The degradation of microcystin-LR (MC-LR) in cyanobacterial aerosol with atmospheric oxidants, such as ozone and OH radicals, was predicted by the Harmful Algal Aerosol Reaction (HAAR) model. The ozonolysis of MC-LR in cyanobacterial aerosol at nighttime and its photooxidation during the daytime was observed in an outdoor chamber. The HAAR model simulates the impact of humidity and aerosol compositions on MC-LR decay. In the model, gas-particle partitioning of atmospheric oxidants onto algal aerosol was kinetically treated using the absorption and desorption processes. In the model simulation, the half-life of MC-LR estimated with its ozonolysis rate constant (3 × 10–11cc/molecules/s) is 4.6 h ± 0.92 at 66 ppb ozone. With the reaction rate constant for MC-LR with OH radicals (6 × 10–7 cc/molecules/s), the estimated half-life of MC-LR during daytime under Florida’s typical summer sunlight is 6 minutes, suggesting that the reaction with OH radicals dominates daytime MC-LR decay. Under moderate sunlight with a typical wind speed (9.2 km/h), the dispersion and HAAR models predict that 25% of aerosolized MC-LR undergoes the atmospheric process within 0.92 km from a bloom source in Florida’s largest lake, suggesting the critical role of the atmospheric oxidation of MC-LR decay.