Tethered Balloons Over Houston Reveal How Air Mass Origins Shape Cloud-Forming Particles

High above the Gulf Coast plains southwest of Houston, a helium-filled aerostat rose and fell through the summer sky more than a hundred times in 2022, carrying instruments that would help settle one of atmospheric science’s most stubborn problems: what actually controls the tiny particles that seed clouds. A new analysis of 149 tethered balloon flights, conducted as part of the U.S. Department of Energy’s TRACER campaign, has now assembled one of the most detailed vertical pictures yet of aerosols and cloud condensation nuclei over a coastal urban environment. The findings, published in Atmospheric Chemistry and Physics, show that the story of pollution and clouds over Houston cannot be told from the ground alone, and that the same weather systems that deliver moisture also deliver the particles that clouds form on.

The TRACER campaign, short for Tracking Aerosol Convection interactions ExpeRiment, occupied the Houston region from 2021 through 2022, with an intensive observing period spanning June through September 2022. Its goal was ambitious: to quantify how aerosol loading and composition modulate the initiation, microphysics, and precipitation of convective clouds in an environment where sea breezes, industrial emissions, and Gulf moisture collide daily. The Department of Energy’s Atmospheric Radiation Measurement user facility deployed its Mobile Facility in La Porte, Texas, and, for the first two weeks of each month, a tethered balloon system at an ancillary site in Guy, Texas, a rural location chosen to capture background air before it mixed with the metropolitan plume.

The balloon itself was a workhorse of precision. Winch-controlled aerostats lifted payloads to roughly 1.5 kilometers, operating below cloud base, carrying sensors for pressure, temperature, humidity, and wind, along with a portable optical particle spectrometer and a condensation particle counter to measure aerosol size distributions and concentrations. Sampling lines were dried to keep relative humidity below 40 percent, and the spectrometer’s size bins were calibrated with ammonium sulfate particles rather than plastic spheres, reducing sizing bias. A size- and time-resolved aerosol collector gathered particles at altitude for later electron microscopy and mass spectrometry at the Environmental Molecular Sciences Laboratory, allowing researchers to identify individual particle types and molecular compositions at different heights.

To make sense of the flights, the team classified the history of every air mass using back-trajectory analysis with the HYSPLIT model driven by high-resolution ERA5 reanalysis data, then applied k-means clustering to sort the trajectories into three distinct groups. The first cluster traced direct marine inflow, with air traveling almost exclusively over the Gulf of Mexico. The second followed more complex paths that crossed the Gulf before curving over land, mixing marine and continental influences. The third originated from the north and west, carrying continental and anthropogenic emissions from the broader Houston metropolitan region and, over five-day histories, from as far away as the central and upper Midwest of the United States and Canada.

Principal component analysis of the meteorological variables within each cluster revealed the physical machinery behind each regime. In the marine cluster, the leading component indexed the strength of the cool, moist, shallow marine layer, while the second captured the diurnal cycle of surface heating. In the anthropogenic cluster, the dominant component behaved like a ventilation index, quantifying whether a deep, well-mixed boundary layer would dilute surface emissions or a shallow, stable layer would trap pollutants near the ground. The mixed cluster was dominated by moisture and energy advection, with a secondary mode reflecting entrainment of drier free-tropospheric air as the boundary layer grew.

The aerosol profiles themselves told sharply different stories depending on air mass origin. Marine-influenced flights showed the lowest concentrations, with cloud condensation nuclei concentrations at 0.8 percent supersaturation staying below 1000 per cubic centimeter in a uniformly mixed layer, consistent with clean oceanic air where sea spray and marine biogenic particles dominate. The urban and anthropogenic cluster, by contrast, reached cloud condensation nuclei concentrations near 3000 per cubic centimeter within the boundary layer, with an accumulation-mode population four to five times larger than the marine case even though mean particle diameters differed by only about 30 percent, a signature of aged, photochemically processed pollution rather than fresh emissions.

The mixed cluster produced the most surprising structure. Near the surface its concentrations were intermediate, but above one kilometer the particle counts rose markedly, with cloud condensation nuclei values exceeding 5000 per cubic centimeter near the boundary layer top. The researchers interpret this elevated layer as evidence of advected, long-range-transported pollution residing near or just above the boundary layer, decoupled from local surface emissions, and possibly of in situ particle growth driven by aqueous-phase chemistry and photochemistry in the humid, sunlit coastal air trapped beneath a temperature inversion. Chemical analysis of samples from anthropogenic-influenced days reinforced the picture of heavy processing: a single representative sample yielded 1020 assigned molecular formulas, including hundreds of organonitrates and organosulfates, hallmarks of extensive atmospheric oxidation.

Converting measured size distributions into cloud condensation nuclei profiles required careful technique. The team extrapolated the spectrometer’s 135 to 3000 nanometer range down to 10 nanometers using constrained lognormal mode fitting, checked the fit against independent total counts from the condensation particle counter, and then applied κ-Köhler theory with ground-based chemical composition from an aerosol chemical speciation monitor to estimate the critical activation diameter at each supersaturation. The authors are candid about the limits of this approach: composition was assumed vertically uniform within a well-mixed boundary layer, an assumption that weakens for decoupled profiles, and refractory species such as black carbon, sea salt, and dust were invisible to the ground instrument, adding uncertainty to the inferred hygroscopicity.

The most striking lesson came from a two-day case study in early September 2022. On 6 September, the balloon sampled two vertically stacked but historically distinct air masses: a recirculating coastal layer near the surface rich in carbonaceous and sodium-bearing particles, and above roughly 1000 meters a cleaner, subsiding marine layer containing long-range-transported dust, confirmed by electron microscopy showing the dust fraction jumping from 9 percent at the ground to 30 percent aloft. Yet despite large convective available potential energy, no clouds formed that day, because a deep layer of extremely dry mid-level air would have eroded any nascent cloud through entrainment. By 7 September, mesoscale advection had transformed everything at once: cloud condensation nuclei concentrations jumped five- to ten-fold above 3000 per cubic centimeter, the lower troposphere moistened dramatically, and colliding cold pools and a bay-breeze front provided the lift that triggered deep convection.

That juxtaposition is precisely why the authors warn against the seductive clean-day-versus-polluted-day comparison. The same mesoscale circulation that imported moisture, removed convective inhibition, and primed the atmosphere for storms also delivered the high-CCN aerosol population, meaning the aerosol and thermodynamic changes were confounded at their source. Thermodynamics and mesoscale dynamics acted as first-order controls on convection, while aerosols modulated droplet number and precipitation efficiency only after storms were assured. For forecasters and climate modelers alike, the message from 149 balloon flights over Houston is clear: in coastal urban environments, boundary-layer circulations, air mass origins, and aerosol populations evolve together, and attributing cloud changes to aerosol forcing requires constraining the meteorology first, treating the circulation-thermodynamics-aerosol system as the integrated whole it truly is.

Subject of Research: Vertical profiles of aerosols and cloud condensation nuclei measured by tethered balloon during the TRACER campaign over Houston

Article Title: Synthesis of the tethered balloon system and other TRACER campaign measurements elucidates aerosol property profiles

Article References: Mei, F., Wang, J., Silber, I., Lata, N. N., Vandergrift, G. W., Li, J., Chen, B., Brooks, S. D., Jensen, M. P., Deng, M., Zhang, D., Dexheimer, D., Schmid, B., Cheng, Z., & China, S. (2026). Synthesis of the tethered balloon system and other TRACER campaign measurements elucidates aerosol property profiles. Atmospheric Chemistry and Physics, 26(19), 13885-13908. https://doi.org/10.5194/acp-26-13885-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13885-2026

Keywords: aerosols, cloud condensation nuclei, TRACER campaign, tethered balloon system, boundary layer, Houston, air mass trajectories, aerosol-cloud interactions, convection, sea breeze, atmospheric chemistry, DOE ARM