Intensity of urban heat island and air quality in Gdansk during 2010 heat wave

During summer unfavorable bioclimatic conditions in cities are primarily related to thermal discomfort caused by the urban heat island, which often is additionally intensified by an increased concentration of air pollution. Temporary deterioration of aero sanitary conditions occurs during summer heat waves and is accompanied by an increased tourist flow toward regions particularly attractive in that time of year, consequently increasing traffic pollution. The objective of the present work is to statistically assess the influence of the intensity of the urban heat island on aero sanitary conditions in the central part of Gdańsk in excessively hot conditions during July 2010. The study is based on hourly air temperature and pollution concentration values included in the Common Air Quality Index: nitrogen dioxide (NO₂), tropospheric ozone (O₃), carbon monoxide (CO), particulate matter PM₁₀ and PM2.5, and sulphur dioxide (SO₂). The results of automatic measurements were obtained from four stations operating within the network of the Agency of Regional Air Quality Monitoring in the Gdańsk metropolitan area (ARMAAG). The intensity of the urban heat island was determined by means of differences of the hourly values of air temperature between stations situated in central and peripheral parts of Gdańsk, including the station in the direct vicinity of Gdańsk Bay. It was found that the urban heat island had a statistically significant influence on thermal sensitivity measured with the use of effective temperature ET. The highest intensity of UHI occurred between the central district (Gdańsk Wrzeszcz) and a typical seaside district (Gdańsk Nowy Port). In approximately 70% of cases, the intensity was in the range 1-3ºC, and its maximum values, over 4ºC, occurred in the morning (7-9 a.m.) and evening (5-8 p.m.). Urban heat island intensity in July 2010 had a statistically significant impact on concentrations of all analyzed pollutants and contributed to the decreased air quality in the centre of Gdańsk. In general, the strongest influence of the urban heat island intensity was manifested with a 1-5 hour delay, depending on the time of day and type of pollution.