The number concentration and properties of aerosol particles serving as cloud condensation nuclei (CCN) are important for understanding cloud properties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure the low-albedo ocean surface. Studies linking aerosol source, composition, and water uptake properties in this region have been conducted primarily during the summertime dust transport season, despite the region receiving a variety of aerosol particle types throughout the year. In this study, we compare size-resolved aerosol chemical composition data to the hygrocopicity parameter κ derived from size-resolved CCN measurements made during the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. We observed unexpected periods of wintertime long-range transport of African smoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN Number concentrations as well as the proportions of dust and smoke particles increased, whereas average κ slightly decreased (κ = 0.46 +/- 0.10) from marine background conditions (κ = 0.52 +/- 0.09) when the particles were mostly composed of marine organics and sulfate. Size-resolved chemical analysis shows that smoke particles were the major contributor to the accumulation mode during long-range transport events, indicating that smoke is mainly responsible for the observed increase in CCN number concentrations. Earlier studies conducted at Barbados have mostly focused on the role of dust in CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Our findings highlight the importance of African smoke for atmospheric processes and cloud formation over the Caribbean. In the file “Dust_Mass_Conc_Royer2022” dust mass concentrations in grams per meter^3 are provided for each day of sampling. These data were used to generate Figure 2a in the manuscript. The file “Particle_Type_#fract_Royer2022” contains data obtained through CCSEM/EDX analysis and used to generate the temporal chemistry plot (Figure 4) provided in the manuscript. The data contains particle numbers for each particle type identified on stage 3 of the sampler, total particle numbers analyzed for the entire stage 3 sample, as well as particle number fractions in % values. In the file “Size-resolved_chem_Royer2022” we provide particle # and number fraction (%) values used to generate size-resolved chemistry plots in the manuscript (Figures 5a and 5b). The file includes all particle numbers and number fractions for sea salt, aged sea salt, dust+sea salt, dust, dust+smoke, smoke, sulfate, and organic particles in each size bin from 0.1 through 8.058 um during cumulative clean marine periods and CAT Event 1 as described in the manuscript. The file “K_at_0.16S_Royer2022” contains κ values calculated at 0.16% supersaturation (S) throughout the entire sampling period. These data were specifically used to generate the plot in Figure 7a. The file “CCN#_at_0.16S_Royer2022” contains cloud condensation nuclei (CCN) values calculated at 0.16% supersaturation (S) throughout the entire sampling period. These data were used to create the CCN portion of the plot in Figure 7b.
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Cloud Microphysical Effects of Aerosol Particle Sources and Marine Boundary Layer Processes
Marine boundary layer (MBL) clouds are an important, though uncertain, part of Earth’s radiative budget. Previous studies have shown sources of aerosol particles in the remote MBL consist of primary sea spray, the oxidation of organic and inorganic vapors derived from the ocean, entrainment from the free troposphere, and anthropogenic pollution. The potential for these particles to become cloud condensation nuclei (CCN) varies largely dependent on their hygroscopic properties. Furthermore, when clouds form, physical processes can alter the optical properties of the cloud. This dissertation aims to identify variations in aerosol sources that affect MBL CCN concentrations and physical processes throughout the cloud lifetime that influence cloud optical properties. Ambient measurements of marine particles and clouds were made throughout two campaigns in the north Pacific and four campaigns in the north Atlantic. Both clean marine and polluted clouds were sampled. In addition, dry MBL particles were measured to identify their chemical composition and size distribution, which is necessary to identify their potential to be CCN active. The organic hygroscopicity influenced CCN concentrations and cloud optical properties significantly for particles that were mostly organic, such as ship stack and generated smoke particles. For a typical range of organic hygroscopicity the amount of reflected solar radiation varied by 2-7% for polluted conditions and less than 1% for clean conditions. Simulated droplet spectral width was shown to be more representative of observations when using a weighted distribution of cloud base heights and updraft velocities, and increased the cloud reflectivity up to 2%. Cloud top entrainment and decoupling of the MBL were found to account for a decrease in cloud radiative forcing. Cloud top entrainment was corrected for homogeneous entrainment and accounted for a decrease in radiative forcing of up to 50 Wm-2. Clustering of individual marine aerosol particles resulted in the identification of particle types derived from dimethyl-sulfide (DMS) oxidation. Two particle types were identified to come from DMS oxidation products and accounted for approximately 25% and 65% of CCN at 0.1% supersaturation during the winter and summer, respectively. One of the particle types was found to be entrained from the free troposphere.
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- Award ID(s):
- 1660509
- NSF-PAR ID:
- 10134732
- Date Published:
- Journal Name:
- UC San Diego Electronic Theses and Dissertations
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. The number concentration and properties of aerosol particles serving ascloud condensation nuclei (CCN) are important for understanding cloudproperties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure thelow-albedo ocean surface. Studies linking aerosol source, composition, andwater uptake properties in this region have been conducted primarily duringthe summertime dust transport season, despite the region receiving a varietyof aerosol particle types throughout the year. In this study, we comparesize-resolved aerosol chemical composition data to the hygroscopicityparameter κ derived from size-resolved CCN measurements made duringthe Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. Weobserved unexpected periods of wintertime long-range transport of Africansmoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN number concentrations as well as the proportions ofdust and smoke particles increased, whereas the average κ slightlydecreased (κ=0.46±0.10) from marine backgroundconditions (κ=0.52±0.09) when the submicron particles were mostly composed of marine organics and sulfate. Size-resolved chemicalanalysis shows that smoke particles were the major contributor to theaccumulation mode during long-range transport events, indicating that smokeis mainly responsible for the observed increase in CCN numberconcentrations. Earlier studies conducted at Barbados have mostly focused onthe role of dust on CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Ourfindings highlight the importance of African smoke for atmospheric processesand cloud formation over the Caribbean.more » « less
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