Analysis of aerosol ammonium nitrate: Departures from equilibrium during SCAQS

The size distribution of the inorganic components of atmospheric aerosols measured during the 1987 Southern California Air Quality Study (SCAQS) are used to investigate the hypothesis that transport limits equilibration of gas- and aerosol-phase ammonium nitrate and influences their size distribution. Estimates of the equilibration time constants at four sites in southern California show a wide range of values; at small values equilibrium is expected whereas at large values departures from equilibrium are expected. An equilibrium indicator is proposed based on the size distributions of aerosol ammonium and nitrate that shows when particles of different size are in mutual equilibrium with respect to ammonium nitrate. Values of the indicator range from unity, demonstrating equilibrium, to less than 0.5, demonstrating that different size particles are not in mutual equilibrium. Comparison of the time constant values to the equilibrium indicator values show a significant correlation, so it is concluded that different size aerosol particles are often not in mutual equilibrium due to transport limitations, supporting the hypothesis. It is concluded that both thermodynamics and mass transport must be considered to predict accurately the size distribution of the volatile inorganics in atmospheric aerosol.