct studies of the reactive uptake of N 2 O 5 to nascent sea spray aerosol that permit assessment of the role that organic molecules present in sea spray aerosol (SSA) may play in suppressing or enhancing N 2 O 5 uptake kinetics. In this study, SSA was generated from ambient seawater and artificial seawater matrices using a Marine Aerosol Reference Tank (MART), capable of producing nascent SSA representative of ambient conditions. The reactive uptake coefficient of N 2 O 5 (γ(N 2 O 5 )) on nascent SSA was determined using an entrained aerosol flow reactor coupled to a chemical ionization mass spectrometer for measurement of surface area dependent heterogeneous loss rates. Population averaged measurements of γ(N 2 O 5 ) for SSA generated from salt water sequentially doped with representative organic molecular mimics, or from ambient seawater, do not deviate statistically from that observed for sodium chloride (γ(N 2 O 5 ) NaCl = 0.01−0.03) for relative humidity (RH) ranging between 50 and 65%. The results are consistent with measurements made under clean marine conditions at the Scripps Institution of Oceanography Pier and those conducted on nascent SSA generated in the marine aerosol reference tank. The results presented here suggest that organic films present on nascent SSA (at RH greater than 50%) likely do not significantly limit N 2 O 5 reactive uptake.

A suite of real-time instruments was used to sample vehicle emissions at the California Air Resources Board Haagen-Smit facility. Eight on-road, spark-ignition gasoline and three alternative vehicles were tested on a chassis dynamometer and the emissions were diluted to atmospherically relevant concentrations (0.5-30 mg/m 3 ). An Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-MS) characterized the real-time behavior of the nonrefractory organic and inorganic particulate matter (PM) in vehicle emissions. It was found that the emission of particulate organic matter (POM) was strongly affected by engine temperature and engine load and that the emission concentrations could vary significantly by vehicle. Despite the small sample size, consistent trends in chemical characteristics were observed. The composition of vehicle POM was found to be related to overall PM mass concentration where the oxygen-to-carbon (O/C) ratio tended to increase at lower concentration and had an average value of 0.057 § 0.047, with a range from 0.022 to 0.15. The corresponding fraction of particle-phase CO 2 C , or f 44 , ranged from 1.1% to 8.6% (average D 2.1%) and exhibited a linear variation with O/ C. The average mass spectrum from all vehicles tested was also compared to those of hydrocarbon-like organic aerosol (HOA) observed in ambient air and the agreement is very high. The results of these tests offer the vehicle emissions community a first glimpse at the real-time chemical composition and variation of vehicle PM emissions for a variety of conditions and vehicle types at atmospherically relevant conditions and without chemical interferences from other primary or secondary aerosol sources.