Ozone Reactivity Measurement of Biogenic Volatile Organic Compound Emissions

Previous research on atmospheric chemistry in the forest environment has shown that the total reactivity by biogenic volatile organic compound (BVOC) emission is not well considered in for-est chemistry models. One possible explanation for this discrepancy is the unawareness and ne-glect of reactive biogenic emission that have eluded common monitoring methods. This ques-tion motivated the development of a total ozone reactivity monitor (TORM) for the direct de-termination of the reactivity of foliage emissions. Emissions samples drawn from a vegetation branch enclosure experiment are mixed with a known and controlled amount of ozone (e.g. re-sulting in 100 ppb of ozone) and directed through a temperature-controlled glass flow reactor to allow reactive biogenic emissions to react with ozone during the approximately 2-minute residence time in the reactor. The ozone reactivity is determined from the difference in the ozone mole fraction before and after the reaction vessel. An inherent challenge of the experi-ment is the influence of changing water vapor in the sample air on the ozone signal. A com-mercial UV absorption ozone monitor was modified to directly determine the ozone differential with one instrument and sample air was drawn through Nafion dryer membrane tubing. These two modifications significantly reduced errors associated with the determination of the reacted ozone compared to determining the difference from two individual measurements and errors from interferences from water vapor, resulting in a much improved and sensitive determina-tion of the ozone reactivity. This paper provides a detailed description of the measurement de-sign, the instrument apparatus, and its characterization. Examples and results from field de-ployments demonstrate the applicability and usefulness of the TORM.

Observing ozone chemistry in an occupied residence

Outdoor ozone transported indoors initiates oxidative chemistry, forming volatile organic products. The influence of ozone chemistry on indoor air composition has not been directly quantified in normally occupied residences. Here, we explore indoor ozone chemistry in a house in California with two adult inhabitants. We utilize space- and time-resolved measurements of ozone and volatile organic compounds (VOCs) acquired over an 8-wk summer campaign. Despite overall low indoor ozone concentrations (mean value of 4.3 ppb) and a relatively low indoor ozone decay constant (1.3 h−1), we identified multiple VOCs exhibiting clear contributions from ozone-initiated chemistry indoors. These chemicals include 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA), nonenal, and C8-C12 saturated aldehydes, which are among the commonly reported products from laboratory studies of ozone interactions with indoor surfaces and with human skin lipids. These VOCs together accounted for ≥12% molecular yield with respect to house-wide consumed ozone, with the highest net product yield for nonanal (≥3.5%), followed by 6-MHO (2.7%) and 4-OPA (2.6%). Although 6-MHO and 4-OPA are prominent ozonolysis products of skin lipids (specifically squalene), ozone reaction with the body envelopes of the two occupants in this house are insufficient to explain the observed yields. Relatedly, we observed that ozone-driven chemistry continued to produce 6-MHO and 4-OPA even after the occupants had been away from the house for 5 d. These observations provide evidence that skin lipids transferred to indoor surfaces made substantial contributions to ozone reactivity in the studied house.

New particle formation from agricultural recycling of organic waste products

Secondary organic aerosols (SOA) are one of the main sources of uncertainty in the current understanding of the Earth’s climate. Agriculture contributes to primary aerosol emissions, but there is no estimate of SOA formation from gaseous precursors. Organic waste products such as sewage sludge are applied to cropland as fertilizers. In this work, we show that sewage sludge is an unaccounted source of nucleation precursors, such as skatole (C9H9N). Skatole emission and nucleation rates up to 1.1 × 106 cm−3 s−1 owing to ozone reactivity were measured in the laboratory. Our results show that SO2 plays a key role in the oxidation of skatole and leads to intensive new particle formation. The products of ozone reactions with skatole and the possible ozonolysis reaction mechanism are discussed. This nucleation mechanism might aid our understanding of the organic waste agricultural recycling contribution to the aerosol balance in the atmosphere. Based on the measured particle emission flux, the surface area of sewage sludge spread in France and the time before sewage sludge incorporation into the soil, a rough estimate of the annual quantity of particles generated by this agricultural activity is in the range of one ton, which represents ~0.03% of the total PM1.0 emissions from the agricultural and forestry sectors in France. As spreading occurs over only a few days (mid-summer), these emissions may locally be of great concern for local and regional air quality during this period of the year.

KINETICS OF OZONE REACTIONS WITH ADENINE AND CYTOSINE IN AQUEOUS SOLUTIONS

The ultraviolet spectroscopy method has been applied to study the kinetics of the ozone reactions with nitrogenous bases (NB), namely adenine and cytosine in aqueous solutions. At the first research stage, the range of NB working concentrations has been determined. It was found that linear dependences between optical densities and concentrations of nitrogenous bases aqueous solutions are quite reliable, with correlation coefficients r ≥ 0.998, are satisfied up to [NB] = 2.3 ∙ 10–4 mol/l. According to the Bouguer-Lambert-Beer law, adenine and cytosine extinction coefficients in aqueous solutions were determined and subsequently used to calculate their residual concentrations. At the next stage, the kinetics of nitrogenous bases ozonized oxidation was studied with equal initial concentrations of the starting substances ([NB]0 = [О3]0). The results revealed that the kinetic consumption curves of the starting reagents are fairly well linearized (r ≥ 0.996) in the second-order reaction equation coordinates. As found with the bubbling installation, 1 mol of the absorbed ozone falls on 1 mol of the used NB. Thus, the reactions of ozone with adenine and cytosine explicitly proceed according to the second-order kinetic laws (the first – according to О3 and the first – according to NB). The rate constants were calculated by the integral reaction equations, the values of which indicate a higher ozone reactivity in relation to nitrogen bases. The temperature dependences of the second-order rate constants was studied ranging 285-309 K, and the activation parameters (pre-exponential factors and activation energies) of the ozone reactions with adenine and cytosine in aqueous solutions were determined.

An instrument for in situ measurement of total ozone reactivity

We present an instrument for the measurement of total ozone reactivity – the reciprocal of the chemical lifetime of ozone (O3) – in the troposphere. The Total Ozone Reactivity System (TORS) was developed with the objective to study the role of biogenic volatile organic compounds (BVOCs) as chemical sinks of tropospheric ozone. The instrument was extensively characterized and tested in the laboratory using individual BVOCs and small plants (lemon thyme, Thymus citriodorus) in a Teflon bag and proved able to measure reactivities corresponding to >4.5×10-5 s−1 (at 5 min averaging time), with an estimated total uncertainty of ∼32%. Such reactivities correspond to >20 ppb of α-pinene or >150 ppb of isoprene in isolation – larger than typical ambient levels but observable in environmental chamber and enclosure experiments as well as in BVOC-rich environments. The functionality of TORS was demonstrated in quasi-ambient conditions with a deployment in a horticultural glasshouse containing a range of aromatic plants. The measurements of total ozone reactivity made in the glasshouse showed a clear diurnal pattern, following the emissions of BVOCs, and are consistent with mixing ratios of tens of parts per billion of monoterpenes and several parts per billion of sesquiterpenes.