Classification of aerosol population type and cloud condensation nuclei properties in a coastal California littoral environment using an unsupervised cluster model
Authors: Atwood Samuel A. , Kreidenweis Sonia M. , DeMott Paul J. , Petters Markus D. , Cornwell Gavin C. , Martin Andrew C. , Moore Kathryn A. 
Published: 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 pathways. In particular, distinct aerosol
populations, associated with diurnal variations in source regions induced by
land- and 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.