A Global Evaluation of Measured Surface and Column Aerosol Optical Properties

E. Boedicker^1,2 and B. Andrews^1,2

¹Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 720-258-6017‬, E-mail: erin.boedicker@noaa.gov
²NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305

Characterizing aerosol optical properties is critical for quantifying their impacts on the radiative energy budget of the atmosphere. This work focuses on the interface of column and ground based measurements of in-situ aerosol optical properties. We present comparisons between aerosol optical depth (AOD) and total scattering. These comparisons were made for sixteen sites within the NOAA Federated Aerosol Network (NFAN) that are, or were during their operation, collocated with Aerosol Robotic Network (AERONET) sites. Selected sites represent a diverse set of land use categories, including, Arctic, marine, continental, high altitude, and urban regions. The shortest overlap period from these sites was approximately six years, while the longest was ~27 years.

The NFAN system measures continuously, however, the AERONET system only measures under some atmospheric conditions. This conditional sampling can mean that the column measurements may not capture airmasses that tend to be associated with particular atmospheric conditions. To reduce discrepancies in the comparisons caused by fog or cloudy conditions (Schutgens et al., 2016) the two data sets were merged based on hourly averages. Data was then only used for direct intercomparison if both AERONET and NFAN data was available in each hourly period. Additionally, only AERONET Level 2 data was used in this analysis.

Seasonal increases in the AOD without accompanying increases in surface scattering indicated large periods of long-range transport (LRT) of aerosol layers above the surface in the Arctic (BRW) and intermittent episodes at one of the costal sites (THD). Peaks in scattering in the winter without increases in AOD were common but varied in magnitude across sites. These likely represent local sources that have less impact when integrating over the column.

Correlation between AOD and surface scattering is highly variable (Figure 1). Linear comparisons for the continental sites showed the strongest relationships between the surface and column. While marine/coastal and polar sites were more likely to have non-significant linear trends. This work is currently investigating the cause of this variation using a variety of approaches. The first being to account for scattering enhancement due to water uptake. NFAN sites maintain a sample humidity of <40%, while AERONET makes measurements at ambient conditions. At sites with high RH this could cause a significant discrepancy between the two measurement sets. Using data from Burgos et al. (2019) for scattering enhancement factors from six sites (BRW, THD, GSN, APP, SGP, and UGR) and weather data from the ASOS network we are currently working to correct the surface measurements for the effects of RH. Additionally, we are exploring the effect of the general local meteorological conditions on this relationship.

Figure 1. Seasonal cycles in the linear correlation coefficient between AOD and surface scattering. Grey markers show yearly data, and the shaded bars and black markers show the interquartile range and median correlation coefficient. Data are shown for four of the sites used in this analysis.