For meteorology and climate variables, a well-developed global observation system with spatial coverage adequate to monitor regional patterns is coordinated by WMO. For atmospheric composition, however, the amount of information available varies significantly by pollutant and region. Countries in North America, Europe and East Asia, have well-developed in-situ ground-based monitoring networks for ground-level O3 and PM, as well as SO2, CO and, in some areas, NO and NO2. For other pollutants, observations tend to be relatively sparse. There is a need for a global catalogue of monitoring station metadata, currently being pursued through expansion of the WMO Global Atmosphere Watch Station Information System (GAWSiS, https://gawsis.meteoswiss.ch) and Observing Systems Capability Analysis and Review (OSCAR, https://oscar.wmo.int) tool. For many regions of the world, however, groundbased networks do not have sufficient density and coverage to characterize spatially representative trends. Observations from satellites, aircraft and other platforms, as well as atmospheric chemistry and transport models, are needed to complement traditional networks.
Existing polar-orbiting satellite instruments provide global observations of a number of important air pollutants (including PM, O3, CO, SO2, NO2, NH3, formaldehyde and CH4) albeit with relatively coarse temporal, spatial and vertical resolution (Duncan et al. 2014; Duncan et al. 2016). In some parts of the world, however, monthly average total column observations from satellites provide the only information available. Current efforts to improve understanding of the relationship between spacebased and ground-based observations should help to fill data gaps in areas with sparse monitoring (e.g. Snider et al. 2015).
Space agencies in the Republic of Korea, the United States of America and Europe are working to deploy a constellation of geostationary satellites over East Asia, North America, Europe, North Africa and the Mediterranean to measure O3, PM and their precursors. In geostationary orbit, these instruments will have much finer temporal and spatial resolution than current polar-orbiting satellites, providing a wealth of information about air pollution over these regions in near real-time (Committee on Earth Observing Satellites 2011).
At the other end of the spectrum of cost and complexity, inexpensive electronic sensors for measuring different pollutants are being developed, marketed to governments, businesses and even individuals, and deployed in a variety of mobile and stationary settings (e.g. Apte et al. 2017). The quality of information varies significantly and is currently low, but efforts are in place to better understand the performance of different sensors, and to develop standardized tests and guidance on how to deploy and use the observations gathered (UNEP 2016; Lewis et al. 2017; US EPA 2018b).
Increasingly, air quality information from ground-based networks as well as air quality forecasts are being made available publicly. The United States of America pioneered such systems with AirNow.gov starting in 1998, and similar information is now available in countries and cities worldwide, as well as through open source platforms (e.g. OpenAQ.org) (see Section 12.2.4).
