LHC. At other times of the day, NOx values at the street canyon
dominated the AQI values and caused a “Medium” LHC. A short
episode of dust flow accompanied by eastern wind which occurred
on the morning of the second day, caused an increase in PM10
values (~100 mG/m3
) between 12:00 and 13:00. However, PM10
values were not high enough to switch the AQI level to “High” LHC,
as long as NOx values were still high. Only when NOx values
decreased below 300ppb (from 22:00 onward) did PM10 values
become the dominant component in controlling the AQI values. At
the urban square, a “Medium” LHC existed at most hours of the day
and night due to high PM10 concentration values. A “Low” LHC
existed for a few hours, between 9:00 to 16:00 of day 1, when PM10
values were not high enough to change the AQI level from “Medium”
to “Low” category. In the urban park, a “Low” LHC existed
most of the day and night hours. Only short episodes of high NOx
values (295e620ppb) decreased the AQI values and the LHC turned
“Medium” and “High”. The notable finding is that during the
episode of dust flow, mitigation of PM10 concentrations by the park
was the most pronounced, decreasing PM10 concentrations over a
short distance of 50 m from a value of 218 mG/m3 in the street
canyon to 64 mG/m3 inside the treed park. Mitigation of NOx concentrations
were less pronounced during this episode.
During the summer, on the first day (June 28th) which was
characterized by typical summer climatic conditions, high PM10
values in the street canyon dominated the AQI values and consequently
a “Medium” LHC existed. In the urban square and the urban
park a “low” level existed most of the time. O3 values at all three
sites were not extremely high, except for 2 h (12:00e13:00) when
O3 values in the park were above 60ppb and the AQI level decreased
to a “Medium” LHC.
O