<p>Atmospheric particles are ubiquitous in the Earth’s atmosphere and have potential to influence atmospheric chemistry, visibility, global climate and human health, particularly downwind from major pollution sources. The main objective of this thesis was to investigate questions pertaining to the microphysical, chemical and optical properties of aerosol particles by using in situ data collected during four experiments carried out in different regions of the Northern Hemisphere.</p><p>The first two papers of this thesis reports on airborne measurements of the aerosol optical properties performed over the North Atlantic and the Los Angeles basin. Airmasses from Europe and North Africa are usually advected in over the North Atlantic, alternating with the background marine conditions. The results showed that the aerosols are not uniformly distributed in the area and variability in the aerosol fields occurs at sub-synoptic scales. It was also observed that the single scattering coefficient varied as the polluted plumes aged, suggesting a relationship between this quantity and transport time. The measurements performed around the Los Angeles basin showed that the area’s complex topography and local meteorological circulations exert a strong control on the distribution of the aerosol in the basin. Large spatio-temporal gradients in the aerosol optical properties were observed along a transect flown from the shore towards the mountains. Profiles flown over sites located on the mountains displayed a stratified configuration with elevated aerosol layers.</p><p>Airborne data of residual particles collected in orographic wave clouds over Scandinavia were analyzed using a single particle analysis technique. Mineral dust, organic aerosols and sea salt were the main group of particles identified. Residuals composed predominantly of mineral dust were found in glaciated clouds while organic residuals were found in liquid clouds. The results suggest that organic material may inhibit freezing and have considerable influence on supercooled clouds that form through heterogeneous pathways.</p><p>The partitioning of the aerosol particles between cloud droplets and interstitial air has been addressed in terms of their microphysical properties using data obtained at a mountain-top site in Sweden during a stratocumulus event. The results showed that the scavenging efficiency varied during the cloud event, and Aitken-mode particles were also efficiently scavenged in addition to accumulation-mode particles. It is hypothesized that alterations of the aerosol chemical composition occurred during the measurement period, modifying the hygroscopic nature of the particles and decreasing their activation diameter.</p>

Cloud reflectivity is a function of cloud liquid water content and droplet number concentration. Since cloud droplets form around pre-existing aerosol particles, cloud droplet number concentration depends on the availability of particles that can serve as cloud condensation nuclei. Given constant liquid water amount, increased availability of cloud condensation nuclei leads to clouds with a greater droplet number concentration, greater total droplet surface area and consequently, greater reflectivity. The change in cloud reflectivity resulting from the increased availability of condensation nuclei is known as the aerosol indirect effect. The aerosol indirect effect ranks as one of the largest sources of uncertainty in current estimates of global climate change, largely due to difficulties in measurement. Changes in cloud reflectivity resulting from the aerosol indirect effect are typically much smaller than the natural background variability observed in clouds. As a result, the modification signal is very difficult to detect against the background noise. Additionally, since atmospheric aerosols are ubiquitous, it is difficult to find polluted and nonpolluted clouds that are sufficiently alike for reasonable comparison. However, ship tracks seen in satellite images present one opportunity to study the aerosol indirect effect in relative isolation. Ship tracks are regions of enhanced reflectivity in marine stratus, resulting from the addition of aerosols from ship exhaust plumes to preexisting clouds. Ship tracks are a common feature of satellite images of the North Pacific. Since the marine atmosphere has comparatively low background aerosol concentrations, the addition of ship exhaust particles can lead to distinct increases in cloud reflectivity. Ship tracks allow for sampling of polluted and nonpolluted clouds from adjacent regions with similar solar and viewing geometry, cloud temperatures and surface properties, and consequently provide a unique opportunity to study the effects of aerosol modification of cloud reflectivity. Using satellite images of the North Pacific in July 1999, over 1000 ship tracks were identified, logged and analyzed, yielding 504 sets of radiance data matching polluted clouds with nearby nonpolluted clouds. It was expected that increasing the size of the region for selection of nonpolluted clouds would increase the variability in observed reflectivity, and make detection of the modification signal more difficult. In order to study this potential effect of domain size for selection of nonpolluted clouds on measurements of the aerosol indirect effect, three data sets were collected, using domain sizes for selection of nonpolluted clouds of 15, 50 and 100 km. Analysis of retrieved optical depth and droplet effective radius for modified and control pixels shows evidence of a 1-5% increase in visible optical depth of marine stratus following modification by addition of ship exhaust particles, but unexpectedly, shows only slight increases in uncertainty with increasing domain size. A subsequent study revealed that autocorrelation lengths of radiances and retrieved cloud properties were only 8-15 km. This indicates that even the 15 km control domain captured much of the background variability present. Domain sizes smaller than 15 km are difficult to sample automatically while avoiding the inclusion of polluted clouds in the nonpolluted cloud sample. As a result, it remains necessary to analyze large numbers of ship tracks to separate the aerosol modification signal from the background variability. / Graduation date: 2003

Atmospheric particles are ubiquitous in the Earth’s atmosphere and have potential to influence atmospheric chemistry, visibility, global climate and human health, particularly downwind from major pollution sources. The main objective of this thesis was to investigate questions pertaining to the microphysical, chemical and optical properties of aerosol particles by using in situ data collected during four experiments carried out in different regions of the Northern Hemisphere. The first two papers of this thesis reports on airborne measurements of the aerosol optical properties performed over the North Atlantic and the Los Angeles basin. Airmasses from Europe and North Africa are usually advected in over the North Atlantic, alternating with the background marine conditions. The results showed that the aerosols are not uniformly distributed in the area and variability in the aerosol fields occurs at sub-synoptic scales. It was also observed that the single scattering coefficient varied as the polluted plumes aged, suggesting a relationship between this quantity and transport time. The measurements performed around the Los Angeles basin showed that the area’s complex topography and local meteorological circulations exert a strong control on the distribution of the aerosol in the basin. Large spatio-temporal gradients in the aerosol optical properties were observed along a transect flown from the shore towards the mountains. Profiles flown over sites located on the mountains displayed a stratified configuration with elevated aerosol layers. Airborne data of residual particles collected in orographic wave clouds over Scandinavia were analyzed using a single particle analysis technique. Mineral dust, organic aerosols and sea salt were the main group of particles identified. Residuals composed predominantly of mineral dust were found in glaciated clouds while organic residuals were found in liquid clouds. The results suggest that organic material may inhibit freezing and have considerable influence on supercooled clouds that form through heterogeneous pathways. The partitioning of the aerosol particles between cloud droplets and interstitial air has been addressed in terms of their microphysical properties using data obtained at a mountain-top site in Sweden during a stratocumulus event. The results showed that the scavenging efficiency varied during the cloud event, and Aitken-mode particles were also efficiently scavenged in addition to accumulation-mode particles. It is hypothesized that alterations of the aerosol chemical composition occurred during the measurement period, modifying the hygroscopic nature of the particles and decreasing their activation diameter.

Atmospheric aerosols may be emitted directly as particles (primary) or formed from gaseous precursors (secondary) from different natural and anthropogenic sources. The highly populated South and East Asia regions are currently in a phase of rapid economic growth to which high emissions of carbonaceous aerosols are coupled. This leads to generally poor air quality and a substantial impact of anthropogenic aerosols on the regional climate. However, the emissions of different carbon aerosol components are still poorly constrained. Water-soluble organic carbon (WSOC) is a large (20-80%) component of carbonaceous aerosols that can absorb solar light and enhance cloud formation, influencing both the direct and indirect climate effects of the aerosols. A novel method for carbon isotope-based studies, including source apportionment, of the WSOC component of ambient aerosols was developed and tested for recovery efficiency and the risk of contamination using both synthetic test substances and ambient aerosols (paper I). The application of this method for the source apportionment of aerosols in South and East Asia shows that fossil fuel input to WSOC is significant in both South Asia (about 17-23%) highly impacted by biomass combustion practices and in East Asia (up to 50%) dominated by fossil energy sources (papers II, III, IV). Fossil fraction in WSOC in the outflow from northern China is considerably larger than what has been measured in South Asia, Europe and USA (paper IV). A trend of enrichment in heavy stable carbon isotopes in WSOC with distance the particles have been transported from the source is observed in the South Asian region (papers II, III). Dual-isotope (Δ14C and δ13C) analysis demonstrates that WSOC is highly influenced by atmospheric aging processes. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Submitted.</p>

Secondary organic aerosol (SOA) generated through the partitioning of gas phase volatile organic carbon compounds (VOCs) into the condensed phase has both epidemiological and climatic impacts through the growth of particulate matter into relevant sizes for respiratory interactions and cloud condensation nuclei activity. Considering the complex chemistry involved with VOC oxidation and subsequent formation of SOA, bulk properties like oxidation state, often represented by O:C ratio, and volatility are used to simplify the representation of SOA in chemical transport models (CTMs) and the like [e.g. Tsimpidi et al. 2010]. This preference for bulk properties is supported by the availability of ambient measurement techniques to constrain model parameters and scenarios. The volatility of SOA is often described by treating it as a mixture of components with differing partitioning coefficients through the volatility basis set (VBS) approach rather than explicitly resolving the complex chemistry [Donahue et al., 2006]. This study presents a method of determining the volatility of an aerosol sample through the use of an optimizing thermodenuder (TD) instrument model that is used to fit laboratory data. Data collected using a volatility tandem differential mobility analyzer (VTDMA) setup consist of inlet and outlet particle size and number concentrations for select dicarboxylic acids - compounds known to contribute to atmospheric SOA. These are interpreted by the model through an iterative optimization routine to obtain estimates of volatility parameters (e.g. saturation concentrations) which are compared to available literature data. The instrument model is currently divided into two decoupled modules. The first resolves the flow field characteristics, obtaining the temperature profile, pressure variations, and radial velocity distribution of the TD, and the second resolves the gas to particle partitioning of aerosol with a given condensed-phase volatility distribution in the TD using the VBS approach as described in the literature. Solving the full hydrodynamic equations for the flow characteristics provides a better numeric representation of entry length and radial velocity variations and is an improvement over similar TD modeling studies in the literature. However, results indicate that coupling the two modules is necessary to more accurately resolve the suppression of evaporation due to buildup of organic vapors in the TD, even at the low mass concentrations involved with the presented experiments.

Atmospheric aerosols are known to exert a significant influence on the Earth's climate system; however, the magnitude of this influence is highly uncertain because of the complex interaction between aerosols and water vapor to form clouds. Toward reducing this uncertainty, this dissertation outlines a series of laboratory and in-situ field measurements, instrument technique development, and model simulations designed to characterize the ability of aerosols to act as cloud condensation nuclei (CCN) and form cloud droplets. Specifically, we empirically quantify the mixing state and thermodynamic properties of organic aerosols (e.g., hygroscopicity and droplet condensational uptake coefficient) measured in polluted and non-polluted environments including Alaska, California, and Georgia. It is shown that organic aerosols comprise a substantial portion of the aerosol mass and are often water soluble. CCN measurements are compared to predictions from theory in order to determine the error associated with simplified composition and mixing state assumptions employed by current large-scale models, and these errors are used to constrain the uncertainty of global and regional cloud droplet number and albedo using a recently-developed cloud droplet parameterization adjoint coupled with the GMI chemical transport model. These sensitivities are important because they describe the main determinants of climate forcing. We also present two novel techniques for fast measurements of CCN concentrations with high size, supersaturation, and temporal resolution that substantially improve the state of the art by several orders of magnitude. Ultimately, this work represents a step toward better understanding how atmospheric aerosols influence cloud properties and Earth's climate.

The aerosol direct and indirect effects were investigated for three specific cases during the March 2000 Cloud IOP at the SGP site by using a modified WRF model. The WRF model was previously altered to include a two-moment bulk microphysical scheme for the aerosol indirect effect and a modified Goddard shortwave radiation scheme for the aerosol direct effect. The three cases studied include a developing low pressure system, a low precipitation event of mainly cirrus clouds, and a cold frontal passage. Three different aerosol profiles were used with surface concentrations ranging from 210 cm-3 to 12,000 cm-3. In addition, each case and each aerosol profile was run both with and without the aerosol direct effect. Regardless of the case, increasing the aerosol concentration generally increased cloud water and droplet values while decreasing rain water and droplet values. Increased aerosols also decreased the surface shortwave radiative flux for every case; which was greatest when the aerosol direct effect was included. For convective periods during polluted model runs, the aerosol direct effect lowered the surface temperature and reduced convection leading to a lower cloud fraction. During most convective periods, the changes to cloud, rain, and ice water mixing ratios and number concentrations produced a nonlinear precipitation trend. A balance between these values was achieved for moderate aerosol profiles, which produced the highest convective precipitation rates. In non-convective cases, due to the presence of ice particles, aerosol concentration and precipitation amounts were positively correlated. The aerosol threshold between precipitation enhancement and suppression should be further studied for specific cloud types as well as for specific synoptic weather patterns to determine its precise values.

This thesis improves the understanding of the fate and transport of aerosols in North America. Aerosols play an important role climate modification, visibility, human health, and regulatory compliance. Through multiple aircraft-based and ground-based field campaigns, in situ ambient bulk aerosol concentrations will be determined across geographically diverse regions of the United States. By examining aerosol composition - specifically, inorganic ions and water-soluble organic carbon - as they are transported across the Pacific Ocean, we can observe background concentrations that may contribute to aerosol loading observed in many US communities. Furthermore, the aerosol continues to be modified by anthropogenic and biogenic emissions, dry and wet deposition, and secondary formation and transformation as it is transported across the continent. To capture some of these dynamic changes, aerosol will be extensively measured near the east coast of the US and Canada, and results may show significant anthopogenic, biogenic, and secondary transformation. Many results from the Northeastern United States and Southeastern Canada will be presented, and a special case study discussing acid-catalyzed formation of secondary organic aerosol in the region of northern Georgia, US is discussed. Lastly, through laboratory- and field-based instrument development, a commonly-deployed instrument is modified for improved measurement of organic carbon and results are presented herein.

This thesis focuses on the characterization of soluble iron in ambient/urban and source emission aerosols, primarily focusing on the sources and atmospheric processes contributing to iron solubility. Multiple techniques, including bulk and single particle measurements, were used to investigate the complex chemistry of iron solubility. A technique was developed and validated (PILS-LWCC), allowing for 12-minute measurements of water-soluble ferrous iron (WS_Fe(II)) in aerosols with a limit of detection of 4.6 ng m-3 and 12% relative uncertainty. The PILS-LWCC was deployed at several urban field sites (Atlanta, GA and Dearborn, MI) and a biomass burning event to determine major sources and atmospheric processes of WS_Fe(II) in aerosols. PILS-LWCC measurements suggest that industrial and biomass burning are sources of WS_Fe(II). In addition, acid-processing mechanisms also appeared to influence WS_Fe(II) concentrations, based on a strong correlation between WS_Fe(II) and SO42- (r2 = 0.76) as well as apparent aerosol acidity (r2 =0.78) during a field campaign in Atlanta, GA. Synchrotron-based techniques, such as X-ray Absorption Near-Edge Structure (XANES) spectroscopy and micro X-ray fluorescence measurements, were also used to identify the chemical composition (redox state and phase) and mixing state (two properties that may influence iron solubility) of source emission and ambient single iron-containing particles. These single particle measurements were used in conjunction with bulk iron solubility to assess the influence of chemical composition and mixing state on iron solubility. Single particle (synchrotron-based) and bulk iron solubility measurements suggested that iron solubility is not primarily driven by chemical composition in source emission and ambient particles. Differences in iron solubility, however, were related to single particle sulfur content in ambient and source emission aerosols, suggesting that similar sources/atmospheric processes control iron solubility in these samples. The relationship between iron solubility and sulfur content corresponded well with bulk ground-based measurements of ambient aerosol using the PILS-LWCC. Combined single particle and bulk online measurements provide compelling evidence that atmospheric acid processing, involving sulfur-containing acids (H2SO4), is an important formation route of soluble iron in ambient urban aerosols. The results of this thesis provide valuable information to further understanding the controls of iron solubility in atmospheric aerosols.

900 archived FRM filters from 15 sites over the southeast during 2007 were analyzed for PM2.5 chemical composition and physical properties. Secondary components (i.e. sulfate aerosol and SOA) were the major contributors to the PM2.5 mass over the southeast, whereas the contribution from biomass burning varied with season and was negligible (2%) during summer. Excluding biomass burning influence, FRM WSOC was spatially homogeneous throughout the region, similar to sulfate, yet WSOC was moderately enhanced in locations of greater predicted isoprene emissions in summer. On smaller spatial scale, a substantial urban/rural gradient of WSOC was found through comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008, indicating important contribution from anthropogenic emissions. A comparative study between Atlanta and LA reveals a number of contrasting features between two cities. WSOC gas-particle partitioning, investigated through the fraction of total WSOC in the particle phase, Fp, exhibited differing relationships with ambient RH and organic aerosols. In Atlanta, both particle water and organic aerosol (OA) can serve as an absorbing phase. In contrast, in LA the aerosol water was not an important absorbing phase, instead, Fp was correlated with OA mass. Fresh LA WSOC had a consistent brown color and a bulk absorption per soluble carbon mass at 365 nm that was 4 to 6 times higher than freshly-formed Atlanta soluble organic carbon. Interpreting soluble brown carbon as a property of freshly-formed anthropogenic SOA, the difference in absorption per carbon mass between the two cities suggests most WSOC formed within Atlanta is not from an anthropogenic process similar to LA.