Detecting supramolecular organic nanoparticles during heat wave.

New particle formation (NPF) represents a major source of tropospheric fine aerosols. A common viewpoint is that NPF hinges thermodynamically on the volatility of condensing species and is unfavorable at high temperatures. From an intensive field campaign, we observed frequent NPF events during a heat wave. Size-resolved chemical composition of nanoparticles down to 3 nanometers was first measured, unraveling a dominant presence of carboxylic acids. Our work uncovers a spontaneous mechanism to produce supramolecular nanoparticles through self-assembly of organic acids. This discovery explains not only the unexpected NPF at high temperatures but also its ubiquitous occurrence under diverse atmospheric conditions. As global warming leads to more frequent and intense heat waves, our findings open avenues for assessing the impacts of aerosols on cloud formation, public health, and climate. Editor's summary: Atmospheric new particle formation is commonly considered to be hindered by high temperatures, an effect stemming from the volatility of the species that condense to form the particles. Zhang et al. report observations of frequent new particle formation events during heat waves. Their measurements reveal a kinetic pathway for supramolecular nanoparticle production by organic acids, an important consideration for assessing the potential impacts of new particle formation on public health and climate in a warming world in which heat waves are becoming more frequent and intense. —Jesse Smith INTRODUCTION: Understanding how aerosols are formed is one of the foremost frontiers in atmospheric research, with profound implications for human and ecosystem health, weather, and climate. As the major contributor to fine aerosols, new particle formation produces >50% of cloud condensation nuclei in the troposphere. Aerosol nucleation is typically described by a two-step process involving a free energy barrier (e.g., nonspontaneous) and a curvature barrier (e.g., volatility-limited). Various chemical species—such as sulfuric acid, ammonia and amines, and oxidized organics—have been proposed to explain its occurrence. Climate change is expected to alter emissions and atmospheric chemistry, thereby affecting the frequency and nature of aerosol formation. RATIONALE: Available experimental, field, and theoretical studies often yield conflicting results, particularly concerning the chemical species responsible for the initial stages of aerosol formation. Owing to the lack of analytical instruments, direct measurements of the chemical composition of sub-10-nm particles remain scarce. To remedy these gaps, we conducted field observations and quantum chemical calculations to elucidate atmospheric chemistry under heat wave conditions. The size-resolved chemical composition of nanoparticles as small as 3 nm was measured, providing mechanistic insights into their formation and growth. RESULTS: We observed frequent new particle formation at high temperatures approaching 40°C. Multifunctional carboxylic acids (e.g., diacids and triacids) were identified as the dominant constituents of nucleation-mode particles (3 to 25 nm), whereas the mass fraction of sulfuric acid increased with size. Amines were detected only in larger particles (>20 nm). We also found that the mass fraction of nonacidic organics, including oligomeric products formed from heterogeneous reactions, increased with size. Nucleation-mode particles exhibited distinct physicochemical properties, notably, low hygroscopicity and density characteristic of their distinctive nanostructures. Detection of gaseous organic acids confirmed their critical role in new particle formation and their origins from photooxidation of anthropogenic and biogenic emissions, such as traffic-related aromatics and biogenic-related isoprene and pinenes. Theoretical calculations showed that diacids and triacids, which have multiple branches for growth and stabilization, readily engage in double-hydrogen-bond formation to yield supramolecular nanoparticles—a process that is jointly augmented by dipole-dipole interaction and electrostatic attraction. CONCLUSION: An unexpected finding from our field observations is the high temperatures at which frequent new particle formation takes place. Such a result cannot be explained by existing theories, especially those involving volatility-based nucleation. In contrast, our work uncovers a spontaneous mechanism in which supramolecular nanoparticles are produced from self-assembly of carboxylic acids through hydrogen-bond interactions. Revelation of the chemical identity of nanoparticles down to 3 nm provides the definitive evidence that multifunctional organic acids contribute dominantly to the formation of nucleation-mode particles. Given that organic acids are ubiquitously produced from photooxidation of anthropogenic and biogenic volatile organic compounds, their molecular self-assembly—in conjunction with sulfuric acid—represents a widespread pathway for new particle formation. Our study not only opens new avenues for assessing the impacts of aerosols on public health, cloud formation, and climate but also has broader implications for many biological, chemical, engineering, medical, and physical disciplines. Supramolecular nanoparticles in air.: Volatile organic compounds are emitted from anthropogenic (e.g., aromatics from traffic) and biogenic (e.g., isoprene from oak trees and pinenes from pine trees) sources. Under intense solar radiation, their oxidation by hydroxyl radicals produces large quantities of organic acids, which undergo self-assembly to form supramolecular nanoparticles. These nanoparticles are prevalent during heat waves, a phenomenon likely to become more frequent in a warming climate. [ABSTRACT FROM AUTHOR]