Measurement of recreational N2O emissions from an urban environment in Manchester, UK

Nitrous oxide (N ₂O) is a potent greenhouse gas that is currently the third largest contributor to anthropogenic radiative forcing. It is also a strong ozone depleting substance. Given this importance, mitigation of N ₂O emissions remains important and sources must be understood in greater detail. In this study, in situ measurements of N ₂O alongside a variety of other trace gases and aerosols were made from a ground-based air quality observation site in an urban environment of Fallowfield, Manchester, United Kingdom over a period of 12 months between October 2020 and October 2021. N ₂O mole fraction was observed to be poorly correlated with other atmospheric pollutant tracers during the measurement period, with little evidence of co-enhancement (and therefore common source relationships) between N ₂O and other local pollutant trace gases and aerosol. Large N ₂O enhancements (> 400 ppb above background) over short time scales (< 2 min) were seen with no co-enhancement of other trace gases and aerosol concentrations, suggesting discrete N ₂O sources in the near vicinity of the measurement site. Measured N ₂O concentrations showed a consistent temporal pattern over day, week, and year timescales with consistently large weekend enhancements observed between the hours of 18:00 and 02:00 local time, suggesting the source of N ₂O may be associated with night-time recreational use by nearby residents. These weekend-night-time temporal patterns were not correlated with other trace gases measured at the same location. Analysis of the air transport history of N ₂O measurements showed high mean nocturnal mole fractions originating from the west and south-west of the observation site, suggesting that emissions may have originated from nearby areas of student accommodation and dense areas of private housing to the west. This study finds evidence for a detectable recreational N ₂O source that appears to be dominant over other potential N ₂O sources for the area studied. Further study is needed to quantify the local and national emission rates of this potentially increasing atmospheric pollution source, and to compare the magnitude of this source to other locations within the UK. The study demonstrates an important need to assess and validate National Atmospheric Emission Inventory (NAEI) estimates for recreational N ₂O emissions.