UPTAKE OF N2O5 AND HNO3 BY AQUEOUS SULFURIC-ACID DROPLETS

Uptake coefficients for gaseous N2O5 and HNO3 into liquid 0.33 H2SO4 mole fraction (73 wt %) droplets at 283 K have been measured. HNO3 in the gas phase was directly observed as a product of N2O5 uptake. The experimental method employs a monodisperse train of droplets (approximately 200-mu-m diameter) in a low-pressure flow reactor. Droplet-trace gas interaction times are approximately 1-2 ms. Uptake coefficients are 0.058 +/- 0.006 for N2O5 and 0.11 +/- 0.01 for HNO3. These values are 40% larger and smaller than for N2O5 and HNO3 uptake, respectively, into pure water. The product branching ratio for gaseous evolution of HNO3 from N2O5 heterogeneous reaction ranged from 0.25 to 0.37 for gas/liquid contact times of (1.1-1.6) x 10(-3)s. Both the reduction in HNO3 uptake coefficient (relative to that on water) and the HNO3 gaseous evolution from N2O5 uptake can be explained by limited HNO3 solubility in aqueous sulfuric acid. A value of 4 (+/- 2) x 10(3) M atm-1 at 283 K is derived for the Henry's law constant based on a time-dependent model of this limitation. Nitric acid production from the heterogeneous reaction of N2O5 with aqueous sulfuric acid may provide an important mechanism for removal of NO(x) from the stratosphere.