For this study, an operational definition consisting of an urban to rural temperature difference greater than 2°C for a minimum of two hours was established to prevent temperature bias caused by local variations in temperature. Observations began on 10 June 2007 and lasted until 10 September 2007. Utilizing a meteorological monitoring network constructed specifically for this study, a distinct temperature difference between downtown Newark and the adjacent rural environment was discovered for 80 of 92 days examined. Heat island development and sustainability occurred at night between 2000 and 0700 hours for the majority of occasions, with only three events existing during daytime hours. Urban heat island intensity reached 3°C or greater during 91% of heat island episodes, 5°C or greater on 45% of occurrences, and 7°C or greater nearly 5% of all occasions. While the mean duration of a heat island episode was 10 hours, events ranged from 2 hours to 19 hours throughout the summer months. The greatest magnitude seen by Newark occurred on 2-3 July 2007, where a maximum Turban-rural reached 7.9°C.
A negative urban heat island, whereby the rural environment remained consistently warmer than the urban environment, existed in Newark on many occasions, mostly in the late morning and early afternoon. The intensity of these negative heat islands was impressive, reaching upwards of 5°C at times. Meteorological factors considered most influential to Newark's urban heat island include wind speed and cloud cover, similar to other UHI studies. A critical wind speed of 5.0 ms-1 was identified, after which an urban to rural temperature difference is unlikely to develop.
Urban fine and coarse particulate matter was collected daily from 11 June 2007 to 1 September 2007. While the summer mean concentration of particulate matter in Newark remained below the standards set up by the U.S. Environmental Protection Agency, this value was exceeded on several occasions. Urban PM shared a statistically significant relationship with urban temperature, presumably due to an amplified number of secondary reactions taking place in the atmosphere under warmer temperatures.
The existence of a relationship between urban heat islands and urban particulate matter has not been given much attention from the scientific community, but is of particular interest to human health and welfare. Although not statistically significant, the results obtained in this study suggest greater concentrations of urban PM in Newark are associated with stronger heat island events. This relationship is further identified through a 10-day intensive period of particulate sampling. The continual presence of a strong urban heat island in Newark provides a good example of human-induced modification to the atmosphere and climate. Not only must we consider the impacts of PM on visibility and climate change, but it is also important for scientists and the public to be aware of an amplified health risk from airborne particulates during the presence of an urban heat island.