Characteristics of PM2.5 aerosol particles during a heat wave in an urban atmosphere in southwest Germany

Aerosol particles have significant impacts on climate, air quality, and human health. Their characteristics are especially important in urban atmospheres during heat waves. Therefore, we conducted a 4-week measurement campaign at an urban kerbside in the city of Karlsruhe in southwest Germany during a heat wave period in July 2019. A high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed in the container to measure non-refractory aerosol compositions of PM_2.5 online. Filter samples were also collected during the campaign, and characterized for oxygenated organic molecular compounds using a chemical ionization mass spectrometer (FIGAERO-CIMS). In addition, a small box with low cost particle sensors and meteorological sensors (solar radiation, temperature, and humidity) was used for spatial resolved measurements employing a bicycle. During our measurement, the total organics, sulfate, nitrate, ammonium, chloride and black carbon contributed on average 58.9%, 17.3%, 5.9%, 5.5%, 0.2% and 12.3% to the particle mass comprising non-refractory components plus black carbon. Positive matrix factorization (PMF) analysis for the AMS organic aerosol (OA) data resolved three factors including hydrocarbon-like OA (HOA), semi-volatile oxygenated OA (SV-OOA) and low-volatility oxygenated OA (LV-OOA). Meteorological effects on aerosol compositions were investigated. Low wind speeds during the whole campaign correspond to major contributions from local emissions. During heat waves, high temperature and low humidity suppressed the formation of nitrate, but facilitated the formation of sulfate and organics. In particular, SV-OOA and LV-OOA showed positive correlations with temperature. The ratios of LV-OOA to SV-OOA strongly correlated with temperature and odd oxygen (O_x = O₃ + NO₂), suggesting fast photochemical transformation of SV-OOA to LV-OOA during heat waves. Furthermore, the relationships between organic aerosol factors and typical organic markers were investigated to study the relative influences of biogenic and anthropogenic emissions on OA formation. Besides, bicycle measurements point to important hot spots of particle pollution. This contribution will discuss the interaction of urban air pollution and heat islands.