The Uptake and Spontaneous Oxidation of HONO to Optically Tweezed Aerosol Droplets is Self-Accelerated by Aerosol Acidity
Luke Monroe, GRAHAM THORNHILL, Jack Hall, Ryan Sullivan, Carnegie Mellon University
Abstract Number: 586
Working Group: Aerosol Chemistry
Abstract
Nitrous acid (HONO) is as an important reservoir species for reactive nitrogen oxides that are ozone precursors in the atmosphere, and a potential source of nitrite in aerosol particles. While HONO is known to undergo numerous important surface chemistry reactions, its uptake and reactivity in tropospheric aerosol has largely been ruled out. This is due to the high acidity of atmospheric aerosol that is assumed to inhibit aerosol uptake by keeping NO2– in its protonated and volatile HONO form. The aerosol optical tweezers with its cavity-enhanced Raman spectroscopy and whispering gallery modes allows us to distinguish between NO2– and NO3– in single-levitated droplets in real-time using measurements that do not perturb the droplet’s properties or reaction kinetics. Evidence for the rapid oxidation of HONO to HNO3 was observed and is attributed to a reaction occurring at the air–water interface involving a HONO dimer. NO2– was never observed while the rapid production of NO3– was found to be a strong function of the pH and buffering capacity of the droplet. HONO oxidation to HNO3 was accelerated in unbuffered droplets compared to buffered droplets that resist acidification. In droplets also containing organonitrate SOA from nitrate oxidation of terpenes, reactive uptake drives morphology changes due to acidification by N2O5 hydrolysis or HONO oxidation to HNO3. The evolution from an aqueous homogeneous to a biphasic core–shell morphology shifted the chemistry from nitrite oxidation to nitrate to the N2O5/NO3• oxidation of the new organic shell phase. Numerous Raman modes from the organic products were observed which provide evidence for a long lifetime of these components versus hydrolysis. These results offer new insights into the nighttime sink processes of HONO and nitrite in coarse mode aqueous aerosol particles. HONO uptake to supermicron aerosol may be an unrecognized sink of NOy that also facilitates multiphase chemistry of HONO.