I think it's non-existent today.
I work at a Dutch municipality and, though things might have changed now, in December 2017 our environmental department concluded that even semi-professional gas or dust sensors were just not good enough to determine the low levels needed for outdoor measurements. (Including everything Libelium offered at that time for CO, NOx and particulate matter.)
It's not my cup of tea, but the reasoning of our environmental department:
For particulate matter Libelium claimed to count PM 1. But outdoor measurements would only make sense for PM 0.1 (particulates smaller than 0.1 micrometer). Also, the weight of the counted particulates is not a constant, so converting the counts into some μg/m3 value, to compare to government regulations, is difficult if not impossible.
For NO2 Libelium claimed an accuracy of 0.1 ppm (parts per million) in ideal conditions. With, for NO2, 1 pbb (parts per billion) being equal to† about 1.88 μg/m3, a claimed accuracy of 0.1 ppm = 100 ppb = about 188 μg/m3. But outdoor hourly averages of NO2 concentrations vary between 1 μg/m3 and 60 μg/m3, and the Dutch government indicators define an upper limit for the annual average of 40 μg/m3. So, a claimed accuracy of 188 μg/m3 does not suffice.
Any other measurements other than (ultra) fine dust and NO2 were deemed not interesting for air quality.
Of course, combining a lot of low-cost measurements and comparing those to the results of (very expensive) professional measurements might still yield useful indicators.
† µg/m3 = ppb × 12.187 × M / T, where µg/m3 is micrograms of gaseous pollutant per cubic meter of ambient air, ppb is parts per billion by volume (i.e., volume of gaseous pollutant per 109 volumes of ambient air), M is the molecular weight of the gaseous pollutant, and T is the temperature in Kelvin. An atmospheric pressure of 1 atmosphere is assumed.
So, for a temperature of 25 °C: 1 ppb SO2 = 2.62 μg/m3; NO2 = 1.88 μg/m3; NO = 1.25 μg/m3; O3 = 2.00 μg/m3; CO = 1.145 μg/m3; Benzene = 3.19 μg/m3.
Source: DCE - Danish Centre For Environment And Energy.
