In memoriam: Peter K. Mueller (1926-2023).

Peter K. Mueller passed away in early February at age 96. He was widely known for his expertise in air measurements and the design and oversight of major air pollution chemistry studies. Peter’s contributions derived from employment in state and federal government, and the private sector. He was a member of the Air & Waste Management Association for more than 70 years, received its 2007 Frank Chambers Excellence in Air Pollution Control Award, and was made an Honorary Member in 2015. His views and a sampling of achievements have been published earlier in A&WMA publications (Hidy et al. 2017; Hidy, Chow, and Watson 2017; Kleinman et al. 2017; Watson et al. 2017). Peter’s involuntary transcontinental migration began with dislocation from Germany followed by a risky immigration to the United States. After enduring Kristallnacht when he was 11, Peter barely escaped the Holocaust, in which many members of his extended family were killed, when Peter and his father immigrated to England in May 1939, not long after his mother, brother, and sister immigrated to the United States. Later, Peter also moved to the United States, where he was drafted by the U.S. Army and served in the Medical Corps. Following this tour, he enrolled as a chemistry major at George Washington University, with subsequent graduate study at Rutgers University, where he focused on aqueous environmental chemistry. After receiving his Ph.D. degree, he moved to the San Francisco Bay Area as a chemist at the California State Air and Industrial Hygiene Laboratory (AIHL). His early experiences shaped his interest in chemistry and public health and led him to exacting standards as an expert in ambient air measurements. His research was aimed at informing policymakers about environmental conditions of concern. Peter was the last of the first generation of environmental scientists who were challenged to characterize and understand photochemical smog. Arie Haagen Smit’s botanical experiments in Southern California led to the discovery of ozone formation from nonmethane hydrocarbons and nitrogen oxides in ambient air (Haagen-Smit 1958). As AIHL director, Peter investigated instrumentation for quantitatively measuring smog products and precursors, recommended those that met quantitative reference standards, and characterized California’s unique air quality problems. In the early 1950s, aerosol scientists recognized the significance of particle size in characterizing atmospheric aerosols. Obtaining size distributions was labor intensive and involved primitive means for sampling and analysis. By the 1960s, new size fractionating systems were being invented such as that of Goetz (1956). Peter and colleagues tested this sampler and improved its design (Mueller, Helwig, and Peterson 1959). As part of this project, they sampled urban air and found that, in contrast to earlier studies, airborne lead was concentrated in respirable particles, not just inhalable particles. This finding had important consequences for lung impacts and identification of gasoline fueled vehicles contributions to suspended particles. This result contributed to the formulation of automobile fuel composition rules later in the 1980s. With mounting public and regulatory pressure to understand the nature of particles forming from gaseous emissions (secondary aerosols), Peter and the AIHL teamed with Kenneth Whitby, Sheldon Friedlander, Robert Charlson, and other investigators beginning in 1969 to conduct experiments on photochemical aerosols across California. The design and conduct of these studies employed unique, complex state of the art instrumentation for gas and particle measurements recorded by micro-computer. These projects (Hidy et al. 1975) advanced knowledge of aerosol gas interactions necessary to formulate effective emission reduction strategies. In 1974, Peter joined Environmental Research and Technology (ERT) to partner with George Hidy in developing a western component of this company. Their goal was to use new concepts in environmental chemistry to inform policymakers of emerging issues. Peter oversaw improvements in ambient and laboratory analyses of air chemistry samples. Among the important achievements was the development of particle carbon analysis methods. JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION 2023, VOL. 73, NO. 5, 333–334 https://doi.org/10.1080/10962247.2023.2202598