A study of nitrogen dioxide (NO₂) periodicity over the United Arab Emirates using wavelet analysis

NO₂ and nitric oxide (NO) are the most reactive gases in the atmosphere. The interaction of NO_x molecules with oxygen, water and other chemicals leads to the formation of acid rain. The presence of NO₂ in the air affects human health and forms a photochemical smog. In this study, we utilize wavelet analysis, namely, the Morlet wavelet, which is a type of continuous wavelet transform, to conduct a spectral analysis of the periodicity of nitrogen dioxide (NO₂). The study is conducted using data from 14 weather stations located in diverse geographic areas of the United Arab Emirates (UAE) over a period of two years (2019 and 2020). We explain and relate the significance of human activities to the concentration level of NO₂, particularly considering the effect of the COVID-19 lockdown to the periodicity of NO₂. The results show that NO₂ concentrations in desert areas such as Liwa and Al Quaa were unaffected by the lockdown period (April-July 2020) resulting from the COVID-19 pandemic. The other stations in the urban areas of Abu Dhabi city, Al Dhafra and Al Ain, showed a reduction in NO₂ during the lockdown. NO₂ is more highly concentrated during winter seasons than during other seasons. The periodicity of NO₂ lasted from a few days up to 16 days in most regions. However, some stations located in the Al Dhafra region, such as Al Ruwais and the Gayathi School stations, exhibited a longer period of more than 32 days with a 0.05 significance test. In the Abu Dhabi region, NO₂ lasted between 64 and 128 days at the Al Mafraq station. The correlation between the NO₂ concentration across several ground stations was studied using wavelet coherence.