A significant source of nitrous oxide from recreational use in Ho Chi Minh City, a future Southeast Asian megacity in Vietnam

Abstract

The first in situ observations of nitrous oxide (N₂O) mole fraction for Ho Chi Minh City are presented for October 2018 to March 2019. The time series of N₂O observations made from the roof of a 10-storey building was dominated by elevated N₂O mole fractions with a maximum of 613 nmol mol⁻¹ in air arriving from the northeast and east. The diurnal pattern of N₂O did not follow the general pattern of other pollutants (i.e., carbon monoxide) associated with urban anthropogenic activity, such as emissions from traffic. The observations revealed an underlying mean daily diurnal cycle with an amplitude of approximately 10 nmol mol⁻¹, consistent with the accumulation of N₂O overnight in a shallow boundary layer. An exception to this was the extremely elevated mean diurnal cycle observed for the northeast sector, where a magnitude of ∼45 nmol mol⁻¹ indicates a strong local source. Using ‘discrete’ sampling, an area of elevated N₂O (mean 4733 nmol mol⁻¹) was identified in an area of the city, the Bùi Viện Walking Street, where N₂O is used for recreational purposes, a practice that is raising serious health concerns globally. Using a simple ‘bottom-up’ approach, estimates of the emission of N₂O from this activity were 0.021 ± 0.010 Gg per year, an emission comparable to the estimated N₂O emissions from the stationary energy sector in HCMC. To investigate this further, the ADMS-Urban dispersion model was used to simulate the in situ observations of N₂O for the measurement site. It was found that the diurnal cycle for each wind sector was reproducible when the emissions of N₂O were assumed to be coming from the Bùi Viện Walking Street; however, the model emissions required for this were approximately one order of magnitude higher than those estimated using the ‘bottom-up’ approach. Despite the disagreement in the magnitude of emissions, continued measurements at the same site could be used to assess the effectiveness of the nationwide ban on N₂O for recreational purposes, as they represent a baseline before the nationwide ban introduced in 2025. Additionally, it is suggested that care must be taken when using atmospheric observations of N₂O and CO to derive excess N₂O/CO molar emission ratios from traffic in tropical Southeast Asian cities. For example, despite being attributed as the largest source of N₂O in HCMC, the observations presented herein show no observable N₂O increase from local rush hour traffic peaks characterised by high CO, likely due to the precision limits of the analyser. Finally, the work presented herein emphasises some of the challenges encountered when estimating emissions of greenhouse gases from heterogeneous sources in urban environments.