Particulate matter (PM) suspended in air varies in size from nanometers to micrometers and contains a wide range of chemical components, including organic compounds, black carbon (soot), inorganic minerals and metals. Atmospheric aerosols are generated from either primary sources like volcanic eruptions, re-suspended soil dust, sea spray, vegetative detritus, fossil fuel and biomass combustion emissions; or secondary atmospheric reactions via gas-to-particle conversion of atmospheric gases. Particle size, abundance, and chemical composition determine how a particle interacts with light and other atmospheric constituents (e.g. gases, water vapor) in addition to its impact on human health. While atmospheric scientists have been working on characterizing atmospheric aerosols for many years, major gaps persist in understanding the properties of many globally-important sources. This dissertation provides new understanding of the chemical composition of biomass burning and sea spray aerosols. PM emissions from biomass burning vary by fuel, and depend on fuel type and composition, moisture content, and combustion conditions. Although biomass smoke is critically important in global climate and local-regional health impacts, the physical and chemical composition of biomass burning aerosol is still not fully understood in the case of peat, agricultural residues and cooking fires. The Fire Laboratory at Missoula Experiments (FLAME) were designed to fulfill these gaps to improve our understanding in both historically undersampled and well-studied fuels while adding new instrumentation and experimental methods to provide previously unavailable information on chemical properties of biomass burning emissions. Globally-important biomass fuels were combusted in a controlled environment, and PM was chemically characterized to compute fuel based emission factors (EF) as the amount of chemical species released per unit mass of fuel burned. We showed that chemical composition of PM varies for different fuel types and certain fuels types (e.g., peat and ocote) emit considerably high concentrations of polycyclic aromatic compounds that are associated with negative health effects. We also showed that PM from biomass smoke contains fluoride for the first time, at approximately 0.1% by weight. With respect to the annual global emissions of PM due to biomass burning, this makes biomass burning an important source of fluoride to the atmosphere. Further, peatland fire emissions are one of the most understudied atmospheric aerosol sources but are a major source of greenhouse gases globally and cause severe air quality problems in Asia. This thesis provides the first field-based emissions characterization study, for samples collected at peat burning sites in Central Kalimantan, Indonesia. Using these EFs and estimates of the mass of fuel burned, it was estimated that 3.2 - 11 Tg of PM2.5 were emitted to atmosphere during 2015 El Niño peat fire episode which is ~10 % of estimated total annual PM flux for biomass burning. Overall, these studies computed more representative EFs for previously undersampled sources like peat, and previously unidentified chemical species like fluoride that can be used to update regional and global emission inventories. The concentration and composition of organic compounds in sea spray aerosol (SSA) alters its optical properties, hygroscopicity, cloud condensation, and ice nucleation properties and thus affects Earth’s radiative budget. In the past, SSA has been difficult to characterize, because of low concentrations relative to background pollutants. Nascent SSA was generated during a mesocosm, using a wave-flume at the University of California, San Diego and was characterized for saccharides and inorganic ions in order to assess their relative enrichment in fine (PM2.5) and coarse (PM10-2.5) SSA and sea surface microlayer (SSML) relative to seawater. For the first time, we showed that saccharides comprise a significant fraction of organic matter in fine and coarse SSA contributing 11 % and 27 %, respectively. Relative to sodium, saccharides were enriched 14-1314 times in fine SSA, 3-138 times in coarse SSA, but only up to 1.0-16.2 times in SSML. The saccharide and ion concentration in SSML and persistent whitecap foam was quantitatively assessed by another mesocosm study performed under controlled conditions. We demonstrated that relative to sodium, saccharides were enriched 1.7-6.4 times in SSML and 2.1-12 times in foam. Higher enrichment of saccharides in foam over the SSML indicates that surface active organic compounds become increasingly enriched on aged bubble film surfaces. Similarly, we showed that fine SSA contains saccharides characteristic of energy-related polysaccharides, while coarse SSA contains saccharides that are characteristic of structure-related polysaccharides. The ultrafiltration studies showed that structure-related polysaccharides effectively coagulate to form large particulate organic matter and size is likely the reason for their exclusion from small SSA. The enrichment of organic species in SSML, foam and SSA led to an enrichment of inorganic ions probably through chelation with organic molecules. Mean enrichment factors for major ions demonstrated the highest enrichment in fine SSA for potassium (1.3), magnesium (1.4), and calcium (1.7). Consequently, due to these organic and inorganic enrichments, SSA develops a significantly different chemical profile compared to seawater. These improved chemical profiles of SSA should be used to develop laboratory proxies to further study the transfer of organic matter across the ocean-air interface and the physical properties of SSA. . Overall, the results presented in this dissertation provide new chemical profiles for previously understudied emission sources like peatland fire emissions, and previously unquantified chemical species like F- in biomass burning emissions and enrichment of saccharides and ions in SSA. These data could be used in updating regional and global emission inventories, atmospheric modeling and human exposure studies.