Gravimetric (gravimetric method)

Gravimetric measurement determines a pollutant concentration by weighing the mass collected. For particulate matter — PM10 and PM2.5 — it is the reference (gold-standard) method in the NAAQS: a known volume of ambient air is pulled through a pre-weighed filter over the averaging period (usually 24 hours), the filter is conditioned and re-weighed, and the mass gained divided by the air volume sampled gives the concentration in µg/m³.

The method is direct and unambiguous — it measures actual particle mass rather than inferring it from an optical or electrical property — which is why it is the legal reference against which continuous and automatic monitors are calibrated. The trade-off is that it is not real-time: a 24-hour sample yields one number a day later, after laboratory conditioning and weighing. Continuous instruments (beta-attenuation, TEOM, optical) give live data but must be validated against the gravimetric reference.

For recyclers, gravimetric measurement is what underlies the PM10 and PM2.5 figures in ambient baseline studies, EIA reports and SPCB-required ambient monitoring. Since particulate — dust from shredding, conveying, storage and combustion — is usually a recycling plant's most prominent and most-complained-about emission, the gravimetric PM number is often the most scrutinised parameter in a plant's ambient data.

The practical relevance is twofold. First, accept that PM compliance is established by gravimetric sampling over the averaging period, so a robust monitoring programme (correctly sited high-volume or fine-particulate samplers, conditioned filters, NABL-accredited weighing) is what proves the case. Second, because dust is the headline parameter, controlling fugitive dust at source (enclosure, water spray, baghouses) is the most cost-effective way to keep the gravimetric numbers — and the neighbours — satisfied.