The study was in Shenyang (41°48′11.75″ N, 123°25′31.18″ E), the capital of Liaoning Province, China. The city has a population of approximately 8.3 million and covers an area of almost 13,000 km2. It has a mid-temperate, continental climate with four distinct seasons. It is hot and humid in summer, cold and dry in winter, and rapid temperature changes in spring and autumn. Average annual temperature is 8.4 °C, with a maximum of 24.7 °C in July. Average annual rainfall is 510–680 mm, most of which falls in July and August. In 2020, Shenyang recorded serious air pollution on 79 days, and on almost 60% of the pollution days, the primary pollutant was PM2.5 (particulate matter with diameter less than 2.5 μm) (Liaoning Shenyang Ecological Environment Monitoring Center 2021).
Five streets with planted trees were selected to compare outdoor thermal comfort and particulate matter (PM) concentrations between points with and without trees (Fig. 1). Observation points without trees were used as controls to assess the influence of street trees on air quality and outdoor thermal comfort. All five sites were chosen with sections with planted and without planted trees on the same side of the street with similar building heights to ensure similar airflow and solar radiation at the two points. The species of trees planted in these streets were Sophora japonica L., Populus alba L., and Ulmus pumila L.
Measurements of particulate matter concentrations and microclimatic factors were carried out over five days in summer and five days in the autumn in 2020. Measurements were taken at 1.5 m above the street between 14:00 and 19:00 local time. The observations were taken at an average breathing height (1.5 m) to assess the direct feeling of thermal comfort and exposure to air pollution.
PM1 (PM with diameter less than 1 μm), PM2.5, PM4 (PM with diameter less than 4 μm), PM10 (PM with diameter less than 10 μm), and TSP (total suspended particles) concentrations were collected at one minute intervals using an AEROCET 831 (Met One Instruments, Inc., a registered ISO 9001 company, 1600 NW Washington Blvd, Grants Pass, OR 97,526, USA) with an accuracy of ± 10%. The AEROCET 831 measures distribution of particle size and converts them to particulate concentrations using a proprietary algorithm. Microclimatic factors, including wind speed and direction, air temperature, relative humidity, and atmospheric pressure, were recorded using a TNHY-5-A-G meteorological parameter instrument (Zhejiang Top Cloud-Agri Technology Co. Ltd., China) at five-minute intervals.
The physiological equivalent temperature (PET) is defined as: “the air temperature at which the human body’s energy budget under conditions free of wind and solar radiation is balanced with its core and skin temperature in the relevant outdoor environment to be assessed”. It is widely used to evaluate outdoor thermal comfort because it is free of behavioral components and describes the thermo-physiological condition. In this study, PET was calculated in a Rayman Pro model using inputs of wind speed, air temperature, relative humidity, and the individual’s data (a 35 year-old man, weight 75 kg, height 1.7 m, activity 80 W, clothing 0.9 clo) (Rodriguez-Algeciras et al. 2021). The thermal perception was divided into slightly warm (23 − 29 °C), warm (29 − 35 °C), hot (35 − 41 °C) and very hot (> 41 °C), according to the PET value (Matzarakis et al. 1999).
The differences in the physiological equivalent temperature, air temperature, relative humidity, wind speed and particulate matter (PM) concentrations between areas without trees and with trees were assessed using a student’s t-test in summer and autumn, respectively. PM concentrations at different perceptions of thermal comfort were also assessed in this manner in the SPSS statistical package for windows 18.0 (SPSS Inc., Chicago, IL, USA). The pairwise Pearson correlation between pairs of PM concentrations, air temperature, relative humidity, wind speed, wind direction, atmospheric pressure and PET was calculated to explore their association (Eslami and Saeed 2018). The differences in Pearson’s correlation and t-test were considered significant at p < 0.05.