Atmospheric particles represent a large source of uncertainty in global radiative modeling, whichis driven largely by an incomplete understanding of the processes responsible for their formation and growth. Particles smaller than 100 nm in diameter, termed ultrafine particles, are of particular interest because of their potential to grow into cloud condensation nuclei (CCN). Their ability to act as CCN depends on numerous factors, including their chemical composition and physical properties. These properties depend on the conditions under which the particles formed, meaning that an improved understanding of these conditions will improve global climate modeling. There is currently a poor understanding of the processes leading to the formation and growth of these small particles in remote locations. This dissertation investigates the processes that lead to ultrafine particles in two remote environments: the Alaskan Arctic and the Amazon Basin.

In Chapter 2, we reported indirect measurements of ultrafine particle composition made duringMarch 2009 in Utqiagvik, Alaska. We compare measurements of ambient size-selected ultrafine particles and those measured during two ultrafine particle growth events, all of which occurred during periods with minimal local emissions. The ambient particles were found to be moderately hygroscopic, with measured hygroscopic growth factors (HGFs) ranging from 1.45 to 1.51 at 90 % RH, and largely volatile. Combining these data, we estimated that the volume of these particles were comprised of oxidized organics (∼70 %) and ammoniated sulfates (∼30 %). The first ultrafine particle growth event was associated with both solar radiation and elevated levels of sulfuric acid at the site, and analysis of air mass back-trajectories indicated that this event was influenced by the upper marine boundary layer above the Arctic Ocean. The second event was not associated with solar radiation or sulfuric acid. Air masses for this event were close to the surface of the Arctic Ocean and passed over open leads, suggesting influence from marine emissions on the observed particle composition. Particles in Event 1 (HGF = 2.1 for 35 nm particles) were more hygroscopic than those in Event 2 (HGF = 1.67 for 15 nm, 1.94 for 35 nm), though both were similarly volatile. From these data we estimated that particles in both events were largely comprised of a highly-hygroscopic and volatile sea salt-like compound (Event 1: 74 %; Event 2: 15 nm - 63 %, 35 nm - 74 %), with particulate volume in Event 1 balanced by sulfuric acid (22 %) and oxidized organics (4 %) and that in Event 2 balanced by a large fraction of oxidized organics (15 nm: 37 %; 35 nm: 26%).

The next two chapters of this thesis focus on measurements from the Amazon Basin. Chapter3 reports measurements of sulfuric acid from the Amazon Basin made during the Green Ocean Amazon (GoAmazon2014/5) field experiment during both the wet and dry seasons. There was little difference in median concentrations measured between the wet (7.82x105 molec cm−3) and dry seasons (2.59x105 molec cm−3). While these measurements are consistent with those made in Hyytiälä, Finland, unlike in Hyytiälä there was no obvious correlation between sulfuric acid and radiation. Additionally, we evaluated the ability of existing sulfuric acid parameterizations, which estimate sulfuric acid concentrations based on the concentrations of its sources and sinks, to estimate measurements from the Amazon Basin. None of the parameterizations effectively estimated nighttime measurements. We hypothesize that nighttime sulfuric acid is produced through both oxidation of sulfur dioxide by hydroxyl radical and a stabilized Criegee intermediate pathway; the former is not included in any current proxy. Several existing proxies are effective for daytime estimates.

Chapter 4 reports measurements of ultrafine particle number-size distribution from the TapajósNational Forest, an eastern Amazonian rainforest, made during the transition from the wet-to-dry season (May - July, 2014). Frequent bursts of ultrafine particles were observed at the measurement site. Ultrafine particle growth events decreased in frequency from May through July, as did the number of particles 5 - 30 nm in diameter. The concentration of particles in this size range was correlated with boundary layer height in May, suggesting that particle formation during the wet- ter times of year occurs from nucleation aloft or atmospheric dilution. This correlation decreases through June and July, indicating that other particle formation mechanism contribute during the drier times of year. Air mass back-trajectories suggest that aged anthropogenic influence becomes stronger at the site throughout this transition, which is supported by the observed increase in particles 100 - 110 nm