A blank period means the Observatory has no eligible stored observation to show for the current selection. Check the station, pollutant and period, load more where available and inspect the source. Missing data is not zero pollution and does not establish that the air was clean.
Observation time describes the measurement; retrieval time describes when it reached this service. Refreshing a page does not create new measurements or backfill a gap. Historical imports and source availability affect what is stored.
Location history brings together recorded station observations, not a representative citywide average. Different stations, reporting patterns, units and averaging windows can make comparisons misleading. Use individual-station comparisons and state the limitations rather than ranking cities or attributing a change to a policy.
Google model estimates are separately labelled for a city-centre point. They are not official station observations and are not inserted into these histories. See Methodology for the distinction between model, observation, retrieval and page-refresh times.