This work is a government work and is not protected by U.S. copyright law. It is available under the Public Domain Mark 1.0. The paper presents the first of a series of studies on aerosol optical properties derived from AERONET measurements. It analyzes monthly aerosol optical depth (AOD) data at 500 nm, the multispectral Angstrom parameter, and precipitable water at nine selected sites with distinct aerosol regimes. The data include 9 primary sites and 21 additional multiyear sites, representing various aerosol sources. The study discusses seasonal trends and climatic averages for each site. The paper describes the instrumentation and methods used, including the CIMEL Sun/sky radiometers part of the AERONET network. It details the calibration and processing of data, including the uncertainty in AOD measurements due to calibration and other factors. The study also discusses the variability of aerosol optical depth and its relationship with precipitable water, noting that they are not strongly correlated. The paper presents data from several locations, including Mauna Loa, Hawaii; Goddard Space Flight Center, Maryland; Sevilleta, New Mexico; H.J. Andrews, Oregon; and Cape Verde. Each section describes the aerosol environment, seasonal variations, and the impact of natural and anthropogenic sources on aerosol loading. The study highlights the importance of long-term monitoring of aerosol optical properties for understanding climate and environmental changes. The data show significant interannual variability in aerosol transport and the influence of regional and global factors on aerosol optical depth. The study concludes that a coordinated network of ground-based and satellite measurements is essential for developing a long-term monitoring system of the Earth's aerosol environment.This work is a government work and is not protected by U.S. copyright law. It is available under the Public Domain Mark 1.0. The paper presents the first of a series of studies on aerosol optical properties derived from AERONET measurements. It analyzes monthly aerosol optical depth (AOD) data at 500 nm, the multispectral Angstrom parameter, and precipitable water at nine selected sites with distinct aerosol regimes. The data include 9 primary sites and 21 additional multiyear sites, representing various aerosol sources. The study discusses seasonal trends and climatic averages for each site. The paper describes the instrumentation and methods used, including the CIMEL Sun/sky radiometers part of the AERONET network. It details the calibration and processing of data, including the uncertainty in AOD measurements due to calibration and other factors. The study also discusses the variability of aerosol optical depth and its relationship with precipitable water, noting that they are not strongly correlated. The paper presents data from several locations, including Mauna Loa, Hawaii; Goddard Space Flight Center, Maryland; Sevilleta, New Mexico; H.J. Andrews, Oregon; and Cape Verde. Each section describes the aerosol environment, seasonal variations, and the impact of natural and anthropogenic sources on aerosol loading. The study highlights the importance of long-term monitoring of aerosol optical properties for understanding climate and environmental changes. The data show significant interannual variability in aerosol transport and the influence of regional and global factors on aerosol optical depth. The study concludes that a coordinated network of ground-based and satellite measurements is essential for developing a long-term monitoring system of the Earth's aerosol environment.