Traffic restrictions during COVID-19 lockdown improve air quality and reduce metal biodeposition in tree leaves

Abstract

The coronavirus disease (COVID-19) has had a great global impact on human health, the life of people, and economies all over the world. However, in general, COVID-19´s effect on air quality has been positive due to the restrictions on social and economic activity. This study aimed to assess the impact on air quality and metal deposition of actions taken to reduce mobility in 2020 in two different urban locations. For this purpose, we analysed air pollution (NO₂, NO, NO_x, SO_2, CO, PM_10, O₃) and metal accumulation in leaves of Tilia cordata collected from April to September 2020 in two cities in northern Spain (Pamplona-PA and San Sebastián-SS). We compared their values with data from the previous year (2019) (in which there were no mobility restrictions) obtained under an identical experimental design. We found that metal accumulation was mostly lower during 2020 (compared with 2019), and lockdown caused significant reductions in urban air pollution. Nitrogen oxides decreased by 33%-44%, CO by 24%-38%, and PM₁₀ by 16%-24%. The contents of traffic-related metals were significantly reduced in both studied cities. More specifically, significant decreases in metals related to tyre and brake wear (Zn, Fe, and Cu) and road dust resuspension (Al, Ti, Fe, Mn, and Ca) were observed. With these results, we conclude that the main reason for the improvement in urban air pollutants and metals was the reduction in the use of cars due to COVID-19 lockdown. In addition, we offer some evidence indicating the suitability of T. cordata leaves as a tool for biomonitoring metal accumulation. This information is relevant for future use by the scientific community and policy makers to implement measures to reduce traffic air pollution in urban areas and to improve environmental and human health.

Keywords: Biomonitoring; COVID-19; Lockdown; Metal deposition; Traffic emissions; Urban pollutants.

Graphical abstract

Fig. 1

Traffic density (vehicles/day) in the sampling points in Pamplona and San Sebastián during the leaf collecting periods in 2019 and 2020.

Fig. 2

Differential effects of location (Pamplona and San Sebastián) and year (2019 and 2020) on heavy metals related to traffic and/or industrial emissions. Data correspond to the mean of five replicates±SE. Results of two-way ANOVA are shown in the upper part of each panel (P < 0.05). NS, not significant.

Fig. 3

Graphic representation of principal component analysis (PCA) and scatter plot distribution of the different elements in Pamplona and San Sebastián during the leaf collecting periods in 2019 and 2020 (April-September). Five replicates were examined per location and sampling point.