Nitrogen Gas Cylinder (236811SG) appears to be contaiminated since original installation - during span , NO2 reading is increasing (up to 42ppb) instead of tending to zero ppb.

06/02/2020 New Cylinder quote requested :  AMLQ12257

13/03/2020 11:10 - 11:50 Nitrogen Cylinder installed 4532 (451/451 ppb)

For more details see Engineers report, Urban Observatory Github

https://github.com/urbanobservatory/deployments/blob/master/Precision%20Station%20Reports/FOUR_LANE/MB13032020_Newcastle%20Uni_Four%20Lane%20Ends_Commissioning%20NO%20cyl.xls