Observation and source apportionment of atmospheric alkaline gases in urban Beijing, China

Atmospheric alkaline gases, including NH₃ and amines, have been demonstrated to play crucial roles in atmospheric chemical processes, from enhancing atmospheric nucleation to promoting secondary particulate matter formation. It has been suggested that new particle formation can contribute significantly to haze formation in Beijing and other megacities in China. Although NH₃ and amines have been ubiquitously detected in the atmosphere, little is known about the specific source profiles and the corresponding source contributions. It is well accepted that agriculture-related emissions dominate the global NH₃ budget in the atmosphere. However, several recent field studies demonstrated that non-agricultural emissions are the primary NH₃ sources in many urban areas in China. Therefore, atmospheric models based on emission inventories concentrated on agricultural emissions may not realistically simulate the atmospheric environment, particularly in the populated megacities of China. Recently, NH₃, amines, amides, and imines emission characteristics of motor vehicles have been determined explicitly through in situ roadside measurements. However, the emission features from many other anthropogenic activities are still unknown. In this study, based on constrained SoFi-PMF analysis, we have investigated the specific impacts of motor vehicle emissions on NH₃ and other alkaline gases in urban Beijing. It was found that motor vehicles can contribute a predominant portion of amines in the urban environment, particularly during daytime. Hence, reducing on-road vehicular emissions can decrease primary emissions of criteria air pollutants (such as NO_x, SO₂, and PM_2.5) and suppress secondary aerosol formation. Although the PMF-based source apportionment analysis may be subjective to individual bias, it provides a valuable opportunity to explore other non-vehicular sources. The septic system in the urban area is recognized as a significant contributor to background NH₃ and amines in urban Beijing. Motor vehicle emissions usually show less seasonal variability. Intense demands for central heating, power generation, and biofuel usage in suburban areas may play a significant role in wintertime haze formation. These emission activities, however, may highly depend on environmental conditions, such as temperature and humidity. Further investigations of the underlying releasing mechanisms and emission intensities are critically needed.