Effects of humidity on consumer particle counter readings

My area currently has poor visibility due to fog. Not surprisingly, my consumer laser particle counter shows very poor air quality: its sensor also measures "visibility" in its optical chamber. At the same time, my local government-run professional station (same elevation etc) shows acceptable air quality (2-3-fold lower counts than what I measure).

This may sound trivial, but it turns out professional laser counters pre-heat the air before sampling. This dries up the air and eliminates the additional counts due to potential condensation (fog is made up of suspended microscopic water droplets):

https://amt.copernicus.org/articles/11/4883/2018/

So I just wanted to put a heads up to all "citizen" AQ enthusiasts like myself. If our instruments show very worrysome numbers, while there are no obvious sources of pollution nearby (chimneys, smokers, diesel trucks, fires etc), and your nearby station numbers are much lower, we need to keep in mind that humidity >75% can affect our measurements. I hope that in the future manufacturers incorporate heaters into consumer devices.

I am also aware of cases when government sensor data were manipulated to lower their counts (previously posted on this sub), or government stations being protected from high local emitters, e.g. by being installed at high elevations (roofs etc). Thus, not all high counts reported by us amateurs are b.s. -- it's all on case by case basis.

Below is the abstract from the 2018 paper I cited above. PDF is obtainable.

"While low-cost particle sensors are increasingly being used in numerous applications, most of them have no heater or dryer at the inlet to remove water from the sample before measurement. Deliquescent growth of particles and the formation of fog droplets in the atmosphere can lead to significant increases in particle number concentration (PNC) and mass concentrations reported by such sensors. We carried out a detailed study using a Plantower PMS1003 low-cost particle sensor, both in the laboratory and under actual ambient field conditions, to investigate its response to increasing humidity and the presence of fog in the air. We found significant increases in particle number and mass concentrations at relative humidity above about 75 %. During a period of fog, the total PNC increased by 28 %, while the PNC larger than 2.5 µm increased by over 50 %. The PM10 concentration reported by the PMS1003 was 46 % greater than that on the standard monitor with a charcoal dryer at the inlet. While there is a causal link between particle pollution and adverse health effects, the presence of water on the particles is not harmful to humans. Therefore, air quality standards for particles are specifically limited to solid particles and standard particle monitoring instruments are fitted with a heater or dryer at the inlet to remove all liquid material from the sample before the concentrations are measured. This study shows that it is important to understand that the results provided by low-cost particle sensors, such as the PMS1003, cannot be used to ascertain if air quality standards are being met."