Synoptic influences on spatial and temporal variability of aerosols across North America

Aerosols are a minor constituent of the Earth’s atmosphere, yet they play an important role in the energy balance of the earth-atmosphere system. Despite their importance, the processes that determine the spatial and temporal variability of aerosols are not well understood. Furthermore, constraints associated with conventional aerosol monitoring techniques result in gaps in aerosol records. This sampling bias has led to questionable inferences about aerosol variability.

This study represents the first large-scale analysis of the role of the atmosphere in understanding the observed temporal and spatial variability in atmospheric aerosols across North America. We used a hybrid synoptic classification procedure, known as the Spatial Synoptic Classification (SSC) scheme, to group surface weather conditions at a particular location into one of several ‘weather types’. We then compared the weather type with the aerosol optical depth (t_a), which is a measure of the total-column amount of aerosols, and Ångström’s wavelength exponent (a), which is a measure of the size distribution of aerosols. Our aerosol data are drawn from two sources: NASA’s Aerosol Robotic Network (AERONET) and the MFRSR network where the periods of record are at least three years in duration.

We found consistent spatial patterns in terms of the relationships between weather type and the aerosol optical depth. For example, most stations experience highest turbidity (optical depth) during the Moist Tropical (MT) weather type and lowest turbidity during Dry Polar (DP) conditions. Seasonal patterns were also evident, with turbidity typically highest in summer and lowest in winter. With regard to the aerosol size distribution, many stations showed an increase in aerosol size during the spring. The causal mechanisms underlying these relationships will be presented.