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LHC. At other times of the day, NOx values at the street canyondominated the AQI values and caused a “Medium” LHC. A shortepisode of dust flow accompanied by eastern wind which occurredon the morning of the second day, caused an increase in PM10values (~100 mG/m3) between 12:00 and 13:00. However, PM10values were not high enough to switch the AQI level to “High” LHC,as long as NOx values were still high. Only when NOx valuesdecreased below 300ppb (from 22:00 onward) did PM10 valuesbecome the dominant component in controlling the AQI values. Atthe urban square, a “Medium” LHC existed at most hours of the dayand night due to high PM10 concentration values. A “Low” LHCexisted for a few hours, between 9:00 to 16:00 of day 1, when PM10values were not high enough to change the AQI level from “Medium” to “Low” category. In the urban park, a “Low” LHC existedmost of the day and night hours. Only short episodes of high NOxvalues (295e620ppb) decreased the AQI values and the LHC turned“Medium” and “High”. The notable finding is that during theepisode of dust flow, mitigation of PM10 concentrations by the parkwas the most pronounced, decreasing PM10 concentrations over ashort distance of 50 m from a value of 218 mG/m3 in the streetcanyon to 64 mG/m3 inside the treed park. Mitigation of NOx concentrations were less pronounced during this episode.
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