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Atmospheric Sciences & Global Change
Research Highlights

May 2016

Why We Care about Carbonaceous Aerosols

Researchers make the most of CARES data to unravel carbon-containing atmospheric particles

Rahul Zaveri and the PNNL G-1 aircraft at the start of CARES
Rahul Zaveri, atmospheric scientist at Pacific Northwest National Laboratory who led the CARES campaign is ready to board the DOE’s G-1 research aircraft. zoomEnlarge Image.

Six years ago, a research dream team descended on the Sacramento, California area. They were equipped with two aircraft, ground-based and skyward pointed instruments, and a crew of 60, including pilots, instrument specialists, engineers, and atmospheric researchers. It would take all of them—and more—to find the secrets of carbon-containing particles that have long been in the top ten of notorious air health hazards. Yet, after clogging lungs, sinuses and more, the remnants of combustion and the dregs of vehicle and power plant exhaust eventually drift out of sight, into the atmosphere for an unwritten second act.

Leading the team to write the second act of these particles were researchers from Pacific Northwest National Laboratory, with co-investigators from NASA, NOAA, the California Air Resources Board, the California Energy Commission, a dozen universities, several national laboratories and private industry partners. The field study was dubbed CARES, short for Carbonaceous Aerosol and Radiative Effects Study.

Using the data collected from CARES, researchers published to date more than 30 papers. These have garnered more than 600 citations advancing our understanding of what the research field calls secondary organic aerosols—or SOA for short—and how the carbon-containing aerosol particles mix in the atmosphere. These particles also influence changes in the climate, including how sunlight energy is disrupted on its path to Earth's surface and their role in forming clouds. (See sidebar, The Life and Times of SOAs.)

The CARES Campaign Charge

Getting the data used in these 30-plus papers, with more to come, required extensive coincident measurements of meteorology, sunlight comings and goings, trace gases, and aerosol particles. The team collected these at two surface sites and aloft, above those sites, using the ARM Aerosol Facility G-1 aircraft. They used the NASA King Air aircraft to collect data profiles in the same regions the G-1 aircraft sampled data on aerosol properties.

Locations of the surface measurement sites and flight paths during CARES
Locations of the surface measurement sites and flight paths during CARES. zoomEnlarge Image.

"It is very exciting to see that CARES data have led to several discoveries in the process-level understanding of carbonaceous aerosols," said Dr. Rahul Zaveri, PI on the CARES campaign and atmospheric scientist at PNNL.

"Major discoveries in SOA formation chemistry and growth dynamics, aging of black carbon and sea salt particles by organics, and the resulting effects on the climate-related properties have, over the last 5 years, motivated a series of focused laboratory studies to further investigate specific processes. In a nutshell, they have inspired new model developments that will lead to improved fidelity of aerosol interactions in climate models."

An Influential Body of Research

Now, in 2016, there is a sizeable body of research, published in journals such as Science, Nature Communications and the Proceedings of the National Academy of Sciences, with solid evidence of the carbonaceous particles' second act. With a rich arsenal of data, the findings can be grouped into several categories, targeting some of the stickiest and toughest science questions about these particles.

A Paradigm Shift in SOA Understanding

Using CARES measurements, Vaden et al. (2011) showed that particle evaporation proceeds slowly-much slower than expected from existing theory. The theory assumes that SOA particles are liquid droplets. The CARES data reveals a different pattern-that SOA particles are more like gooey orbs.

Subsequent studies, such as O'Brien et al. (2014), compared CARES data with those from other campaigns and showed that the carbon-based particles found in the air are much stickier and have a higher surface tension than those typically generated by laboratory experiments.

These papers and similar findings published led to a paradigm shift in the way models describe SOA formation and evaporation, including a new treatment of SOA that is currently being implemented in DOE's Accelerated Climate Model for Energy (ACME).

Analyses of the ground and aircraft data performed by Setyan et al. (2012), Shilling et al. (2013), and Kleinman et al. (2016) showed that organic aerosol production increased when human-caused emissions from Sacramento mixed with air rich in isoprene, an organic compound wafting from many plants that originate in the area's foothills. Researchers are not yet certain about the chemical mechanism responsible for this increase. They are certain that the result of the mixing is missing in current climate models. These findings are forming the basis for new laboratory studies aimed at identifying the chemical pathways they can implement in models.

organic aerosol concentrations during CARES
Organic Aerosol Concentrations as a function of carbon monoxide (CO) that show the highest concentrations associated with mixtures of biogenic and anthropogenic emissions (from Shilling et al. 2013. zoomEnlarge Image to view entire graph..

Zaveri et al. (2012) described the frequent occurrence of new particle formation events during CARES and Setyan et al. (2014) found that interaction of the region's natural emissions with the urban-sourced plumes also enhanced new particle formation and growth. The growth of new particles, they found, is driven by the condensation of oxygen-rich organic particles, and to a lesser extent, ammonium sulfate, an emitted compound typical over agricultural areas.

Using CARES campaign data, other studies have evaluated how models perform. Lupascu et al. (2015) showed that when model formulas include low-volatility organic vapors, the results affect the way particles grow and develop. Fast et al. (2014) found that modeled concentrations of organic aerosols were lower than the CARES measurements, possibly due to missing reactions in the modeled atmosphere.

Fast et al. (2012, 2014) showed that mountain venting processes, where circulation is redirected due to topology, contributed to the aged aerosol layers observed by (Scarino et al. 2014) in the valley atmosphere. These layers are captured by the moving air into the growing atmospheric boundary layer the next day. Climate models, with their coarse resolution, typically simulate the transport of aerosols away from California too quickly and at the wrong altitudes. They also show that errors in calculating long-range transport of aerosols from Asia confound quantifying any regional-scale variations in aerosol's effects on Earth's energy budget. Further, these errors block determination of the relative role of emissions from local and distant sources during the relatively "clean" conditions observed during CARES.

Aerosol Mixing State: What's in those SOA?

Cahill et al. (2012) showed that the majority (88%) of carbonaceous particles in California are internally mixed, but varied regionally. Northern California is dominated by mixtures of organics with sulfates (typical of rural areas) and southern California having larger fractions of nitrate and soot (to be expected over urban areas).

Moffet et al. (2013) showed that as the Sacramento plume ages there is an increase in organic carbon content and homogeneous organic particles that lack an inorganic component. They found fewer carbon-carbon double bonds in CARES particles compared to more polluted regions. This finding suggests that SOA chemistry in central California behaves quite differently than in other regions.

seasalt and SOA as a function of temperature and relative humidity based on CARES
Sea Salt and SOA Phase of mixtures of sea salt and SOA as function of temperature and relative humidity based on CARES and laboratory data (from Wang et al., 2014). zoomEnlarge Image.

Laskin et al. (2012) and Wang et al. (2014) revealed a surprising existence of internally mixed sea salt and organic particles in the CARES region—150 kilometers (93 miles) from the ocean. They showed that chloride in sea salt reacts with organic acids that subsequently release volatile hydrogen chloride gas into the atmosphere. This reaction leaves behind particles stripped of chloride yet rich in organic salts. The lingering salty particles change the acidity, water attractiveness, and sunlight channeling properties of the aged particle population that travels from the Pacific Ocean to inland California. The researchers note that the recycling of hydrogen chloride and nitric acids from these particles may have important implications for atmospheric chemistry in all coastal environments.

Aerosols: Sun Lens and Cloud Creators

As published in Science, Cappa et al. (2012) used the CARES data to show that increased radiative absorption by these particles is small and increases weakly as particles photochemically age. The weakness suggests that climate models may overestimate the warming effect of soot in certain cases. However, the fact that CARES was conducted in a dry environment with low relative humidity affects the generality of these findings.

Chemical Ageing of Aerosols
Measured absorption enhancement (y-axis) as a function of chemical aging (x-axis), showing that absorption in ambient aerosols increases with age in contrast to model calculations (from Cappa et al., 2012).

In fact, a subsequent study conducted by Liu et al. (2015) and published in Nature Communications, contrasts the CARES measurements with those obtained from the 2012 Clean Air for London (ClearfLo) campaign to show that aerosol coatings influence black carbon absorption and the form and structural details of the mixing state may be specific to the source and region where the mixing occurs. They conclude that additional measurements in a range of environments are needed.

Kassianov et al. (2012) revealed the value of accounting for coarse particles, typically neglected with routine measurements. They showed how neglecting these coarse particles leads to overestimates of aerosols' direct radiative forcing power, by as much as 45%, despite their low aerosol optical depth. During CARES, coarse particles contributed more than 50% of the total aerosol volume up to 85% of the time. The finding advocates for evaluating climate models' ability to simulate the entire size distribution and not just the fine mode fraction when assessing uncertainties in aerosol radiative forcing.

Mei et al. (2013) found that due to the high fraction of organics, the ability of active particles that act as cloud seeds (condensation nuclei, or CCN) to attract water vapor (hygroscopicity) varied at a rate that was surprisingly much lower than assumed for continental regions. They showed that the low organic hygroscopicities during the campaign had a strong impact on calculated CCN concentrations. Over 90% of the particles sized between 100 and 171 nm were CCN active, suggesting most of them were aged.

Atkinson et al. (2015) came to similar conclusions, except that they also found relatively large hygroscopic values for supermicron particles. This finding suggests the presence of sea salt, specifically chloride displacement by nitrate and SOA. This was also illustrated by Laskin et al. (2012).

Providing Insights in the Future

Six years after the campaign, data collected by CARES continues to inspire researchers. While the data has filled in knowledge gaps, there is still much to learn from analyzing the extensive set of measurements. Still to tackle are determining the exact chemical processes that increase SOA from the bio-based and human-caused emissions.

Researchers also need to take advantage of information gathered by instruments, such as the single-particle mass spectrometer, to understand the best way to add complexity in model representations of aerosol mixing. And finally, researchers can tease out more from the data to understand how particles grow from nano to larger sizes, and how these affect the climate. Many of the new insights into SOA, aerosol mixing state, and aerosol optical properties have yet to be included in climate models.

Publications Cited List

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The Life and Times of SOAs

The CARES field campaign was designed to increase scientific knowledge about the evolution of black carbon, primary organic aerosols (POA), and secondary organic aerosols (SOA) from both human-caused and natural (biogenic) sources. Black carbon and primary organic aerosols are emitted directly into the atmosphere through vehicle exhaust, meat cooking, and biomass burning. Secondary organic aerosols are formed through complex physical and chemical interactions between pre-existing aerosols in the atmosphere and trace organic gases emitted from both human-caused and natural sources.

Download PDF list of all cited CARES papers.