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| Titel |
Sensitivity of free tropospheric carbon monoxide to atmospheric weather states and their persistency: an observational assessment over the Nordic countries |
| VerfasserIn |
M. A. Thomas, A. Devasthale |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 21 ; Nr. 14, no. 21 (2014-11-03), S.11545-11555 |
| Datensatznummer |
250119135
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| Publikation (Nr.) |
 copernicus.org/acp-14-11545-2014.pdf |
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| Zusammenfassung |
Among various factors that influence the long-range transport of pollutants
in the free troposphere (FT), the prevailing atmospheric weather states
probably play the most important role in governing characteristics and
efficacy of such transport. The weather states, such as a particular wind
pattern, cyclonic or anticyclonic conditions, and their degree of
persistency determine the spatio-temporal distribution and the final fate of
the pollutants. This is especially true in the case of Nordic countries,
where baroclinic disturbances and associated weather fronts primarily
regulate local meteorology, in contrast to the lower latitudes where a
convective paradigm plays a similarly important role. Furthermore, the
long-range transport of pollutants in the FT has significant contribution to
the total column burden over the Nordic countries. However, there is
insufficient knowledge on the large-scale co-variability of pollutants in
the FT and atmospheric weather states based solely on observational data
over this region. The present study attempts to quantify and understand this
statistical co-variability while providing relevant meteorological
background.
To that end, we select eight weather states that predominantly occur over
the Nordic countries and three periods of their persistency (3 days, 5 days,
and 7 days), thus providing in total 24 cases to investigate sensitivity of
free tropospheric carbon monoxide, an ideal tracer for studying pollutant
transport, to these selected weather states. The eight states include four
dominant wind directions (namely, NW, NE, SE and SW), cyclonic and
anticyclonic conditions, and the enhanced positive and negative phases of
the North Atlantic Oscillation (NAO). For our sensitivity analysis, we use
recently released Version 6 retrievals of CO at 500 hPa from the Atmospheric
Infrared Sounder (AIRS) onboard Aqua satellite covering the 11-year period from
September 2002 through August 2013 and winds from the ECMWF's ERA-Interim
project to classify weather states for the same 11-year period.
We show that, among the various weather states studied here, southeasterly
winds lead to highest observed CO anomalies (up to +8%) over the Nordic
countries while transporting pollution from the central and eastern parts of
Europe. The second (up to +4%) and third highest (up to +2.5%) CO
anomalies are observed when winds are northwesterly (facilitating
inter-continental transport from polluted North American regions) and during
the enhanced positive phase of the NAO respectively. Higher than normal CO
anomalies are observed during anticyclonic conditions (up to +1%)
compared to cyclonic conditions. The cleanest conditions are observed when
winds are northeasterly and during the enhanced negative phases of the NAO,
when relatively clean Arctic air masses are transported over the Nordic
regions in the both cases. In the case of nearly all weather states, the CO
anomalies consistently continue to increase or decrease as the degree of
persistency of a weather state is increased. The results of this sensitivity
study further provide an observational basis for the process-oriented
evaluation of chemistry transport models, especially with regard to the
representation of large-scale coupling of chemistry and local weather states
and its role in the long-range transport of pollutants in such models. |
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