###### Abstract  
The aim of this work was to analyze the changes in the emissions from the transport sector during the COVID-19 lockdown in Colombia. We compared estimated emissions from road transportation of four groups of pollutants, namely, greenhouse gases (CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\)O), ozone precursor gases (CO, NMVOC, NOx), aerosols (BC, PM\({}_{2.5}\), PM\({}_{10}\)), and acidifying gases (NH\({}_{3}\), SO\({}_{2}\)), during the first half of 2020 with values obtained in the same period of 2018. The estimate of emissions from road transportation was determined using a standardized methodology consistent with the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories and the European Environment Agency/European Monitoring and Evaluation Program. We found a substantial reduction in GHG emissions for CH\({}_{4}\), N\({}_{2}\)O, and CO\({}_{2}\) by 17%, 21%, and 28%, respectively. The ozone precursors CO and NMVOC presented a decrease of 21% and 22%, respectively, while NOx emissions were reduced up to 15% for the study period. In addition, BC decreased 15%, and there was a reduction of 17% for both PM\({}_{10}\) and PM\({}_{2.5}\) emissions. Finally, acidifying gases presented negative variations of 19% for SO\({}_{2}\) and 23% for NH\({}_{3}\) emissions. Furthermore, these results were consistent with the Ozone Monitoring Instrument (OMI) satellite observations and measurements at air quality stations. Our results suggest that the largest decreases were due to the reduction in the burning of gasoline and diesel oil from the transport sector during the COVID-19 lockdown. These results can serve decision makers in adopting strategies to improve air quality related to the analyzed sector. COVID-19; lockdown; acidifying gases; aerosols; greenhouse gases; ozone precursors; road transportation; Colombia +
Footnote †: journal: Applied Sciences  
constrained. Consequently, educational institutes and non-essential factories remained closed, public events were cancelled, and work at home was implemented, to prevent the further spread of the COVID-19 pandemic. The anthropogenic changes caused by the lockdown led to a decline in industrial production and energy consumption, up to 30% in some countries [6; 7]. Energy demand has been altered drastically worldwide, and due to forced confinement, many international borders were closed and populations were isolated in their homes [8]. This led to a change in some consumption patterns for energy, e.g., those related to the transport sector, because of a reduction in mobility. These restrictions on economic activity during the pandemic have reduced NO2 emissions in China, Europe and the United States during COVID-19 [9]. Mobility has also been one of the things most affected by the COVID-19 restrictions. The changes in patterns of mobility indicate a reduction in vehicular traffic; as a consequence, a decrease in emissions associated with this sector is to be expected, given that the greenhouse gas (GHG) emissions from road and aviation transportation make up 72% and 11% of all GHG emissions, respectively [10]. Consequently, containment measures implemented in various countries have shown changes in the air quality [11; 12; 13; 14]. The use of fossil fuels by road vehicles is the main source of four groups of pollutants, including GHG [10], including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O); ozone precursor gases, such as carbon monoxide (CO) [15], non-methane volatile organic compounds (NMVOC) [16], and nitrogen oxides (NOx) [17; 18]; aerosols, including black carbon (BC) [10] and particulate matter (PM2.5, PM10) [15]; acidifying gases, such as ammonia (NH3) [19] and sulfur dioxide (SO2) [20]. GHG emissions, such as CO2, are mainly produced by power generation and road transport. Other GHG emissions, such as CH4, are generated by fermentation processes, fossil fuel extraction and use, landfills and waste. In addition, N2O is produced from soil emissions [21].