Atmospheric aerosols have a profound effect on climate by scattering and absorbing solar and terrestrial radiation. They can be predominantly scattering (e.g., sea-salt aerosol), predominantly absorbing (e.g., black carbon), or have intermediate optical properties (e.g., light-absorbing "brown carbon" organic aerosol). One of the possible mechanisms by which optical properties of secondary organic aerosols (SOA) can be changed is reactive uptake of ammonia (NH3) by the SOA compounds. Our hypothesis is that ammonia can affect SOA in three independent ways: (1) it reduces acidity of aerosols containing sulfuric acid, which suppresses acid-catalyzed SOA growth reactions; (2) it increases partitioning of semivolatile organic compounds into the particulate phase by forming less volatile ammonium carboxylates; (3) it reacts with carbonyl compounds leading to the formation of various nitrogen-containing organic compounds (NOC), which are now known to account for a significant (~30%) fraction of the total atmospheric particulate nitrogen. The first two effects may change the size of the particles, as well as the hygroscopicity and the real refractive index of the material from which the particles are made. The third effect may change the imaginary refractive index of particles because certain types of NOC compounds act as strong absorbers of visible radiation. For example, previous laboratory studies showed that model SOA generated from biogenic precursors becomes light absorbing and changes color from white to brown when exposed to ammonia. The ammonia emissions are expected to increase worldwide because of the intensifying agricultural use of ammonia-based fertilizers. At the same time, the emissions of VOC precursors are also expected to increase because of the rising trends in the temperature. Therefore, there is an urgent need to understand the effect of ammonia on optical and hygroscopic properties of SOA. We propose to carry out a first systematic experimental investigation of the effect of ammonia on the light-absorption and scattering by SOA. This will represent a team effort combining smog chamber methods (Nizkorodov group), real-time ammonia detectors (Bertram group), instruments for real-time characterization of both real and imaginary refractive indexes of particles (Cappa group) and hygroscopicity (Bertram and Cappa groups), and the unique molecular-level characterization capabilities of high-resolution mass spectrometry available at EMSL (A. Laskin and J. Laskin groups). This study will not only provide semi-empirical data useful for the parameterization of the effect of ammonia on optical properties of SOA leading to applications in modelling, but also detailed molecular level information about the types of products reactions of ammonia with SOA may generate.