Optical signal suffers from fluctuations in both intensity and phase due to the random refractive index variations induced by atmospheric turbulence.Meanwhile,system performance degradation due to urban fog/haze also should not be ignored.Given slowly time-varying characteristics of turbulent phase variations and laser phase noise,decision-aided,maximum likelihood (DAML) phase estimation algorithm is employed in free space coherent optical communication system to improve system performance.Then,BER performance of DAML system with DQPSK modulation is investigated considering the influences of atmospheric turbulence,laser phase noise and weather conditions.Transmission distance and wavelength selectivity is also analyzed in fog/haze weather.Simulation results show that turbulent phase noise can be ignored with less than 0.1 dB signal-to-noise ratio penalty compared with no turbulence.And in fog and haze weathers,free space coherent optical communication with DAML algorithm has longer transmission distance than OOK IM/DD free space optical communication,at least 500 meters.DAML system with 1 550 nm wavelength has better performance than that with 850 nm wavelength in haze and light fog weathers.But system performance is independent of wavelength in thick fog weather when visibility is less than 200 meters.