Ammonium is an important neutralizing species for sulfate and has a significant effect upon hygroscopicity. It is usually assumed that particulate ammonium concentrations are controlled by equilibrium with the gas phase. Quinn et al. [1992] made simultaneous measurements of the relevant oceanic, atmospheric, and particulate concentrations of ammonia to ascertain the degree to which ammonia approached equilibrium between these phases. The degree of neutralization of the particulate matter appeared to affect equilibrium, with equilibrium observed only for those particles which are not fully neutralized. The importance of particle hygroscopicity to visibility and climate effects assessments suggests the need for further research in this area.
The cycles of reactive nitrogen are of interest in understanding the oxidative capacity of the atmosphere. Nitric acid vapor and particulate nitrate are the stable end products of NOx oxidation. Lee et al. [1994] report three years of measurements of free tropospheric total nitrate at Mauna Loa Observatory. High nitrate concentrations were associated with transport pathways from Asia and North America. However, other factors---vertical mixing, stratospheric sources, and precipitation removal---appeared to be more important in determining observed concentrations.