Intra-annual variations of regional aerosol optical depth, vertical distribution, and particle types from multiple satellite and ground-based observational datasets.

Yu Gu,Lei Huang,Kuo-Nan Liou,Hui Su,Xuehua Fan,Bin Zhao,Zhe Jiang,David J Diner,Ali H Omar,Jonathan H Jiang,Yoshi Takano

doi:10.5194/acp-18-11247-2018

Yu Gu, Lei Huang + Show 9 more

Open Access

https://doi.org/10.5194/acp-18-11247-2018

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Abstract

The climatic and health effects of aerosols are strongly dependent on the intra-annual variations in their loading and properties. While the seasonal variations of regional aerosol optical depth (AOD) have been extensively studied, understanding the temporal variations in aerosol vertical distribution and particle types is also important for an accurate estimate of aerosol climatic effects. In this paper, we combine the observations from four satellite-borne sensors and several ground-based networks to investigate the seasonal variations of aerosol column loading, vertical distribution, and particle types over three populous regions: the Eastern United States (EUS), Western Europe (WEU), and Eastern and Central China (ECC). In all three regions, column AOD, as well as AOD at heights above 800 m, peaks in summer/spring, probably due to accelerated formation of secondary aerosols and hygroscopic growth. In contrast, AOD below 800m peaks in winter over WEU and ECC regions because more aerosols are confined to lower heights due to the weaker vertical mixing. In the EUS region, AOD below 800m shows two maximums, one in summer and the other in winter. The temporal trends in low-level AOD are consistent with those in surface fine particle (PM2.5) concentrations. AOD due to fine particles (< 0.7 μm diameter) is much larger in spring/summer than in winter over all three regions. However, the coarse mode AOD (> 1.4 μm diameter), generally shows small variability, except that a peak occurs in spring in the ECC region due to the prevalence of airborne dust during this season. When aerosols are classified according to sources, the dominant type is associated with anthropogenic air pollution, which has a similar seasonal pattern as total AOD. Dust and sea-spray aerosols in the WEU region peak in summer and winter, respectively, but do not show an obvious seasonal pattern in the EUS region. Smoke aerosols, as well as absorbing aerosols, present an obvious unimodal distribution with a maximum occurring in summer over the EUS and WEU regions, whereas they follow a bimodal distribution with peaks in August and March (due to crop residue burning) over the ECC region.

Highlights

Aerosols have adverse effects on human health (Lelieveld et al, 2015) and play a key role in Earth’s climate through aerosol–radiation interactions (McCormick and Ludwig, 1967) and aerosol–cloud interactions (Twomey, 1977; Albrecht, 1989; Garrett and Zhao, 2006)
The Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) retrievals for each site are averaged within a 2 h window centered on the satellite overpass times, and compared with the satellite AOD retrievals in a 1◦ × 1◦ grid box that contains the corresponding AERONET site
For the Western Europe (WEU) and Eastern and Central China (ECC) regions, Multi-angle Imaging SpectroRadiometer (MISR), Moderate resolution Imaging Spectroradiometer (MODIS)/Terra, and MODIS/Aqua reveal consistent seasonal patterns in which AOD peaks in spring and/or summer and reaches its lowest valley in winter

Summary

Introduction

Aerosols have adverse effects on human health (Lelieveld et al, 2015) and play a key role in Earth’s climate through aerosol–radiation interactions (McCormick and Ludwig, 1967) and aerosol–cloud interactions (Twomey, 1977; Albrecht, 1989; Garrett and Zhao, 2006). The wide ranges of particle optical properties and size distribution mean that even for the same AOD, different aerosol types have different effects on the magnitude, and the sign, of aerosol radiative forcing (IPCC, 2013; Gu et al, 2006; Garrett et al, 2004). The health impacts of aerosols are only associated with those present near the surface, where they are inhaled For these reasons, systematic analyses of the intra-annual variations of aerosol vertical distribution and particle types, in addition to total column AOD, are necessary to improve our understanding of aerosol climatic and health effects. We investigate the seasonal variations of aerosol column loading, vertical distribution, and particle types using multiple satellite and ground-based observational datasets covering the period from 2007 to 2016.

Satellite data

Seasonal variations of column AOD

Seasonal variations of aerosol loadings as a function of height

Seasonal variations of aerosol types

Conclusions and implications