MtoA supports Maya particles and nparticles, however, there are some current limitations:

Render Points As

Min Particle Radius

Sets the minimum radius for the size of the particles.

Radius Multiplier

Applies a global multiplier to the radius size of the particles. 

Max Particle Radius

Sets the maximum radius for the size of the particles.

Min Pixel Width

Min Pixel Width doesn't work on particles with varying radius over time (which is the case when motion blur is enabled). Also, for the automatic transparency to work, it needs a special material that supports the internal geo_opacity shader such as the Standard Hair shader.

Interpolated Motion Blur

When the motion blur keys from the scene don't match the time where the nParticle cache was sampled, artifacts will appear. The Interpolated Motion Blur check will avoid this situation by using the nearest samples from the cache. This option requires that the nCache Evaluation Interval attribute be the same as the nCache Evaluate Every # frame(s) option that was used to create the nCache.

 

Delete Dead Particles

This option disables the rendering of particles that exceed the maximum particle Lifespan range when using motion blur.

 

Volume Step Size

Particles switch to rendering in volume mode when using this attribute. This controls the rate at which the volume shader gets called. The value indicates the average distance between adjacent volume shader samples along a given ray, in world space units. Generally, you will want this setting to be as large as possible before visibly noticeable artifacts occur, and in the case of voxelized volumes no smaller than about half the size of your voxels (in world space). Low Step Sizes will increase render times. For example, when the step size is 0.1 and the volume is 10 units big in world space, there are approximately 100 primary samples and therefore the volume shader is called 100 times.

Cloud texture connected to scattering & attenuation of volume collector

The volume step size has a fixed size, therefore, the larger the container the more steps will be taken and the longer it will take to render. Also, beware that if the step size is too small your render times will increase. If you cannot see anything rendered, you may need to reduce the Step Size value.

A Maya scene that demonstrates nparticles rendered as volumes can be found here.

Workflow Issues

Particle Sampler Info

MtoA does not export the particleSamplerInfo node. It exports some "userDataXXX" nodes and they require that the info is exported within the particles object. 

The example below has a particleSamplerInfo connected to the Uv Co-ordinates of a ramp texture and the Out Uv Type is set to Normalized Age:

The particleSamplerInfo is ignored and the particles remain the same color over time

 

By default, this will not work. However, it can be achieved by selecting the particleShape node, and in the Arnold section, in the Export Attributes, type 'age':

Adding 'Age' to the Arnold Export Attributes for the ParticleShape node fixes the connection to the ParticleSamplerInfo node

A Maya scene file that demonstrates the above particle sampler info workflow can be found here.

OpacityPP with particles will only be exported if the Opacity in the Arnold section of the particleShape is disabled.

Using Maya's Particle Instancer in combination with Stand-ins is a powerful way to produce scenes with a lot of complexity.

Stand-ins connected to particles via Particle Instancer (rollover image)