  Previous studies have shown that a hierarchical network comprising a number
of compliant reference stations and a much larger number of low-cost sensors
can deliver reliable air quality data at high temporal and spatial resolution
for ozone at neighbourhood scales. Key to this framework is the concept of a
proxy: a reliable (regulatory) data source whose results have sufficient
statistical similarity over some period of time to those from any given
low-cost measurement site. This enables the low-cost instruments to be
calibrated remotely, avoiding the need for costly on-site calibration of dense
networks. This paper assesses the suitability of this method for local air
pollutants such as nitrogen dioxide which show large temporal and spatial
variability in concentration. The proxy technique is evaluated using the data
from the network of regulatory air monitoring stations measuring nitrogen
dioxide in Southern California to avoid errors introduced by low-cost
instrument performance. Proxies chosen based on land use similarity signalled
typically less than 0.1 percent false alarms. Although poor proxy performance
was observed when the local geography was unusual (a semi-enclosed valley) in
this instance the closest neighbour station proved to be an appropriate
alternative. The method also struggled when wind speeds were low and very local
sources presumably dominated the concentration patterns. Overall, we
demonstrate that the technique can be applied to nitrogen dioxide, and that
appropriate proxies can be found even within a spatially sparse network of
stations in a region with large spatio-temporal variation in concentration.
