There has been a substantial reduction in toxic air emissions in the United States and Canada during the past four decades. Despite impressive progress, toxic air emissions continue to be of concern in major urban areas. To better understand the current situation, this issue includes a special grouping of 10 papers focusing on topics such as air toxics, mobile sources, and urban air quality. Two of the papers in this edition are based on presentations given at the Coordinating Research Council’s (CRC) Mobile Source Air Toxics (MSATs) Workshop, which was held at the California Air Resources Board (CARB) headquarters in Sacramento, CA, February 17–19, 2015. The objective of the CRC workshop was to bring together interested parties from government, industry, and academia to review the status and knowledge gaps regarding MSATs. The workshop consisted of one plenary and six technical sessions, which explored regulatory aspects, vehicle emissions, air quality and exposure measurements, atmospheric and air quality modeling, and accountability. The workshop agenda and proceedings are available on the CRC Web site at www.crcao.org. Some of technical challenges discussed during this workshop included:
Baseline information needed to demonstrate the effectiveness of the mobile source regulatory programs in producing the intended benefits;
Observations and methods to isolate the mobile source signal from other emission sources (i.e., biogenic, power plants, and area source);
Innovative analysis of observational data from the current air quality monitoring network and air quality model outputs in assessing the impact of promulgated regulatory programs; and
Gaps in observations of some tracers to properly delineate the impact of emission controls on the mobile source sector versus other sectors.
Meanwhile, the downward trend in mobile source emissions is expected to continue as older vehicles in the on-road fleet are replaced with newer vehicles with more modern emission controls and as cleaner fuels are available.
Participation in workshops like this, with knowledge exchanged among all stakeholders, along with peer-reviewed research publications will help achieve and sustain long-term air quality improvement and quality of life for future generations.