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 800 m 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 800 m 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.
Climatological Aspects of Aerosol Physical Characteristics in Tunisia Deduced from Sun Photometric Measurements
