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Abstract:
Multi-axis differential optical absorption spectroscopy (MAX-DOAS)
observations of trace gases can be strongly influenced by clouds and
aerosols. Thus it is important to identify clouds and characterize their
properties. In a recent study Wagner et al. (2014) developed a cloud
classification scheme based on the MAX-DOAS measurements themselves with
which different "sky conditions" (e.g., clear sky, continuous clouds,
broken clouds) can be distinguished. Here we apply this scheme to
long-term MAX-DOAS measurements from 2011 to 2013 in Wuxi, China (31.57
degrees N, 120.31 degrees E). The original algorithm has been adapted to
the characteristics of the Wuxi instrument, and extended towards smaller
solar zenith angles (SZA). Moreover, a method for the determination and
correction of instrumental degradation is developed to avoid artificial
trends of the cloud classification results. We compared the results of
the MAX-DOAS cloud classification scheme to several independent
measurements: aerosol optical depth from a nearby Aerosol Robotic
Network (AERONET) station and from two Moderate Resolution Imaging
Spectroradiometer (MODIS) instruments, visibility derived from a
visibility meter and various cloud parameters from different satellite
instruments (MODIS, the Ozone Monitoring Instrument (OMI) and the Global
Ozone Monitoring Experiment (GOME-2)). Here it should be noted that no
quantitative comparison between the MAX-DOAS results and the independent
data sets is possible, because (a) not exactly the same quantities are
measured, and (b) the spatial and temporal sampling is quite different.
Thus our comparison is performed in a semi-quantitative way: the MAXDOAS
cloud classification results are studied as a function of the external
quantities. The most important findings from these comparisons are as
follows: (1) most cases characterized as clear sky with low or high
aerosol load were associated with the respective aerosol optical depth
(AOD) ranges obtained by AERONET and MODIS; (2) the observed dependences
of MAX-DOAS results on cloud optical thickness and effective cloud
fraction from satellite confirm that the MAX-DOAS cloud classification
scheme is sensitive to cloud (optical) properties; (3) the separation of
cloudy scenes by cloud pressure shows that the MAX-DOAS cloud
classification scheme is also capable of detecting high clouds; (4) for
some cloud-free conditions, especially with high aerosol load, the
coincident satellite observations indicated optically thin and low
clouds. This finding indicates that the satellite cloud products contain
valuable information on aerosols.
