Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Abstract

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Figure 1

Schematic of nighttime NO₃-BVOC chemistry.

Figure 2

Condensed reaction mechanism for isoprene and β-pinene oxidation via NO₃ (adapted from Schwantes et al., 2015 and Boyd et al., 2015). For brevity, only products generated from the dominant peroxy radicals (RO₂) are shown. R′ represents an alkoxy radical, carbonyl compound, or hydroxy compound. Two of the largest uncertainties in β-pinene oxidation are shown in red: (1) quantification of product yields from the RO₂+ HO₂ channel and (2) identification of carbonyl products formed from RO₂ reaction with NO₃, RO₂, or HO₂ (see text for more details).

Figure 3

Uptake coefficients, γ (NO₃), for the interaction of NO₃ with single-component organic surfaces. Details of the experiments and the references (corresponding to the x-axis numbers) are given in Table S1 in the Supplement.

Figure 4

(a) Correlation of OH versus NO₃ radical rate constants in the aqueous phase for the respective compound classes. The linear regression fits for the different compound classes are presented in the same color as the respective data points. The black line represents the correlation of the overall data. (b) Comparison of modeled, aqueous-phase reaction fluxes (mean chemical fluxes in mol cm⁻³ s⁻¹ over a simulation period of 4–5 days) of organic compounds with hydroxyl (OH) versus nitrate (NO₃) radicals distinguished by different compound classes (urban CAPRAM summer scenario).

Figure 5

(a) Average mass concentrations (in μg m⁻³, ambient temperature and pressure) of submicrometer particulate organic nitrates (NO_{3, org}) and particulate inorganic nitrates (NO_{3, inorg}) in different months at multiple sites. The concentrations correspond to mass concentrations of –ONO₂ functionality. Note that the y axis is different for sites with total nitrates greater than 1 μg m⁻³ (shaded). Detailed information and measurements for each site are provided in Table S5. (b) Percentage (by mass; cyan) of submicrometer particulate organic nitrate aerosols in ambient organic aerosols in different months at multiple sites. Detailed information and measurements for each site are provided in Table S5.

Figure 6

The real part of refractive index (RI) (mr) for biogenic SOA compiled from several chamber studies. The legend specifies the precursor type and oxidation pathway as well as the reference. The figure is reprinted with permission from Moise et al. (2015).