High Time Resolution Nucleation Rates from Newly Developed Aerosol Particle Instrumentation

<p dir="ltr">Atmospheric aerosols play an important role in Earth’s climate. Aerosol particles can directly influence climate by scattering and absorbing light or indirectly influence climate by serving as cloud condensation nuclei (CCN). CCN can alter cloud properties, such as cloud brightness, albedo, and lifetime, affecting the amount of solar radiation reflected by clouds and, ultimately, Earth’s radiative balance. Over half of CCN originate from new particle formation (NPF). NPF is the process where gas molecules nucleate and form detectable atmospheric particles approximately 1 nm in diameter. While NPF is an extremely important atmospheric process, its parameters of nucleation and growth rates, the rates at which newly formed particles appear and grow to larger sizes, remain poorly understood, largely due to instrumentation constraints. Traditional instruments used to measure particle size distributions have poor time resolution and low sensitivity in the sub-3 nm size range. </p><p dir="ltr">The purpose of my thesis is to (1) improve the instruments and methods used to sample newly formed particles, enhancing both time resolution and sensitivity, and (2) compare nucleation rates calculated with these newly developed particle-based instruments and methods to nucleation rates calculated with gas vapor concentrations measured from mass spectrometry. A major focus has been the development of particle sampling methods that do not rely on charged particles. Charging particles in the sub-3 nm size range is extremely inefficient (< 1%) and prone to particle composition-based biases. To that extent, I developed a novel particle separation method based on particle diffusion with particle transmission efficiency similar to charged mobility separation methods. However, I also describe multiple improvements that could improve the transmission efficiency of my method by orders of magnitude. </p><p dir="ltr">The main instrument of my thesis is the condensation particle counter (CPC). The CPC grows particles to optically detectable sizes by condensing a working fluid, often water or butanol, onto the sampled particles. I have shown that the CPC can measure the concentration of particles as small as single molecules of sulfuric acid. I also demonstrate two methods to measure size distributions of freshly nucleated particles using CPCs. First is a method that sizes particles based on the size of the grown droplets. The second method uses an array of physically identical CPCs operated at different detection capabilities. This method sizes particles using the differential measurements between each CPC. The array of CPCs was compared to traditional particle instruments and found to detect both fast fluctuations in the concentration of newly formed particles and particles that were otherwise missed. </p><p dir="ltr">The newly developed CPC array was deployed during a field campaign in Pittsburgh, PA, alongside a chemical ionization mass spectrometer (CIMS). I calculated high-time resolution particle nucleation rates, the rate at which newly formed particles appear, from both particle concentrations and gas phase precursors. Particle-based nucleation rates were calculated using the measured concentrations of the smallest particles. Gas vapor-based nucleation rates were calculated by modeling the formation of discrete sulfuric acid–dimethylamine clusters using an acid-base nucleation model. The nucleation rates were found to agree well during sulfuric acid-driven nucleation events. The comparison study also demonstrated that in urban atmospheres, newly formed particles cannot be assumed to be well mixed and particle nucleation cannot be assumed to occur at steady-state.</p>