Low temperature growth of sub 10 nm particles by ammonium nitrate condensation

Abstract

Co-condensation of nitric acid and ammonia vapors to form ammonium nitrate transforms from a fully semi-volatile behavior when it is relatively warm (273 K and above, typical of the seasonal planetary boundary layer) into effectively non-volatile and irreversible uptake for the limiting vapor when it is cold (well below 273 K, typical of the upper troposphere and occasionally the wintertime boundary layer). This causes the system to switch in character from the one governed by semi-volatile equilibrium (how it is usually portrayed) to the one governed by irreversible reactive uptake to even the smallest particles. Uptake involves an activation diameter, which can be as small as 1 nm for typical vapor concentrations, and subsequent growth rates can be very high, exceeding 1000 nm h⁻¹. In addition to this somewhat surprising behavior, the system provides an exemplary case for semi-volatile reactive uptake within the context of volatility and saturation ratios.