The impact of anthropogenic aerosol on climate forcing remains uncertain largely due to inadequate representation of natural aerosols in climate models. The marine boundary layer (MBL) might serve as a model location to study natural aerosol processes. Yet source and sink mechanisms controlling the MBL aerosol number, size distribution, chemical composition, and hygroscopic properties remain poorly constrained. Here aerosol size distribution and water uptake measurements were made aboard the R/V Hi'ialakai from 27 June to 3 July 2016 in the subtropical North Pacific Ocean. Size distributions were predominantly bimodal with an average integrated number concentration of 197 ± 98 cm−3. Hygroscopic growth factors were measured using the tandem differential mobility analyzer technique for dry 48, 96, and 144 nm particles. Mode kappa values for these were 0.57 ± 0.12, 0.51 ± 0.09, and 0.52 ± 0.08, respectively. To better understand remote MBL aerosol sources, a new algorithm was developed which decomposes hygroscopicity distributions into three classes: carbon‐containing particles, sulfate‐like particles, and sodium‐containing particles. Results from this algorithm showed low and steady sodium‐containing particle concentrations while the sulfate‐like and carbon‐containing particle concentrations varied during the cruise. According to the classification scheme, carbon‐containing particles contributed at least 3–7%, sulfate‐like particles contributed at most 77–88% and sodium‐containing particles at least contributed 9–16% to the total aerosol number concentration. Size distribution and hygroscopicity data, in conjunction with air mass back trajectory analysis, suggested that the aerosol budget in the subtropical North Pacific MBL may be controlled by aerosol entrainment from the free troposphere.

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Atmospheric measurements of aerosol size‐resolved hygroscopicity at submicron sizes are carried out at the United States Army Corps of Engineers Field Research Facility in Duck, North Carolina. The scientific aim of the field deployment is to gain improved understanding of the springtime advection of aerosols from the East Coast of the United States over the Atlantic and help to constrain assessments of anthropogenic particle contributions to the marine boundary layer aerosol budget. Air mass back trajectories show that the aerosol sampled at the coast is largely of continental origin that either gets transported directly from the land or spends some time over the Atlantic Ocean. Aerosol size‐resolved hygroscopicity measurements are consistent with air masses of both continental and marine background that are heavily influenced by the continental outflow. Aitken and accumulation mode mean diameters range from 49.1 ± 1.7 nm to 66.9 ± 0.8 nm and 142.8 ± 1.1 nm to 155.0 ± 2.8 nm, respectively. Hygroscopicity distributions for 96 nm, 188 nm, and 284 nm dry‐sized particles show the mode hygroscopicity parameter range from 0.20 ± 0.01 to 0.54 ± 0.03, suggesting the presence of anthropogenic aerosols. We have used the method described by Royalty et al. (2017) to decompose the hygroscopicity distributions into three distinct classes based on the ambient aerosol hygroscopic properties relative to the hygroscopic properties of a reference compound. The method shows that continental outflow heavily influences aerosol chemical and physical properties at the East Coast, with hygroscopicities of submicron aerosols consistent with sulfate‐containing species (62% to 83%), with small contributions from sodium‐ and carbon‐containing particles (up to 9% and 37%, respectively).

Local sources of particles and precursor gases have long been considered as the major control for the ground‐level particle number concentration in an urban environment. Here we show the existence of two distinct sources. The first source was detectable during morning and afternoon rush hours and was defined by high black carbon concentrations. Particle number concentration inversely correlated with the local planetary boundary layer height. The particle size distributions were characterized by a wide range of modal diameters and did not exhibit detectable modal growth. This source was attributed to vehicular emissions. The second source yielded particle number concentration comparable to those during the rush hours and was detected six times over the 3‐week measurement campaign. Small particles produced by this source were recorded during the midday after the diminishment of the rush‐hour traffic effects. The particles exhibited prolonged modal growth over 8 hr, which may indicate a regional scale nucleation event. The data suggest that these particles were likely formed above the nocturnal boundary layer after sunrise and were subsequently transported to the surface through convective mixing. Overall, the nocturnal and convective boundary layer evolution was found to be closely associated with the of small particle event and the most important factor affecting the ground‐level particle number concentration. Shallow nocturnal boundary layers trapped pollution near the ground leading to particle number concentrations over 104 cm−3.