Particles are detected with laser scattering to measure PM1 PM2.5
and PM10
Air Monitoring 19 ,
. Scattered light is measured either from single
particles (Alphasense, Particle Plus), providing real time particle diameter distributions or scattering from an entire ensemble, calibrated to correlate with PM2.5
(Plantower, Sensirion, Tera,
Shinyea…). Both methods have a temperature dependence which can be compensated, but at humidity above 60 %RH the particles adsorb water7
, swelling to a larger apparent diameter. Equivalent
grade PM instruments (typically $10,000) include high power inlet heaters to dry particles, but this is missing from low cost PM monitors due to power and cost restrictions, Tera being an exception. Climates with high humidity require either particle heating or humidity correction algorithms with their limitations; if not corrected, PM values will be seriously overestimated at high humidity, especially above 85%RH. Pressure variations are not a direct problem but the air fl ow rate must be controlled and ambient pressure will slightly affect the air fl ow rate.
Non-dispersive infrared (NDIR) spectroscopy is the most popular technology for measuring CO2
, a critical gas for climate warming
and a product of combustion. Temperature compensation is critical, but most manufacturers take care of this by expensive factory multipoint temperature calibration. Humidity does not directly affect NDIR at the CO2
wavelength, but fog and
rain can condense on the optics causing errors. Pressure has a linear effect on the observed NDIR light absorbance, so must be corrected, usually with a low cost absolute pressure sensor. If there is no pressure correction then measurements are referenced to the ambient pressure at the time of factory calibration and should be post-process corrected.
Weather, Climate and Environmental Conditions
Daily weather will affect low cost sensors with hot or cold temperature, or dry or high humidity. We cannot predict weather weeks ahead, but we do know the climate which is the average weather behaviour for any location on this planet. In the long term, climate with high temperatures, frequent and rapid humidity changes or long term high humidity will affect the overall performance of AQ networks. But climates are also defi ned by their seasons7
.
Away from the equator there are four seasons, but when discussing environmental conditions both Spring and Autumn are considered to be equivalent: the average temperature will start from either high or low and fi nish the season either low or high; pressure, temperature and humidity fl uctuations are frequent. Winter and Summer have more stable conditions with less fl uctuations. Equatorial locations have either no seasonal differences or simply wet and dry seasons.
Climate can be classifi ed in many ways, the Köppen–Geiger climate classifi cation is the most common classifi cation. The table above lists the 20 classifi cations which are habitable.
How do climates compare?
Let us look at three very different climates: very humid Glasgow (Cfb: courtesy Ricardo plc), Hot and humid Hong Kong (Cwa: courtesy Dane Westerdahl) and hot Mediterranean Los Angeles (Csa: courtesy Raul Dominguez SCAQMD). Diurnal patterns for Hong Kong and Diamond Bar in Los Angeles both show expected midday temperature rises but the average temperature and range of humidities are quite different. Likewise, there are clear seasonal differences between January and July.
We can see that there are differences in both the average temperature and humidity and the daily fl uctuations. Annual profi les for the year of 2021 (below) for the two locations show the wide range and variations of temperature and relative humidity, with Hong Kong on the average 15°C hotter and 30% higher humidity.
It would be surprising if an AQ sensor system network that was calibrated in one climate remained accurate in another climate. The red plots to the left show diurnal, weekly and monthly relative humidities in Glasgow in 2021. It is best to visit in the spring. While diamond Bar in Los Angeles averages about 50% RH, Glasgow averages between 65% and 85% RH.
When we look at the time traces of temperature and relative humidity, it is clear that some climates can present severe environmental conditions for low cost AQ sensor systems.
Are fi eld trials in one climate valid?
Dozens of peer reviewed papers have published results from fi eld trials of low cost AQ sensors systems, although there have been
WWW.ENVIROTECH-ONLINE.COM
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