Optical properties of humic-like substances (HULIS) in biomass-burning aerosols

Smoke from Amazon fires turns the sky brown. For years, atmospheric scientists assumed they understood what that brown meant — black carbon, the sooty residue of combustion, absorbing sunlight and warming the air. But there is another class of material in that smoke, chemically closer to the dark humic acids in forest soil than to soot. Until a team led by András Hoffer measured it directly, nobody knew whether it absorbed light in any meaningful quantity. That material has a name: humic-like substances, or HULIS. It turns out to matter far more than anyone expected, but not in the part of the spectrum you might guess. HULIS are high-molecular-weight, largely water-soluble organic compounds that form during biomass burning and accumulate in the fine fraction of smoke aerosol. They belong to the broader category atmospheric chemists call brown carbon — light-absorbing organic material that is chemically and optically distinct from black carbon. The key difference is abundance versus strength. Black carbon absorbs light intensely but makes up a smaller fraction of the particulate mass. HULIS absorbs more weakly per gram, but is far more abundant. Whether that abundance could compensate for the weaker absorption — and make HULIS a meaningful player in atmospheric radiation — was the open question. The basic optical parameters needed to answer it, including the index of refraction, had never been measured. The September 2002 Large-Scale Biosphere-Atmosphere Experiment in Amazonia — SMOke aerosols, Clouds, rainfall and Climate, or LBA-SMOCC — gave Hoffer and colleagues the material they needed. Intense, widespread biomass burning during that period loaded the Amazonian atmosphere with fine-fraction carbonaceous aerosol, maximizing the presence of HULIS and making it possible to isolate enough material for rigorous optical characterization. The team collected aerosol at a tropical pasture site in Rondônia, Brazil, using a dichotomous virtual impactor that separated fine particles smaller than 2.5 micrometers from coarser material. Filters ran for roughly ten hours during the day and twelve hours at night. From twenty-six fine-fraction samples, the team extracted HULIS using a one-step solid-phase isolation procedure — dissolving the filter material in ultrapure water, acidifying the extract to a pH of 2, and separating the HULIS fraction on a polymer sorbent column. Combined daytime extracts yielded 10.8 milligrams of HULIS; nighttime extracts yielded 20.3 milligrams. To measure optical properties, the dried HULIS was redissolved and atomized into laboratory aerosol particles. These were dried to a relative humidity of about 26 percent and passed through sizing and optical instruments. Scanning electron microscope images confirmed the particles were spherical — an important detail, because the core analytical method, Mie theory closure, assumes spherical geometry. The closure approach works like this: you measure the particle size distribution and the scattering and absorption coefficients independently, then you ask what complex index of refraction a Mie calculation would need to reproduce those measurements simultaneously. You step through candidate index values until the calculated and measured optical coefficients agree to within half a percent. The real part of the index describes how much the material bends and scatters light; the imaginary part describes how much it absorbs. The central result from those closure calculations: at 532 nanometers — green light — the average complex index of refraction for HULIS was 1.65 minus 0.0019i for daytime samples and 1.69 minus 0.0016i for nighttime samples. The propagated measurement uncertainties were about 3.7 percent for the real part and 12.5 percent for the imaginary part. The real parts, between 1.65 and 1.69, are substantially higher than water's 1.33, meaning HULIS bends light significantly and scatters it. But the imaginary parts are tiny. The corresponding mass absorption coefficients were just 0.031 and 0.029 square meters per gram for day and night respectively. The single-scattering albedo — the fraction of light interaction that goes to scattering rather than absorption — was about 0.98. HULIS, at green wavelengths, barely absorbs at all. The small day-to-night difference in the index is itself informative. The composition and optical behavior of HULIS were essentially stable across the diurnal cycle in these samples, suggesting that whatever processes produced HULIS during the day and night resulted in chemically similar material. So far, the story seems to close quickly: HULIS scatters well but absorbs almost nothing at visible wavelengths. Not a radiatively important absorber. Case closed — except it isn't. The key is the absorption Ångström exponent. This is the number that describes how steeply absorption changes with wavelength. The larger the exponent, the more sharply absorption rises as you move toward shorter, bluer, ultraviolet wavelengths. For black carbon and diesel soot, that exponent is close to 1, meaning absorption is nearly flat across the spectrum. For HULIS, Hoffer and colleagues measured absorption Ångström exponents of 6.4 for the daytime samples and 6.8 for the nighttime samples. They used 6 and 7 as lower and upper bounds in their calculations. That is a steep climb. Starting from a mass absorption coefficient of roughly 0.03 square meters per gram at 532 nanometers and projecting that spectral slope toward shorter wavelengths, HULIS reaches roughly 2 to 3 square meters per gram at 300 nanometers — two orders of magnitude larger. At 300 nanometers, in the near-ultraviolet, HULIS goes from an afterthought to a dominant absorber. Under the biomass-burning conditions measured during LBA-SMOCC — using an average fine total carbon concentration of 42.7 micrograms per cubic meter, with 10 percent as black carbon and 35 percent of total carbon as HULIS carbon — Hoffer and colleagues calculate that HULIS accounts for 35 to 50 percent of aerosol light absorption at 300 nanometers. That is not a rounding error. That is half the absorption budget at a wavelength where models had nothing. Integrated across wavelengths from 300 to 1200 nanometers, HULIS contributes only 6.4 to 8.6 percent of total aerosol absorption — modest in terms of total solar energy. But that integrated number is dominated by the visible and near-infrared, where HULIS is weak. In the ultraviolet, where photochemistry operates, the picture is entirely different. The 300-nanometer region matters because it drives tropospheric photochemistry. Ultraviolet photons at these wavelengths produce hydroxyl radicals, the primary oxidant that breaks down pollutants and greenhouse gases in the lower atmosphere. Hoffer and colleagues note that light-absorbing organic compounds like HULIS can initiate the formation of hydroxyl and hydroperoxy radicals directly. If HULIS is absorbing up to half the ultraviolet light reaching the aerosol at 300 nanometers, then it is not just warming the air — it is reshaping the radical budget and altering how quickly the atmosphere chemically processes what's in it. The practical consequence is that climate and chemistry models built around black carbon as the dominant absorber will underestimate ultraviolet attenuation in smoke-filled air. They will calculate too much ultraviolet light reaching the lower troposphere, and they will misestimate photochemical reaction rates. Hoffer and colleagues suggest HULIS could account for at least part of what they call "the up to now unexplained fraction of light absorption observed in the troposphere." That is a careful claim, but it is grounded in measurements: the mass absorption coefficients, the refractive indices, the Ångström exponents — these are not modeled quantities; they are directly measured from isolated Amazon aerosol. As biomass burning expands globally, the HULIS fraction of pyrogenic aerosol will matter more. The brown haze above the Amazon in September 2002 was doing something to ultraviolet light that, until Hoffer and colleagues took it apart, isolated it, aerosolized it, and ran the numbers, had been entirely invisible to atmospheric science. A weak absorber in the green, yes. But in the ultraviolet — where photochemistry runs the atmosphere's chemistry — HULIS was hiding in plain sight, responsible for as much light absorption as everything else combined. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.