Surface adsorbed organics affect the heterogeneous and photochemical reactivity of atmospheric aerosols, thereby impacting tropospheric photochemical cycles and altering aerosol properties. Due to the diverse composition of surface adsorbed organics in the troposphere, our understanding of this chemistry is far from complete. This project describes condensed-phase infrared spectroscopic studies aimed at identifying condensed-phase products, quantifying reaction kinetics, and elucidating reaction mechanisms by means of isotopic experiments. Specific systems of interest include: lignin pyrolysis products, nitrophenols, and functionalized polycyclic aromatic compounds, including humic and fulvic acids, exposed to nitrogen dioxide or ozone under dark and light conditions; and a comparison of the heterogeneous reactivity of these organics adsorbed on tropospheric aerosol surrogates.
Broader impacts include (1) Supporting the first active research program in physical/atmospheric chemistry at Drew University, a small liberal arts college. (2) Providing nine undergraduate summer research positions over the three-year duration of the project. Under the guidance of the principal investigator, undergraduate students at Drew will play an integral role in all aspects of the project, including experimental design, data acquisition and analysis, and communication of results to the scientific community as journal articles with undergraduate co-authors and as presentations at professional meetings. (3) Providing one local high school teacher a research opportunity for three summers. Preference will be given to a teacher from a district with a high percentage of students from groups underrepresented in science. Atmospheric science curricular material will be co-developed and co-implemented by the teacher and principal investigator. (4) Supporting curricular expansions in environmental and atmospheric chemistry by integrating research and teaching to enhance the Drew University chemistry major and by supporting a new major in environmental studies and sustainability. Support will contribute to the momentum of this new major to attract and support undergraduate students interested in environmental sciences.
Forest fires and other forms of biomass combustion emit an estimated 51 Tg of volatile and semi-volatile organics into earth’s atmosphere annually, where they impact atmospheric chemistry, earth’s radiative balance (i.e., climate), and human health. Many critical questions regarding the environmental and health impacts of these emissions remain unanswered because of their diverse and complex chemical compositions, which often vary with transport through the atmosphere via reactions with trace pollutant gases such as ozone and nitrogen dioxide. One major class of organic compounds emitted by biomass combustion is lignin pyrolysis products, which include an array of compounds such as phenols, methoxyphenols (guaiacols), and nitrophenols. Figure 1 shows ten prevalent lignin pyrolysis products which were the subject of our specific experiments. Figure 2 depicts major forms of airborne particulates such as salt particles from sea spray and mineral particles from dust storms. We found that these volatile and semi-volatile organics emitted by forest fires readily condensed from the gas phase onto the surfaces of common atmospheric particulates. This gas-to-surface process, known as adsorption, resulted in an approximate monolayer coating of the organic on salt and mineral particulates. Since airborne particulates often serve as nucleation sites that facilitate the condensation of water vapor forming cloud droplets and ice crystals, we investigated whether these organic coatings affect this process of water uptake. The presence of catechol organic coatings inhibited the adsorption/condensation of water on mineral particles, which implies that if present in the atmosphere, these organics will diminish the ability of these particles to form ice crystals. No such effect was observed when catechol formed a coating on sodium chloride, a dominant salt particle substrate. Thus, organic coatings alter the water uptake of certain airborne particulates but not all; the chemical composition of the underlying particle plays an important role in understanding the impact of these organics. Once these organics adsorb onto salt and mineral aerosols, they will readily react with common pollutants such as ozone (O3) and nitrogen dioxide (NO2), the two main components of photochemical smog. Ozone is well known to cleave carbon-carbon double bonds (C=C), and consistent with this textbook reaction we observed the rapid reactivity of ozone with eugenol (see Figure 1). Assuming an ozone concentration representative of moderately polluted urban areas, this reaction will consume half of the adsorbed molecules, known as the atmospheric lifetime, in 3-7 hours for eugenol adsorbed on mineral and salt particulates. Ozone can also cleave the aromatic ring of these organic coatings by attacking across adjacent –OH and/or –OCH3 groups. Measured atmospheric lifetimes ranged from 5-18 hours for catechol to 1-2 days for guaiacols; solar radiation increased the rate of guaiacol ring cleavage by a factor of 4. Since the average lifetime for particulates suspended in the atmosphere is on the order of a week, this chemistry will alter the organic coatings relatively early in their lifetime. Although similar chemistry can happen to these volatile organics when in the gaseous state, this chemistry is much faster when adsorbed on salt and mineral particle surfaces. We also measured the impact of nitrophenolic compounds on the amount of solar radiation absorbed by salt and mineral particulates. Initial experiments demonstrated that such nitrophenols can be produced by reactions with NO2. For instance, catechol + NO2 produced nitrocatechol, although the relative amount of this product varied with the strength of basic surface sites for salt and mineral particulates. Mineral dust aerosols coated with nitrophenols will absorb more solar radiation due to acid-base chemistry between the weakly acidic organics and basic surface sites on mineral surfaces, which formed nitrophenolates (i.e., the conjugate base of nitrophenols) that absorb more visible radiation than nitrophenols. Such solar absorption properties indicate that the microenvironment of the airborne particulate (for example, mineral vs. salt, pH of aqueous droplets) alter the extent of absorption vs. scattering which controls the amount of heating vs. cooling atmospheric aerosols contribute to factors controlling earth’s climates. This research demonstrates that volatile organics emitted by forest fires and other forms of biomass combustion will alter the climate-relevant properties of mineral particulates they encounter, especially when transported through polluted urban environments. For salt particulates, these organics do not alter the water uptake properties and exert a smaller effect on the amount of solar radiation absorbed by sea salt aerosol. A major theme identified by these cumulative results is that the chemical composition of the particulate’s surface controls the chemistry of these adsorbed organic coatings, and these organic coatings in turn alter the chemical and physical properties of salt and mineral particulates suspended in earth’s lower atmosphere.
