About

Christopher D. Cappa is a professor with a Ph.D. from UC Berkeley and a B.S. from Hope College. He leads a research group focused on problems at the intersection of air pollution and climate change. His lab includes graduate students from diverse programs such as Civil and Environmental Engineering, Atmospheric Sciences, and Agricultural and Environmental Chemistry at UC Davis. Professor Cappa actively mentors both graduate and undergraduate students, encouraging those interested in environmental and atmospheric sciences to engage with his research. His group has produced numerous alumni who have gone on to careers in academia, government agencies, and industry, reflecting the applied nature and impact of his research. Prospective students interested in working with him are invited to contact him directly to explore opportunities in his lab.

Research topics

• Chemistry
• Environmental science
• Meteorology
• Physics
• Geology
• Medicine
• Environmental chemistry
• Organic chemistry
• Geography
• Computer Science
• Materials science
• Nuclear engineering
• Psychology
• Atmospheric sciences
• Audiology
• Optics
• Acoustics
• Mechanics
• Anesthesia
• Composite material
• Climatology
• Oceanography
• Virology
• Biomedical engineering

Selected publications

• Characterization of carbonaceous aerosols during TRACER-CAT

  2026-01-13

  reportOpen access1st authorCorresponding

  Absorbing aerosols (AA) have an important impact on the global radiation budget and cloud properties. The composition and properties of AA can vary substantially throughout the atmosphere, depending on the particle source and the influence of chemical aging. Uncertainties associated with the radiative effects of AA remain substantial. A key contributor to this uncertainty is understanding the extent to which coatings in general, and water uptake especially, alters absorption by AA particles and how this depends on particle composition. We deployed new and existing experimental tools during the Tracking Aerosol Convection Interactions Experiment (TRACER) campaign in Houston, TX as part of the Carbonaceous Aerosols Thrust (CAT) to provide detailed characterization of aerosol optical, chemical, and physical properties. Our TRACER-CAT measurements complemented and expanded on the planned TRACER instrumentation, allowing for more detailed characterization of aerosol properties of relevance to cloud development (a core focus of TRACER), such as the composition of particles that can act as cloud condensation nuclei, than would otherwise be available. Our measurements have allowed for assessment of the relationship(s) between AA optical properties (with a focus on absorption) and the chemical and physical characteristics (including the mixing state of black carbon (BC) containing particles). These field observations occurred in collaboration with Los Alamos National Laboratory in summer 2022 during the TRACER intensive operating period. The instrumentation we co-deployed provided for measurement of (i) multi-wavelength dry aerosol absorption, scattering, and extinction, (ii) the size-dependent composition and abundance of sub-micron aerosol, differentiating between those particles that do and do not contain BC, (iii) BC-specific concentrations and size distributions, (iv) particle size, and (v) the first field measurements at an ARM site of the influence of RH on multi-wavelength absorption by ambient AA. We have leveraged the natural variability of the atmosphere and of aerosol sources in the Houston region to (i) specifically disentangle contributions to light absorption from BC, absorbing organic carbon (brown carbon), and coatings on BC, (ii) characterize the mixing state of BC and assess the factors that give rise to compositional differences between BC-containing and BC-free aerosol, and (iii) establish how water uptake influences absorption and how any such effect depends on particle composition and BC mixing state. Overall, our study contributed to the mission of the Atmospheric System Research program in multiple ways. Through the deployment of complementary, advanced instrumentation for characterization of a wide range of aerosol properties our work helped to maximize the scientific impact of the TRACER campaign. Our work also allowed for development of new insights into the relationship(s) between aerosol composition, hygroscopicity, and the mixing state of BC with aerosol optical properties. Through this, our work has provided knowledge that can improve understanding and model representation of aerosol processes as they affect the Earth’s radiation budget.

  PublisherOA PDFDOI

• Chamber-based insights into the factors controlling epoxydiol (IEPOX) secondary organic aerosol (SOA) yield, composition, and volatility

  UNC Libraries · 2026-02-11

  articleOpen access

  We present measurements utilizing the Filter Inlet for Gases and Aerosols (FIGAERO) applied to chamber measurements of isoprene-derived epoxydiol (IEPOX) reactive uptake to aqueous acidic particles and associated secondary organic aerosol (SOA) formation. Similar to recent field observations with the same instrument, we detect two molecular components desorbing from the IEPOX SOA in high abundance: C5H12O4 and C5H10O3. The thermal desorption signal of the former, presumably 2-methyltetrols, exhibits two distinct maxima, suggesting it arises from at least two different SOA components with significantly different effective volatilities. Isothermal evaporation experiments illustrate that the most abundant component giving rise to C5H12O4 is semi-volatile, undergoing nearly complete evaporation within 1 h while the second, less volatile component remains unperturbed and even increases in abundance. We thus confirm, using controlled laboratory studies, recent analyses of ambient SOA measurements showing that IEPOX SOA is of very low volatility and commonly measured IEPOX SOA tracers such as C5H12O4 and C5H10O3, presumably 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diols, result predominantly from thermal decomposition in the FIGAERO-CIMS. We infer that other measurement techniques using thermal desorption or prolonged heating for analysis of SOA components may also lead to reported 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diol structures. We further show that IEPOX SOA volatility continues to evolve via acidity-enhanced accretion chemistry on the timescale of hours, potentially involving both 2-methyltetrols and organosulfates.

  PublisherDOI

• Characterizing the impact of water uptake on light absorption by aerosol particles

  2026-01-14

  reportOpen access1st authorCorresponding

  Absorbing aerosols (AA) have an important impact on the global radiation budget. The composition and properties of AA can vary substantially throughout the atmosphere, depending on the particle source and the influence of chemical aging. Uncertainties associated with the radiative effects of AA remain substantial. A key contributor to this uncertainty is understanding the extent to which water uptake alters absorption by AA particles and how this depends on particle composition. We have used new and existing experimental tools to systematically characterize the relationship(s) between AA chemical and physical characteristics, relative humidity, and mixing of AA with other aerosol components through a combination of laboratory and field measurements. We have developed fundamental understanding through laboratory experiments that will enable interpretation of field observations at DOE ARM sites and facilitate process-based improvements in the simulation of absorption by carbonaceous aerosols in climate models. In the laboratory experiments we have considered a variety of different AA types, including fullerene soot (a surrogate for refractory black carbon, or BC) and nigrosine (a surrogate for partially soluble brown carbon, or BrC). By examining a range of AA types we will develop insight into how the chemical and physical properties of AA particles (e.g. whether they are refractory or soluble) impact the influence of water uptake on absorption. These AA particles will also be mixed—both internally and externally—with compounds having a wide range of hygroscopic properties to establish how the impact of water uptake on absorption will vary with photochemical processing. These laboratory observations have been compared with commonly used theoretical models, enabling development of empirical models that will facilitate improved representation of absorption by AA particles in regional and global climate models. Overall, our study has contributed to the mission of the Atmospheric System Research program by quantifying how interactions between aerosols and radiation depend on relative humidity. Through this, our work will improve understanding and model representation of aerosol processes as they affect the Earth’s radiation budget.

  PublisherOA PDFDOI

• Changes in Light Absorption and Chemical Properties for Biomass Burning Organic Aerosol over Long Time Scales

  ACS ES&T Air · 2025-04-07 · 3 citations

  articleSenior author

  Photolysis driven fragmentation is known to decrease the light absorbing properties of brown carbon (BrC) molecules in organic aerosol particles, but chemical changes and the lifetimes for the more photorecalcitrant fraction are not well understood. In this study, we probe the photoaging behavior of biomass burning organic aerosol (BBOA) particles collected on filters during the two Fire Influence on Regional and Global Environments Experiment (FIREX) campaigns in 2016 and 2019. We evaluate changes in the chemical properties during direct photolysis on the filters and probe the photobleaching rates over extended time periods in dilute aqueous solutions. We find that most of the laboratory burns (FIREX 2016) increase the average carbon oxidation state of the BBOA over 1–3 days of photolysis. We also introduce a new characterization method called Solar Flux-Weighted Mass Absorption Cross-section (SFW-MAC) that describes the full absorption properties of the mixture. We find that estimates from our photolysis rates are in the same range as ambient lifetimes. Our results show that longer term aging studies of more than 4 days are needed to fully capture the bleaching rate of atmospheric BrC aerosols, especially for photorecalcitrant BrC that will play a larger role farther away from the original emission source.

  PublisherDOI

• Studying Aerosol, Clouds, and Air Quality in the Coastal Urban Environment of Southeastern Texas

  Bulletin of the American Meteorological Society · 2025-08-04 · 3 citations

  article

  Abstract A multi-agency succession of field campaigns was conducted in southeastern Texas during July 2021 through October 2022 to study the complex interactions of aerosols, clouds and air pollution in the coastal urban environment. As part of the Tracking Aerosol Convection interactions Experiment (TRACER), the TRACER- Air Quality (TAQ) campaign the Experiment of Sea Breeze Convection, Aerosols, Precipitation and Environment (ESCAPE) and the Convective Cloud Urban Boundary Layer Experiment (CUBE), a combination of ground-based supersites and mobile laboratories, shipborne measurements and aircraft-based instrumentation were deployed. These diverse platforms collected high-resolution data to characterize the aerosol microphysics and chemistry, cloud and precipitation micro- and macro-physical properties, environmental thermodynamics and air quality-relevant constituents that are being used in follow-on analysis and modeling activities. We present the overall deployment setups, a summary of the campaign conditions and a sampling of early research results related to: (a) aerosol precursors in the urban environment, (b) influences of local meteorology on air pollution, (c) detailed observations of the sea breeze circulation, (d) retrieved supersaturation in convective updrafts, (e) characterizing the convective updraft lifecycle, (f) variability in lightning characteristics of convective storms and (g) urban influences on surface energy fluxes. The work concludes with discussion of future research activities highlighted by the TRACER model-intercomparison project to explore the representation of aerosol-convective interactions in high-resolution simulations.

  PublisherDOI

• Kinetic Modeling of Secondary Organic Aerosol in a Weather-Chemistry Model: Parameterizations, Processes, and Predictions for GOAmazon

  ACS ES&T Air · 2025-01-24 · 2 citations

  articleOpen access

  Secondary organic aerosol (SOA) forms and evolves in the atmosphere through many pathways and processes, over diverse spatial and time scales. Hence, there is a need to represent these widely varying kinetic processes in large-scale atmospheric models to allow for accurate predictions of the abundance, properties, and impacts of SOA. In this work, we integrated a kinetic, process-level model (simpleSOM-MOSAIC) into a weather-chemistry model (WRF-Chem) to simulate the oxidation chemistry and microphysics of atmospheric SOA. simpleSOM-MOSAIC simulates multigenerational gas-phase chemistry, autoxidation reactions, aqueous chemistry, heterogeneous oxidation, oligomerization, and phase-state-influenced gas/particle partitioning of SOA. As a case study, the integrated WRF-Chem-simpleSOM-MOSAIC (WC-SSM) model was used to simulate the photochemical evolution downwind of a large city (Manaus, Brazil) in the Amazon and, in turn, study the anthropogenic and biogenic interactions in an otherwise pristine environment. Consistent with previous work, we found that OA was enhanced by up to a factor of 4 in the urban plume due to elevated hydroxyl radical (OH) concentrations, relative to the background, and that this OA was dominated by SOA from biogenic precursors (80%). In addition to accurately simulating the OA enhancement in the urban plume, the model reproduced the magnitude of the OA oxygen-to-carbon (O:C) ratio and broadly tracked the evolution of the aerosol number size distribution. Our work highlights the importance of including an integrated, kinetic representation of SOA processes in an atmospheric model.

  PublisherDOI

• Reconciling measurements of black carbon in a cold and dark urban environment: Insights from the ALPACA 2022 campaign in Fairbanks, Alaska

  2025-11-24

  articleOpen access

  Fairbanks, Alaska experiences severe winter pollution under cold, dark conditions, making it a unique location for studying black carbon and brown carbon optical properties. The 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) campaign deployed multiple instruments, including filter-based (AE33, MAAP) and in-situ (PAAS-4λ, PAX) instruments during January–February to characterize aerosol absorption. Black carbon contributed 2–5%, and organic aerosol 40–75% of total PM2.5 mass during campaign, with brown carbon representing only a small, weakly absorbing fraction of the organic component. Instrument inter-comparisons revealed significant discrepancies: the AE33 with default multiple scattering parameter (CAE33= 1.57), reported ~2.5 times higher absorption than the PAAS-4λ and PAX across 405–880 nm. A revised multiple scattering parameter (C*AE33= 3.64) improved agreement to within 10–20%. In contrast, MAAP (637 nm) which does not require multiple-scattering correction, exhibited ~1.9 times higher absorption than corrected AE33 (referred as AE33*) at 660 nm. Despite elevated organic concentrations, brown carbon contributed only ~20% to total absorption at 370 nm, suggesting a dominant role of black carbon to light absorption, with limited variability in absorption Ångström exponent (1.2–1.8). EPA thermal-optical elemental carbon concentrations were ~2 times higher than corrected AE33* black carbon during elevated pollution, highlighting systematic biases. These findings demonstrate the critical need for context-specific instrument calibrations and multi-method validation to accurately quantify black and brown carbon absorption in Arctic wintertime conditions.

  PublisherOA PDFDOI

• Airborne Hexavalent Chromium Nanoparticles Detected Around Cleanup Zones for the 2025 Los Angeles Wildfires

  Communications Earth & Environment · 2025-08-26

  preprintOpen access

  <title>Abstract</title> The 2025 Eaton and Palisades fires in Los Angeles exemplify destructive wildfires that increasingly threaten cities across the globe. The dangers during active burning are obvious, but the threats to public health during the post-burn cleanup phase are still being discovered. Here we report that airborne chrome (Cr) and silver (Ag) bearing nanoparticles (Dp&lt;56 nm) were found in the debris cleanup zones around the Eaton and Palisades wildfires. These nanoparticles may have originated from corrosion inhibitors added to aerial fire suppressants dropped over the region which reacted under high temperatures during the fire, but further studies are required to test this hypothesis. The airborne chrome was predominantly in the carcinogenic +6 oxidation state (hexavalent chromium). The hexavalent chromium concentrations averaged 13.7 ± 6.2 ng / m3, below the NIOSH workplace exposure limit of 200 ng / m3 but above the US EPA screening levels for indoor air (0.1 ng / m3 for cancer; 3 ng / m3 for non-cancer effects). Outdoor nanoparticles can infiltrate indoors, and simple transport calculations indicate that hexavalent chromium-containing nanoparticles could travel several km downwind from the cleanup zone. Although the health effects from inhaled hexavalent chromium nanoparticles are uncertain, caution is warranted given that nanoparticles can easily cross cell membranes and circulate throughout the body. Residents near the cleanup zones should be aware of the potential risk so that they can take steps to reduce exposure and healthcare providers should monitor for possible health effects in these regions.

  PublisherOA PDFDOI

• Wind-driven emission of marine ice-nucleating particles in the Scripps Ocean-Atmosphere Research Simulator (SOARS)

  Atmospheric chemistry and physics · 2025-03-14 · 7 citations

  articleOpen access

  Abstract. Sea spray aerosol (SSA) represents one of the most abundant natural aerosol types, contributing significantly to global aerosol mass and aerosol optical depth, as well as to both the magnitude of and the uncertainty in aerosol radiative forcing. In addition to its direct effects, SSA can also serve as ice-nucleating particles (INPs), which are required for the initiation of cloud glaciation at temperatures warmer than ca. −36 °C. This study presents initial results from the CHaracterizing Atmosphere-Ocean parameters in SOARS (CHAOS) mesocosm campaign, which was conducted in the new Scripps Ocean-Atmosphere Research Simulator (SOARS) wind–wave channel. SOARS allows for isolation of individual factors, such as wave height, wind speed, water temperature, or biological state, and can carefully vary them in a controlled manner. Here, we focus on the influence of wind speed on the emission of SSA and INPs. In agreement with recent Southern Ocean measurements, online INP concentrations during CHAOS showed an increasing relationship with wind speed, whereas offline CHAOS INP concentrations did not, which may be related to sampling inlet differences. Changes in the INP activated fraction, dominant INP particle morphology, and INP composition were seen to vary with wind. Seawater ice-nucleating entity concentrations during CHAOS were stable over time, indicating that changes in atmospheric INPs were driven by wind speed and wave-breaking mechanics rather than variations in seawater chemistry or biology. While specific emission mechanisms remain elusive, these observations may help explain some of the variability in INP concentration and composition that has been seen in ambient measurements.

  PublisherOA PDFDOI

• Optimization of Ventilation and Filtration System Operation in Classrooms to Minimize Airborne Infectious Disease Transmission, Particulate Matter Exposure, and Energy Consumption

  SSRN Electronic Journal · 2024-01-01

  preprintOpen access
  PublisherDOI

Recent grants

• Heterogeneous Hydroxyl (OH) Chemistry and Aerosol Optical Properties: Direct and Indirect Connections

  NSF · $280k · 2009–2013

• CAREER: Organic Aerosol Volatility, Phase and Partitioning

  NSF · $669k · 2012–2019

Frequent coauthors

• R. C. Cohen

  University of California, Berkeley

  66 shared

• D. A. Lack

  65 shared

• Richard J. Saykally

  62 shared

• Timothy H. Bertram

  University of Wisconsin–Madison

  57 shared

• T. B. Onasch

  Aerodyne Research

  53 shared

• Kanako Sekimoto

  Yokohama City University

  52 shared

• J. L. Jiménez

  51 shared

• Kimberly A. Prather

  Scripps Institution of Oceanography

  50 shared

Labs

• Cappa, Christopher CappaPI

Education

• Ph.D., Civil Engineering

  University of California, Davis

  2005

• M.S., Civil Engineering

  University of California, Davis

  2001

• B.S., Civil Engineering

  University of California, Davis

  1999