###### Abstract
Ozone precursor gases, such as CO, are emitted by incomplete fuel combustion of road transport as well as industrial processes [22]. NMVOCs are important air pollutants because of their contributions of secondary compounds (aerosols and ozone), generated from gasoline combustion [16; 23; 24]. The emissions of NOx (NOx = NO + NO2) mainly include biomass burning and fuel combustion (e.g., power plant combustion, industrial emissions and transportation emissions) [25]. Aerosol emissions are contributed mostly as by-products of combustion from thermal power stations, vehicle engines and factories [26], with on-road vehicles being the source of fine particulate matter (PM2.5) [27]. In addition, one of the main anthropogenic emissions sources of BC is the incomplete combustion of fossil fuels (especially diesel) in vehicles [10]. Acidifying gases are emitted by the combustion of biomass and fossil fuels as well as by industrial activity [19; 20]. NH3 emissions related to road traffic are due to use of catalytic NOx reduction systems on light and heavy-duty vehicles [19], whose devices use an injection of urea or ammonia [28]. Recent studies showed that the containment measures to minimize the spread of SARS-CoV-2 have resulted in reductions of 15% to 40% in industrial sectors and temporarily reduced China's CO2 emissions by 25%. The European Public Health Alliance (EPHA) states that, in Italy, the urban NO2 pollution comes mainly from traffic, especially diesel vehicles, which are also a major source of particulate matter; the COVID-19 pandemic has resulted in a remarkable drop in these pollutants. France also showed a drop in NOx emissions as a result of the reduction in economic activities and transportation. During the spread of the COVID-19 pandemic in New York, traffic levels were estimated to be down 35% compared with the previous year; significant decreases in the emissions of CO and CO2 were registered, with a 5-10% reduction in CO2 [26]. Some studies have examined the effects of the COVID-19 lockdown on urban mobility [18; 29; 30; 31]. The data show that mobility has dropped around the world as the spread of the virus has increased; public transportation systems were the most affected due to users refusing to use them in order to avoid social contact, and therefore the risk of contagion [32]. Other studies have shown an improvement in air quality in some Colombian cities due to mobility restrictions during the COVID-19 lockdown [33; 34]. However, these studiesdid not look at the changes in atmospheric emissions associated with the observed air quality changes. Google, in its COVID-19 Community Mobility Reports for Colombia ([[https://www.google.com/covid19/mobility/](https://www.google.com/covid19/mobility/)]([https://www.google.com/covid19/mobility/](https://www.google.com/covid19/mobility/))), reports that, in April 2020, the country saw the biggest reduction in visits to retail and recreation places (77%), transport stations (77%), parks (67%), grocery stores and pharmacies (59%), and workplaces (58%), while the trend of mobility in residential areas increased by 28%. At the beginning of May, the opening of some economic sectors caused an increase in mobility in relation to the previous month, especially in workplaces (17%), grocery stores and pharmacies (13%), retail and recreation places (10%), and transport stations (9%). Therefore, the aim of this study is to analyze the changes in the emissions associated with road transportation during the COVID-19 lockdown in Colombia, comparing these emissions with values obtained in the same period of 2018 for four groups of pollutants, namely, GHGs (CH\({}_{4}\), CO\({}_{2}\), N\({}_{2}\)O), ozone precursor gases (CO, NMVOC, NOx), aerosols (BC, PM\({}_{10}\), PM\({}_{2.5}\)), and acidifying gases (NH\({}_{3}\), SO\({}_{2}\)). The results can serve decision makers in the development of strategies to improve air quality related to the road transport sector in Colombia. This article is ordered as follows.