Optical Properties of Atmospheric Aerosol in Maritime Environments

Abstract

Systematic characterization of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. We report the results of a comprehensive optical and physical characterization of ambient aerosol in 5 key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2-5 years. The results are compared with aerosol optical depths and size distributions reported in the literature over the last 30 years. Aerosol found over the tropical Pacific Ocean (at 3 sites between 20°S and 20°N), still resembles mostly clean background conditions dominated by maritime aerosol. The optical thickness is remarkably stable with mean value of τ a (500 nm)=0.07, mode value at τ am =0.06 and standard deviation of 0.02 to 0.05. Average Ångström exponent ranged from 0.3 to 0.7 characterizes the wavelength dependence of the optical thickness. Over the tropical to subtropical Atlantic (2 stations at 7°S and 32°N) the optical thickness is significantly higher: τ a (500 nm)=0.14 and τ am =0.10 due to the frequent presence of dust, smoke and urban/industrial aerosol. For both oceans the atmospheric column aerosol is characterized by a bimodal log-normal size distribution with a fine mode at effective radius R eff =0.11±0.01 µm and coarse mode at R eff =2.1±0.3 µm. A review of the published 150 historical ship measurements from the last three decades shows that τ am was around 0.07 to 0.12 in general agreement with the present finding. The information should be useful as a test-bed for aerosol global models and aerosol representation in global climate models. With global human population expansion and industrialization, these measurements can serve in the 21st century as a basis to assess decadal changes in the aerosol concentration, properties and radiative forcing of climate.

Relating aerosol absorption due to soot, organic carbon, and dust to emission sources determined from in-situ chemical measurements

“…1), and the dominance of sea salt could be important. However, Smirnov et al (2002Smirnov et al ( , 2011 studied optical properties in maritime environment and found that the AOD has a mean value of 0.07 with standard deviation of 0.03 to 0.05. This means that in a clean maritime environment, dominated by sea salt, the AOD is far below the limit for AERONET Level 2.0 data.…”

Dust optical properties over North Africa and Arabian Peninsula derived from the AERONET dataset

“…Such a criteria ensures that the fine mode aerosols are mostly excluded from this analysis and the results are representative of nearly "pure" dust, even though the fine mode dust is also being excluded. Although sea salt aerosol is also mainly in the coarse mode and has smallÅngström exponent (∼0.3-0.7; Smirnov et al, 2002b), its contribution over these dust dominated sites is largely negligible. Both ALL and DU data are first sampled from the AERONET Version 2 quality assured Level 2 inversion data taken at 15 min time intervals.…”

Origin and pathways of the mineral dust transport to two Spanish EARLINET sites: Effect on the observed columnar and range-resolved dust optical properties

“…Measurements of aerosol properties over the Mediterranean region have been carried out since the 1980s (Smirnov et al, 2002), but most of them were occasional and lasted only for short periods. Other measurements covered longer time periods (Cachorro et al, 2000;Formenti et al, 2001;Sabbah et al, 2001;Tanre et al, 2001;Gerasopoulos et al, 2003;Israelevich et al, 2003;Esposito et al, 2004;Pavese et al, 2009).…”