The objective of this study was to evaluate university students’ understanding of atmospheric environmental issues according to gender and attending (or not) a college level environmental course (CLEC) including topics such as global warming (GW), greenhouse effect (GE), ozone layer depletion (OLD) and acid rain (AR), and to investigate their misconceptions about the examined content using three-tier diagnostic test. In this study, the survey method was used. The sample of the study consisted of 170 students enrolled in the College of Science at San Jose State University in the USA. To collect the data, “The Atmosphere-Related Environmental Problems Three-Tier Diagnostic Test (AREPDiT)” was used. The data were statistically analyzed using variance analysis (two-way ANOVA) to determine possible differences in students’ understanding of atmospheric environmental issues. In addition, they were descriptively analyzed to find out what misconceptions students had. The results of the analysis indicated that male students’ conceptual understanding of atmospheric environmental issues was significantly higher than that of their female counterparts and that students who had attended a CLEC had a higher conceptual understanding of atmospheric environmental issues than those who had not. It was also found that university students had various misconceptions about atmospheric environmental issues.