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Among various factors that influence the long-range transport of pollutantsin the free troposphere (FT), the prevailing atmospheric weather statesprobably play the most important role in governing characteristics andefficacy of such transport. The weather states, such as a particular windpattern, cyclonic or anticyclonic conditions, and their degree ofpersistency determine the spatio-temporal distribution and the final fate ofthe pollutants. This is especially true in the case of Nordic countries,where baroclinic disturbances and associated weather fronts primarilyregulate local meteorology, in contrast to the lower latitudes where aconvective paradigm plays a similarly important role. Furthermore, thelong-range transport of pollutants in the FT has significant contribution tothe total column burden over the Nordic countries. However, there isinsufficient knowledge on the large-scale co-variability of pollutants inthe FT and atmospheric weather states based solely on observational dataover this region. The present study attempts to quantify and understand thisstatistical co-variability while providing relevant meteorologicalbackground.  To that end, we select eight weather states that predominantly occur overthe Nordic countries and three periods of their persistency (3 days, 5 days,and 7 days), thus providing in total 24 cases to investigate sensitivity offree tropospheric carbon monoxide, an ideal tracer for studying pollutanttransport, to these selected weather states. The eight states include fourdominant wind directions (namely, NW, NE, SE and SW), cyclonic andanticyclonic conditions, and the enhanced positive and negative phases ofthe North Atlantic Oscillation (NAO). For our sensitivity analysis, we userecently released Version 6 retrievals of CO at 500 hPa from the AtmosphericInfrared Sounder (AIRS) onboard Aqua satellite covering the 11-year period fromSeptember 2002 through August 2013 and winds from the ECMWF's ERA-Interimproject to classify weather states for the same 11-year period.  We show that, among the various weather states studied here, southeasterlywinds lead to highest observed CO anomalies (up to +8%) over the Nordiccountries while transporting pollution from the central and eastern parts ofEurope. The second (up to +4%) and third highest (up to +2.5%) COanomalies are observed when winds are northwesterly (facilitatinginter-continental transport from polluted North American regions) and duringthe enhanced positive phase of the NAO respectively. Higher than normal COanomalies are observed during anticyclonic conditions (up to +1%)compared to cyclonic conditions. The cleanest conditions are observed whenwinds are northeasterly and during the enhanced negative phases of the NAO,when relatively clean Arctic air masses are transported over the Nordicregions in the both cases. In the case of nearly all weather states, the COanomalies consistently continue to increase or decrease as the degree ofpersistency of a weather state is increased. The results of this sensitivitystudy further provide an observational basis for the process-orientedevaluation of chemistry transport models, especially with regard to therepresentation of large-scale coupling of chemistry and local weather statesand its role in the long-range transport of pollutants in such models

atmospheric chemistry and physics

Sensitivity of free tropospheric carbon monoxide to atmospheric weather states and their persistency: an observational assessment over the Nordic countries