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Abstract. Cold frontal passages usually promote quick removal of atmospheric pollutants overNorth China (e.g. the Beijing–Tianjin–Hebei region). However, in the Yangtze RiverDelta (YRD), cold fronts may bring air pollutants from the polluted North China Plain(NCP), thereby deteriorating the air quality in the YRD. In this study, a cold frontalpassage and a subsequent stable weather event over YRD during 21–26 January 2015 wasinvestigated with in situ observations and Weather Research and Forecasting – CommunityMultiscale Air Quality Modeling System simulations. Observations showed a burst ofPM 2.5 pollution and an obvious southward motion of PM 2.5 peaks on the afternoonof 21 January, suggesting a strong inflow of highly polluted air masses to YRD by a coldfrontal passage. Model simulations revealed an existing warm and polluted air mass overYRD ahead of the frontal zone, which climbed to the free troposphere along the frontalsurface as the cold front passed, increasing the PM 2.5 concentration at highaltitudes. Strong north-westerly frontal airflow transported particles from the highlypolluted NCP to the YRD. As the frontal zone moved downstream of YRD, high pressure tookcontrol over the YRD, which resulted in a synoptic subsidence that trapped PM 2.5 inthe boundary layer. After the cold frontal episode, a uniform pressure field took controlover the YRD. Locally emitted PM 2.5 started to accumulate under the weak winds andstable atmosphere. Tagging of PM 2.5 by geophysical regions showed that thePM 2.5 contribution from the YRD itself was 35 % and the contribution from theNCP was 29 % during the cold frontal passage. However, under the subsequent stableweather conditions, the PM 2.5 contribution from the YRD increased to 61.5 % andthe contribution from the NCP decreased to 14.5 %. The results of this study indicatethat cold fronts are potential carriers of atmospheric pollutants when there are strongair pollutant sources in upstream areas, which may deteriorate air quality in downstreamregions.

Potential impacts of cold frontal passage on air quality over the Yangtze River Delta, China