Particulate matter (PM)2.5 is a common air pollutant known to induce damages in the respiratory, cardiovascular, and nervous systems. Previous study has shown that acute and high-level PM insult could significantly aggravate the severity of LPS-induced acute lung injury (ALI). However, humans typically experience more chronic and low-level PM, of which the effect on ALI is yet unclear. Here, we varied the concentration of PM from low, medium, to high, which was given to mice via intratracheal instillation for a short period of time. Compared to the saline-treated mice, mice with medium or high PM treatment presented significantly higher mortality rate, weight reduction, and bronchoalveolar lavage (BAL) protein concentration during ALI, while mice with low PM treatment did not demonstrate significant differences from saline-treated mice. However, when the PM was given for an elongated period of time, PM, even at the low level, significantly aggravated ALI severity. Furthermore, the PM-mediated changes were sustained even after PM withdrawal. We also examined the CD4 T cells in saline- or PM-treated mice. We found that, although PM did not significantly change the number of lung-infiltrating CD4 T cells, it significantly altered the composition of lung-infiltrating CD4 T cells, characterized by having a higher T-bet/Foxp3 ratio in the PM-treated group compared to the saline-treated group. Additionally, the Treg-mediated suppression was reduced in PM-treated mice. The effect of PM on CD4 T cells depended on the concentration of PM and the duration of the treatment, and was independent of the PM withdrawal. Overall, these results demonstrated that chronic and low-level PM was sufficient at aggravating ALI and altering pulmonary CD4 T cells, and the effect could be sustained even after PM withdrawal.