Has this happened to you? You've set up your particle system to follow a motion path and orient to motion, such as with a line of arrows, only to have the first particle in line flip around and face the wrong direction.
What's happening is that Particular uses the vector information from your light's animated position in order to determine the direction your particles should be facing. The vector is determined by the speed and direction between point A and point B. On the very first frame of your composition, there's no vector information, because the light hasn't moved yet. So, to solve this issue, set a keyframe for your Particles/sec to the amount you wish it to be on Frame 1, then move back to Frame 0 and set another keyframe at 0. This way, no particles will get generated until Frame 1 when the vector information has been established.
*Set a Particles/sec keyframe at frame 1 for the amount of particles you desire
*Move back to frame 0 and set the Particles/sec to 0
For a more advanced solution on this subject as well as further information on working with motion paths, check out Harry Frank's Trapcode Particular 3 Training - Motion Paths video. Should you need additional assistance, please feel free to contact the support team.