problem with blitting render target onto backbuffer

Hi, i've got a problem with blitting a render target onto backbuffer. Basically, i create a separate render target and set it to the current render target. I render particle effects and set the original backbuffer to the current render target. After that, I blit the separate render target onto backbuffer by rendering a full screen quad. The problem is that the final result looks different than what i would see if i just directly render particle effects onto backbuffer. It looks more transparent .. overall, RGBA value of blitted pixels looks decreased. Have anyone seen this problem before? thanks.