Atmospheric aerosols are liquid or solid particles that are suspended in the air. Despite their small size (10 nm - 10μm) and low concentration in the air, their presence has pro- found impacts on earth climate and human health. Aerosols can influence earth’s radiative balance through direct or indirect mechanisms. Both aerosol’s direct and indirect radiative forcing effect are poorly quantified in global radiative transfer models. Aerosol chemistry and physics research on aerosol’s direct and indirect radiative forcing effect would improve our understanding of the earth climate system. In addition, this research could provide strategies to combat the urgent concern of climate change and air pollution.Organic aerosols, which account for a large fraction of atmospheric aerosols, have the most complex composition thanks to the rich variety of organic molecules. Depending on their chemical and microphysical properties, organic aerosols can participate in various atmospheric processes resulting in various kinds of impacts. The complex nature of aerosols is a key source of error in assessing their climate impact. Additional studies on organic aerosols and how they interact with light, in particular, would help to provide insight into their roles in influencing climate. In this work, research relevant to both direct and indirect forcing of atmospheric aerosol was conducted. Three particular areas of work are documented: 1) Development of an instrument called a Polarized Nephelometer that measures the angular distribution of light scattered by an aerosol. 2) Measurement of photolytic mass loss processes of humic substances as surrogates for organic particulate matter. 3) Investigation of aqueous photolysis of water- soluble particulate matter from simulated prescribed and wildfire biomass burning.