Exploring Nighttime Uptake of HONO and Nitrite Oxidation on Coarse Mode Aqueous Aerosol Particles Utilizing the Aerosol Optical Tweezers

Luke Monroe, Jack Hall, Graham Thornhill, RYAN SULLIVAN, Carnegie Mellon University

     Abstract Number: 552
     Working Group: Aerosol Chemistry

Abstract
Nitrous acid (HONO) acts as an important reservoir species for inorganic nitrogen in the atmosphere and a potential source of nitrite in aerosol particles. HONO rapidly photolyzes in the morning to provide a significant source of NO_x and hydroxyl radical prior to peak daylight production. Sink processes for HONO can take place in the gas phase but are often accelerated by surface adsorption, which can be pH dependent as HONO is volatile in its protonated state. Direct measurement of these species is difficult due to the instability and surface chemistry of nitrous acid in instrument inlets. The aerosol optical tweezers (AOT) can use cavity-enhanced Raman spectroscopy and whispering gallery modes to probe chemical information, droplet size, and droplet refractive index of single-levitated droplets in real-time using measurements that do not perturb the droplet’s properties or reaction kinetics. To better understand the ability of HONO to partition to supermicron aqueous aerosols and the kinetics of nitrite oxidation to nitrate at night, the AOT was used to investigate trapped droplets of varying chemical compositions, morphology, and pH under HONO exposure in oxic and anoxic conditions. The droplet accelerated kinetics of nitrite oxidation in a droplet were investigated under conditions simulating polluted and pristine atmospheric conditions and compared to bulk rates. The results presented here offer new insight on the nighttime sink processes of HONO and nitrite in coarse mode aqueous aerosol particles, suggesting that HONO uptake to supermicron aerosol may be an unrecognized sink of NO_y that also facilitates aqueous chemistry of nitrite.