Classification of aerosol population type and cloud condensation nuclei properties in a coastal California littoral environment using an unsupervised cluster model

Atmos. Chem. Phys., 19, 6931–6947, 2019 https://doi.org/10.5194/acp-19-6931-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Classification of aerosol population type and cloud condensation nuclei properties in a coastal California littoral environment using an unsupervised cluster model Samuel A. Atwood1 , Sonia M. Kreidenweis1 , Paul J. DeMott1 , Markus D. Petters2 , Gavin C. Cornwell3 , Andrew C. Martin4,a , and Kathryn A. Moore1,3 1 Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA 2 Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC, USA 3 Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, USA 4 Climate Atmospheric Science and Physical Oceanography, Scripps Institution of Oceanography, La Jolla, CA, USA a currently at: Department of Geography, Portland State University, Portland, OR, USA Correspondence: Sonia M. Kreidenweis () Received: 15 December 2018 – Discussion started: 3 January 2019 Revised: 3 April 2019 – Accepted: 25 April 2019 – Published: 23 May 2019 Abstract. Aerosol particle and cloud condensation nuclei (CCN) measurements from a littoral location on the northern coast of California at Bodega Bay Marine Laboratory (BML) are presented for approximately six weeks of observations during the boreal winter–spring as part of the CalWater-2015 field campaign. The nature and variability of surface (marine boundary layer, MBL) aerosol populations were evaluated by classifying observations into periods of similar aerosol and meteorological characteristics using an unsupervised cluster model to derive distinct littoral aerosol population types and link them to source regions. Such classifications support efforts to understand the impact of changing aerosol properties on precipitation and cloud development in the region, including during important atmospheric river (AR) tropical moisture advection events. Eight aerosol population types were identified that were associated with a range of impacts from both marine and terrestrial sources. Average measured total particle number concentrations, size distributions, hygroscopicities, and activated fraction spectra between 0.08 % and 1.1 % supersaturation are given for each of the identified aerosol population types, along with meteorological observations and transport pathways during time periods associated with each type. Five terrestrially influenced aerosol population types represented different degrees of aging of the continental outflow from the coast and interior of California, and their appearance at the BML site was often linked to changes in wind direction and transport path- ways. In particular, distinct aerosol populations, associated with diurnal variations in source regions induced by landand sea-breeze shifts, were classified by the clustering technique. A terrestrial type representing fresh emissions, and/or a recent new particle formation event, occurred in approximately 10 % of the observations. Over the entire study period, three marine-influenced population types were identified that typically occurred when the regular diurnal land and sea-breeze cycle collapsed and BML was continuously ventilated by air masses from marine regions for multiple days. These marine types differed from each other primarily in the degree of cloud processing evident in the size distributions, and in the presence of an additional large-particle mode for the type associated with the highest wind speeds. One of the marine types was associated with a multi-day period during which an atmospheric river made landfall at BML. Differences between many of the terrestrial and marine population types in total CCN number concentrations active at a specific supersaturation were often not as pronounced as the associated differences in the corresponding activated fraction spectra, particularly for supersaturations below about 0.4 %. This finding was due to the generally higher number concentrations in terrestrial air masses offsetting the lower fraction of particles activating at low supersaturations. At higher supersaturations, CCN concentrations for aged terrestrial types were typically above those of the marine types due to their higher number concentrations. Published by Copernicus Publications on behalf of the European Geosciences Union. 6932 1 S. A. Atwood et al.: Classification of aerosol using a cluster model Introduction Atmospheric rivers (ARs) are tropical moisture advection phenomena that can account for large fractions of the wintertime precipitation in California (Ralph et al., 2004; Dettinger et al., 2011). The winter–spring 2015 CalWater-2015 study (Ralph et al., 2015), and coordinated US Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility Cloud Aerosol Precipitation Experiment (ACAPEX) (Leung, 2016) that included aircraft- and ship-based observations in the same region, were designed to probe the atmospheric conditions in and around ARs, and to provide new observations of the characteristics of regional aerosols that may interact with these atmospheric moisture features and thereby influence the downwind formation of precipitation. As part of the CalWater-2015 study, groundbased aerosol observations were conducted at the Bodega Bay Marine Laboratory (BML), a coastal California site that is suitable for observation of aerosols in landfalling marine air masses, and in mixtures of marine and continental air. In marine regions impacted by continental outflow, aerosol chemical and microphysical properties, including particle number concentrations and size distributions, are often moderated by impacts from terrestrial sources (Nair et al., 2013; Wex et al., 2016; Zhao et al., 2016; Phillips et al., 2018). For example, freshly emitted sea spray aerosol particles comprise a mixture of salts with generally high hygroscopicities (κ ∼ 0.6–1.2), and co-emitted organic species with lower hygroscopicities (κ ∼ 0–0.3) (Prather et al., 2013; Quinn et al., 2014) – classified using the κ hygroscopicity parameter (Petters and Kreidenweis, 2007). Bulk hygroscopicity values above 1 are infrequently observed, but have been reported for some background- and precipitation-impacted marine aerosol populations (Good et al., 2010; Prather et al., 2013). As the aerosol ages, chlorine replacement by uptake of acidic gases can reduce the hygroscopicity of the salts (FinlaysonPitts and Pitts, 1999; Song and Carmichael, 1999). Aging and organic components result in reported κ values in the range of 0.4 to 0.7 for more pristine or background marine aerosol populations (Keene et al., 2007; Bates et al., 2012; Prather et al., 2013; Quinn et al., 2014; Zhang et al., 2014; Forestieri et al., 2016; Atwood et al., 2017; Royalty et al., 2017; Phillips et al., 2018). In contrast, average hygroscopicities for continental aerosol populations are oft (...truncated)