While the number of times Alexandra exceeds the National Environmental Standard for air quality - no more than one day a year over the daily concentration of 50mcg/m3 - is similar to last year, 49 so far this year compared with 47 last year, the peaks were declining.
Council air quality scientist Deborah Mills said the maximum level of PM10 (fine particles smaller than 10 microns across) recorded this year so far was 106mcg/m3, one of the lowest recorded. During the past 10 years, the average maximum was 129mcg, with the highest level recorded 193mcg.
''It is in the reduction of these highest numbers that we are still seeing improvement.''
The climatology analysis for Alexandra was not due to be completed until after winter ended.
Various scenarios kept smoky emissions such as that featured in the photograph from dispersing, such as a very still day, she said.
Another factor was the formation of a temperature inversion, which could occur when there were cold overnight temperatures, usually less than 0degC, coupled with calm conditions and a clear sky.
Last Saturday, the data showed it was likely to have been an inversion in the morning with corresponding high hourly PM10 during the hours from 7am to 9am.
''But by the afternoon the weather changed and the winds had picked up and PM10 did not have its usual peak during evening hours.''
