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Particulate matter (PM10 and PM2.5) is monitored at five sites across the region. Monitoring instruments at Lower Hutt, Upper Hutt and Wainuiomata have reached the end of their operational life and are being replaced with instruments that use different measurement technology. Before replacement, the new and old instruments were operated side by side at each monitoring site for at least one year to compare their PM measurements and develop corelation factors. These factors are used to adjust data from the replacement instruments from 1 September 2025 so that data from the new instrument is comparable to the old instrument. This data adjustment is needed for continuity of the monitoring record and confidence that observed air quality trends reflect real changes rather than instrument effects.