The Ångström coefficient as an indicator of non-marine particles in ANAM (Advanced Navy Aerosol Model)

Abstract

The Advanced Navy Aerosol model (ANAM), being a modified version of the Navy Aerosol Model (NAM), is a wellknown engineering tool providing a quick and reasonable estimate of the aerosol extinction in the marine near-surface environment on the basis of simple meteorological input data. The original NAM consists of 3 lognormal distributions, which describe freshly produced marine aerosols, aged marine aerosols (produced elsewhere and advected to the measurement site) and a background concentration of marine aerosols. The ANAM adds a 4th lognormal mode to NAM to account for the largest marine particles. To account for non-marine particles, a special lognormal mode, called “dust mode” was included in NAM. The relative importance of the dust mode versus the marine background concentration is governed by a special input parameter known as the air mass parameter (AMP). Unfortunately, the AMP is ill-defined and the NAM user community has found it difficult to attribute a proper value to the AMP. This inconvenience became even more stressing when NAM was used for assessing aerosol extinction in the coastal zone. To overcome this inconvenience, a new approach is suggested which involves replacement of the AMP by the Angstrom coefficient. The advantage is that the latter parameter can be directly measured and has a physical relation to the aerosol size distribution. When the particle size distribution is dominated by small particles, usually associated with pollution, the Angstrom coefficients are high; in clear conditions they are usually low. Therefore this parameter is a good tracer of the aerosols originated over land and hence a good replacement for the AMP.