Uptake of Gas-Phase Species by 1-Octanol. 1. Uptake of α-Pinene, γ-Terpinene, p-Cymene, and 2-Methyl-2-hexanol as a Function of Relative Humidity and Temperature

Foliar limonene uptake scales positively with leaf lipid content: "non-emitting" species absorb and release monoterpenes

Monoterpenes synthesized and released by emitting vegetation can be taken up by neighboring non-emitting plants, but the uptake capacity of non-emitting species has not been studied extensively. We investigated the foliar uptake potential of the hydrophobic monoterpene limonene in 13 species of contrasting leaf structure and lipid content to determine the structural and chemical controls of monoterpene uptake. Leaf dry mass per unit area (M(A,D)) varied 6.5-fold, dry to fresh mass ratio (D(F)) 2.7-fold, lipid content per dry mass (L(M)) 2.5-fold and per unit area (L(A)) 4.6-fold across the studied species. Average foliar limonene uptake rate (U(A)) from air at saturating limonene partial pressures varied from 0.9 to 6 nmol m(-2) s(-1), and limonene leaf to air partition coefficient (K(FA), ratio of limonene content per dry mass to limonene partial pressure) from 0.7 to 6.8 micromol kg(-1) Pa(-1). U(A) and K(FA) scaled positively with leaf lipid content, and were independent of D(F), indicating that variation in leaf lipid content was the primary determinant of species differences in monoterpene uptake rate and K(FA). Mass-based limonene uptake rates further suggested that thinner leaves with greater surface area per unit dry mass have higher uptake rates. In addition, limonene lipid to air partition coefficient (K(LA)=K(FA)/L(M)) varied 19-fold, indicating large differences in limonene uptake capacity at common leaf lipid content. We suggest that the significant uptake of hydrophobic monoterpenes when monoterpene ambient air concentration is high and release when the concentration is low should be included in large-scale monoterpene emission models.

Uptake of Organic Gas Phase Species by 1-Methylnaphthalene

Organic compounds are a significant component of tropospheric aerosols. In the present study, 1-methylnaphthalene was selected as a surrogate for aromatic hydrocarbons (PAHs) found in tropospheric aerosols. Mass accommodation coefficients (alpha) on 1-methylnaphthalene were determined as a function of temperature (267 K to 298 K) for gas-phase m-xylene, ethylbenzene, butylbenzene, alpha-pinene, gamma-terpinene, p-cymene, and 2-methyl-2-hexanol. The gas uptake studies were performed with droplets maintained under liquid-vapor equilibrium conditions using a droplet train flow reactor. The mass accommodation coefficients for all of the molecules studied in these experiments exhibit negative temperature dependence. The upper and lower values of alpha at 267 and 298 K respectively are as follows: for m-xylene 0.44 +/- 0.05 and 0.26 +/- 0.03; for ethylbenzene 0.37 +/- 0.03 and 0.22 +/- 0.04; for butylbenzene 0.47 +/- 0.06 and 0.31 +/- 0.04; for alpha-pinene 0.47 +/- 0.07 and 0.10 +/- 0.05; for gamma-terpinene 0.37 +/- 0.04 and 0.12 +/- 0.06; for p-cymene 0.74 +/- 0.05 and 0.36 +/- 0.07; for 2-methyl-2-hexanol 0.44 +/- 0.06 and 0.29 +/- 0.06. The uptake measurements also yielded values for the product HD(l)(1/2) for most of the molecules studied (H = Henry's law constant, D(l) = liquid-phase diffusion coefficient). Using calculated values of D(l), the Henry's law constants (H) for these molecules were obtained as a function of temperature. The H values at 298 K in units 10(3) M atm(-1) are as follows: for m-xylene (0.48 +/- 0.05); for ethylbenzene (0.50 +/- 0.08); for butylbenzene (3.99 +/- 0.93); for alpha-pinene (0.53 +/- 0.07); for p-cymene (0.23 +/- 0.07); for 2-methyl-2-hexanol (1.85 +/- 0.29).

Uptake and reaction of atmospheric organic vapours on organic films

Films composed in whole or in part of organic compounds represent an important atmospheric interface. Urban surfaces are now known to be coated with a film ("grime") whose chemical composition somewhat resembles that of urban atmospheric aerosols. Such films may act as media in which atmospheric trace gases may be sequestered (leading to their removal from the gas phase); they may also act as reactive media, either as a "solvent" or as a source of reagents. Organic coatings on aqueous surfaces are also important, not just on ocean and lake surfaces ("biofilms") but also on the surfaces of fogwaters and atmospheric aerosol particles. We have initiated experimental uptake studies of trace gases into simple proxies for urban organic films using two techniques: a Knudsen cell effusion reactor and a laser-induced fluorescence method. We will discuss our first results on non-reactive uptake of organic compounds by organic films we use as proxies for urban grime coatings. In general, the measured uptake coefficients appear to track the octanol-air partition coefficients, at least qualitiatively. We have also measured the kinetics of reactions between gas-phase ozone and small polycyclic aromatic hydrocarbons (PAHs), when these are adsorbed at the air-aqueous interface or incorporated into an organic film. Reactions at the "clean" air-water interface and at a coated interface consisting of a monolayer of various amphiphilic organic compounds all follow a Langmuir-Hinshelwood mechanism, in which ozone first adsorbs to the air-aqueous interface, then reacts with already adsorbed PAH. By contrast, the reaction in the pure organic film occurs in the bulk phase. Under some circumstances, heterogeneous oxidation of PAHs by ozone may be as important in the atmosphere as their gas phase oxidation by OH.