Aerosols are suspensions of solid and/or liquid particles in the air. Atmospheric aerosols influence Earth’s climate by scattering and absorbing light and by modifying the properties of clouds, including the albedo (reflectivity) of clouds and the persistence of clouds against precipitation development. The influences of anthropogenic aerosols on climate are considered one of the greatest contributors to uncertainty in present understanding of climate change. In addition, aerosols strongly impact air quality and human health. According to estimates from the World Health Organization, particle pollution contributes to approximately 7 million premature deaths each year, making it one of the leading causes of worldwide mortality. Understanding and quantifying these important effects of aerosols require the knowledge of aerosol properties, and their spatial and temporal distributions, which are driven by a wide range of processes, from formation of the smallest particles from gas phase precursors (i.e., nucleation and new particle formation) up through removal of aerosol particles by drizzle and rain.

Aerosol processes and the effects of aerosols on clouds and climate

One of our research foci is to understand the key processes that drive the properties and evolution of aerosols, and to elucidate and quantify the effects of aerosols on clouds and climate. We accomplish this by deploying cutting-edge instruments in well-designed field observations, focusing on climatically important regions.

As climate change is referenced to the climate state in pre-industrial era, understanding aerosol properties and processes under natural conditions is critical for a reliable assessment and quantification of climate change. Using data collected in the Amazon basin, we discovered that small particles of high concentration are transported from the free troposphere into the boundary layer through downdrafts associated with precipitation of deep convective systems. These small particles then grow and subsequently maintain the cloud condensation nuclei (CCN) population in the Amazon boundary layer under natural conditions (Wang et al. 2016, Nature). We also participated in a study that shows the ultrafine aerosol particles from pollution can intensify deep convective clouds (DCCs) over the Amazon basin (Fan et al., 2018, Science)

Figure 1.  Processes that help maintain the CCN population in the Amazon atmospheric boundary layer under natural conditions. Condensational and coagulational growth of new particles formed in the outflow region of earlier convective clouds leads to high concentrations of small particles in the free troposphere. These particles descend from the free troposphere into the boundary layer during rainfall. In the boundary layer, the small particles grow into CCN of larger diameter. The mass for the particle growth derives from the oxidation of biogenic volatile organic compounds (VOC) emitted by the forest.

Examples of our recently completed, ongoing, and upcoming field studies:

Development of novel instruments

Measurement capabilities shape research, and scientific advances often follow breakthroughs in measurement technologies. One focus of our research is to develop (1) advanced instruments for high time-resolution measurements of aerosol size spectrum, hygroscopic growth, and morphology, and (2) miniature and portable aerosol instruments for deployments onboard UAV and/or in networks.

One instrument we developed is the Fast Integrated Mobility Spectrometer (FIMS) for rapid measurement of sub-micrometer aerosol size spectrum. FIMS measures the concentration of different particle size simultaneously from the displacement of charged particles in an electric field. This novel instrument allows size spectrum measurement with a time resolution of 1 Hz, about two orders of magnitude faster than conventional aerosol mobility sizing instruments (Kulkarni and Wang 2006; Olfert et al., 2008; Wang et al., 2017; Wang et al., 2018). The capability is being expanded to rapid measurements of aerosol hygroscopic growth and morphology (Pinterich et al., 2017; Wang et al., 2019).

Figure 2. Schematic of the Fast Integrated Mobility Spectrometer (FIMS) and an example image showing particle displacement in an electric field.  

Impact of aerosols on air quality

In urban areas, aerosol properties vary over small spatial scales and short time periods, because the properties at a specific location often depend strongly on local emission sources and atmospheric flow conditions.  Traditional air quality monitoring stations are spread sparsely around or within cities, therefore cannot capture the spatial and temporal variability of aerosols needed for accurate assessment of human exposure and health effect. We develop low cost miniature sensors and portable instruments and deploy them in networks to characterize urban aerosols with high spatial and temporal resolution.  In addition, Fast response instruments, including the FIMS we have developed, will be deployed on mobile sampling platforms (e.g.,  mobile vans) to study the spatial and temporal variability of aerosol properties and the processes that drive the variability in areas where high human exposure is expected.

Figure 3. Ultrafine particle concentration maps for Zurich (Switzerland) showing strong spatial variations (Source: Hasenfratz et al., 2015).