As far as I know the inlet of the monitor can be heated, so high humidity should not be a problem. You might apply a Nafion dryer connected to the inlet as alternative.  Anyway, it's sensible to check carefully the specs of your monitor or contact your supplier.

Dear Marcel thanks for the reply. But the point for which I am concerned isn't  just the inlet temperature or the humidity of the instrument but the the generalised percentage of the humidity in ambient air. Higher humidity may increase the agglomeration of the finer particulate matter inferring with the precision of the instrument at lower levels. My concern is at which level of humidity the monitoring of the finer particles must be stopped?

What do you mean with agglomeration?

The clustering up of the finer particles to form larger sized particles in presence of moisture.

Depends what is being measured and where the intake is.  If the inlet is on a tower (10 meters) it may need to be heated.  For visibility monitoring 60% RH is generally the cut off.  Readings taken with ambient RH over 60% are generally thrown out.

The clustering for instance of soot agglomerates results in SMALLER particles

Dear Ted Porwoll

Thanks for your answer. I think  I have got my answer for the time being. But as you stated that the if the instrument is placed on the tower then the inlet has to be heated, but what if in case I have to measure PM 1, PM 2.5 and PM 10 simultaneously. Can the heating segregate the particles in their respective sizes. or simply how does the instrument will read out the final readings, wouldn't the smaller particles coagulate to form the larger one in presence of moisture.

Dear Harry ten Brink

Thanks for the answer, but can you please explain this concept a bit in detail. or if you could provide me any literature supporting or explaining the same.

The clustering of agglomerates has been extensively studied in nuclear reactor safety

In simulations of an accident large agglomerates of metal oxides are formed and there is also steam formation and condensation on the agglomerates. this is research of the late seventies

We also studied the behaviour of metal oxides iron-oxide zinc copper at conditions of 100% humidity and interaction with gases like SO2 and NO2. We noticed from electron microscope photo's of the collected aerosol that the fluffy aggregates had contracted into framboids with a lower aerodynamic diameter but only at saturation or supersaturation conditions

We published this work but I do not have copies.

I am currently looking for info also on the web and found an OECD report I have to read

http://www-ramanathan.ucsd.edu/files/pr140.pdf

this is for soot agglomerates and their change in properties after "clustering"

To be complete:

Clustering into a framboid-like structure of agglomerates leads to:

smaller 1) physical size and 2) mobility size

larger 3) aerosdynamic size

When the size of the agglomerate is of the order of the wavelength of the measuring light: larger optical size

1.108 works well below 80% RH. High RH can damage the optics. 

Thanks Mr Brink I think, for the time being I have got the answer, thanks for sharing the papers, I will definitely get back to you if in doubt.

Thanks!