Assessment of cloud-related fine-mode AOD enhancements based on AERONET SDA product
AERONET (AErosol RObotic NETwork), which is a network of ground-based sun photometers, produces a data product called the aerosol spectral deconvolution algorithm (SDA) that utilizes spectral total aerosol optical depth (AOD) data to infer the component fine- and coarse-mode optical depths at 500 nm...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/5991/2017/acp-17-5991-2017.pdf |
| Summary: | AERONET (AErosol RObotic NETwork), which is a network of ground-based sun
photometers, produces a data product called the aerosol spectral deconvolution
algorithm (SDA) that utilizes spectral total aerosol optical depth (AOD) data
to infer the component fine- and coarse-mode optical depths at
500 nm. Based on its assumptions, SDA identifies cloud optical depth as the
coarse-mode AOD component and therefore effectively computes the fine-mode
AOD also in mixed cloud–aerosol observations. Therefore, it can be argued
that the more representative AOD for fine-mode fraction should be based on
all direct sun measurements and not only on those cloud screened for
clear-sky conditions, i.e., on those from level 1 (L1) instead of
level 2 (L2) in AERONET. The objective of our study was to assess, including
all the available AERONET sites, how the fine-mode AOD is enhanced in cloudy
conditions, contrasting SDA L1 and L2 in our analysis. Assuming that the
cloud screening correctly separates the cloudy and clear-sky conditions, then
the increases in fine-mode AOD can be due to various cloud-related processes,
mainly by the strong hygroscopic growth of particles in the vicinity of
clouds and in-cloud processing leading to growth of accumulation mode
particles. We estimated these cloud-related enhancements in fine-mode AOD
seasonally and found, for instance, that in June–August season the average
over all the AERONET sites was 0.011, when total fine-mode AOD from L2 data
was 0.154; therefore, the relative enhancement was 7 %. The enhancements were
largest, both absolutely and relatively, in East Asia; for example, in
June–August season the absolute and relative differences in fine-mode AOD,
between L1 and L2 measurements, were 0.022 and 10 %, respectively.
Corresponding values in North America and Europe were about 0.01 and 6–7 %.
In some highly polluted areas, the enhancement is greater than these
regional averages, e.g., in Beijing region and in June–July–August (JJA) season the corresponding
absolute values were about 0.1. It is difficult to separate the fine-mode AOD
enhancements due to in-cloud processing and hygroscopic growth, but we
attempted to get some understanding by conducting a similar analysis for
SDA-based fine-mode Ångström exponent (AE) patterns. Moreover, we exploited a
cloud parcel model, in order to understand in detail the relative role of
different processes. We found that in marine conditions, were aerosol
concentration are low and cloud scavenging is efficient, the AE changes in
opposite direction than in the more polluted conditions, were hygroscopic
growth of particles leads to a negative AE change. |
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| ISSN: | 1680-7316 1680-7324 |