In [11]: air_quality["month"] = air_quality["datetime"].dt.month

In [12]: air_quality.head()
Out[12]: 
    city country                  datetime location parameter  value   unit  month
0  Paris      FR 2019-06-21 00:00:00+00:00  FR04014       no2   20.0  µg/m³      6
1  Paris      FR 2019-06-20 23:00:00+00:00  FR04014       no2   21.8  µg/m³      6
2  Paris      FR 2019-06-20 22:00:00+00:00  FR04014       no2   26.5  µg/m³      6
3  Paris      FR 2019-06-20 21:00:00+00:00  FR04014       no2   24.9  µg/m³      6
4  Paris      FR 2019-06-20 20:00:00+00:00  FR04014       no2   21.4  µg/m³      6