The characteristic of aerosols in the northern America based on the Angstrom exponent from AERONET measurements

Angstrom exponent (at 440-870 nm, hereafter AE) is the parameter showing the variation of aerosol optical depth (AOD) according to the wavelength. AE is considered as a proxy to diagnose the size of aerosol: small AE for coarse-mode and large AE for fine-mode aerosols. Also, the wavelength dependence of AE can be used to estimate the type of aerosol. In this study, the regional characteristics of the aerosol in the northern America were examined mainly based on the analysis of AE from the ground-based columnar measurements, AERONET, during 2008-2010. To focus on the pattern at the city region, AERONET dataset was selected at the five urban and rural sites in the northern America (including Mexico). Generally AE shows the positive correlation with AOD, meaning that the emission of fine mode aerosol contributes to the AOD increase. But for high AOD cases, sometimes AOD negatively correlates with AE implying the influence of aerosol growth to the pollution. Among ten sites, Mexico City only shows this pattern a little bit, probably due to the large amount of hygroscopic secondary organic aerosols. In addition, AE generally shows the positive correlation with AE ratio (AE at 500-870nm divided by AE at 380-500 nm) that also provides the information of aerosol type: e.g., AE ratio > 1 for biomass burning or urban condition and AE ratio < 1 for dusty condition. Seasonally, the small AE and AE ratio less than 1 appear in spring but vice versa in summer, showing the role of coarse-mode dust aerosols in spring and fine-mode anthropogenic emission in summer. However there are exceptional cases for some urban sites, which will be more investigated in detail.