The lifetime of nitrogen oxides in an isoprene-dominated forest

The lifetime of nitrogen oxides (NO_x) affects the concentration and distribution of NO_x and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NO_x is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NO_x and NO_x reservoirs. We find that the lifetime of NO_x during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates (ΣANs). During SOAS, ΣAN production was rapid, averaging 90 ppt h^(−1) during the day, and occurred predominantly during isoprene oxidation. Analysis of the ΣAN and HNO_3 budgets indicate that ΣANs have an average lifetime of under 2 h, and that approximately 45 % of the ΣANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that ΣAN hydrolysis is the largest source of HNO_3 and the primary pathway to permanent removal of NO_x from the boundary layer in this location. Using these new constraints on the fate of ΣANs, we find that the NO_x lifetime is 11 ± 5 h under typical midday conditions. The lifetime is extended by storage of NO_x in temporary reservoirs, including acyl peroxy nitrates and ΣANs

Importance of biogenic volatile organic compounds to acyl peroxy nitrates (APN) production in the southeastern US during SOAS 2013

Gas-phase atmospheric concentrations of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN) were measured on the ground using a gas chromatograph electron capture detector (GC-ECD) during the Southern Oxidants and Aerosols Study (SOAS) 2013 campaign (1 June to 15 July 2013) in Centreville, Alabama, in order to study biosphere&ndash;atmosphere interactions. Average levels of PAN, PPN, and MPAN were 169, 5, and 9&thinsp;pptv, respectively, and the sum accounts for an average of 16&thinsp;% of NOy during the daytime (10:00 to 16:00 local time). Higher concentrations were seen on average in air that came to the site from the urban NOx sources to the north. PAN levels were the lowest observed in ground measurements over the past two decades in the southeastern US. A multiple regression analysis indicates that biogenic volatile organic compounds (VOCs) account for 66&thinsp;% of PAN formation during this study. Comparison of this value with a 0-D model simulation of peroxyacetyl radical production indicates that at least 50&thinsp;% of PAN formation is due to isoprene oxidation. MPAN has a statistical correlation with isoprene hydroxynitrates (IN). Organic aerosol mass increases with gas-phase MPAN and IN concentrations, but the mass of organic nitrates in particles is largely unrelated to MPAN.</p

The lifetime of nitrogen oxides in an isoprene-dominated forest

The lifetime of nitrogen oxides (NOx) affects the concentration and distribution of NOx&nbsp;and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NOx&nbsp;is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NOx&nbsp;and NOx&nbsp;reservoirs. We find that the lifetime of NOx&nbsp;during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates (&Sigma;ANs). During SOAS, &Sigma;AN production was rapid, averaging 90 ppt h-1 during the day, and occurred predominantly during isoprene oxidation. Analysis of the &Sigma;AN and HNO3 budgets indicate that &Sigma;ANs have an average lifetime of under 2 h, and that approximately 45 % of the &Sigma;ANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that &Sigma;AN hydrolysis is the largest source of HNO3&nbsp;and the primary pathway to permanent removal of NOx&nbsp;from the boundary layer in this location. Using these new constraints on the fate of &Sigma;ANs, we find that the NOx&nbsp;lifetime is 11&thinsp;&plusmn;&thinsp;5 h under typical midday conditions. The lifetime is extended by storage of NOx&nbsp;in temporary reservoirs, including acyl peroxy nitrates and &Sigma;ANs.</p

The lifetime of nitrogen oxides in an isoprene-dominated forest

The lifetime of nitrogen oxides (NO_x) affects the concentration and distribution of NO_x and the spatial patterns of nitrogen deposition. Despite its importance, the lifetime of NO_x is poorly constrained in rural and remote continental regions. We use measurements from a site in central Alabama during the Southern Oxidant and Aerosol Study (SOAS) in summer 2013 to provide new insights into the chemistry of NO_x and NO_x reservoirs. We find that the lifetime of NO_x during the daytime is controlled primarily by the production and loss of alkyl and multifunctional nitrates (ΣANs). During SOAS, ΣAN production was rapid, averaging 90 ppt h^(−1) during the day, and occurred predominantly during isoprene oxidation. Analysis of the ΣAN and HNO_3 budgets indicate that ΣANs have an average lifetime of under 2 h, and that approximately 45 % of the ΣANs produced at this site are rapidly hydrolyzed to produce nitric acid. We find that ΣAN hydrolysis is the largest source of HNO_3 and the primary pathway to permanent removal of NO_x from the boundary layer in this location. Using these new constraints on the fate of ΣANs, we find that the NO_x lifetime is 11 ± 5 h under typical midday conditions. The lifetime is extended by storage of NO_x in temporary reservoirs, including acyl peroxy nitrates and ΣANs

Cation-cation contact pairing in water: guanidinium.

The formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized by X-ray absorption spectroscopy and first-principles spectral simulations based on molecular dynamics sampling. Observed concentration-induced nitrogen K-edge resonance shifts result from π* state mixing and the release of water molecules from each first solvation sphere as two solvated guanidinium ions associate into a stacked pair configuration. Possible biological implications of this counterintuitive cation-cation pairing are discussed

Cation-cation contact pairing in water: guanidinium.

The formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized by X-ray absorption spectroscopy and first-principles spectral simulations based on molecular dynamics sampling. Observed concentration-induced nitrogen K-edge resonance shifts result from π* state mixing and the release of water molecules from each first solvation sphere as two solvated guanidinium ions associate into a stacked pair configuration. Possible biological implications of this counterintuitive cation-cation pairing are discussed

Organic nitrate contribution to the oxidized nitrogen budget at the 2013 Southern Oxidant and Aerosol Study (SOAS)

Org. nitrates (RONO_2) that are formed from the OH-initiated or NO_3-initiated oxidn. of biogenic alkenes can be important reservoirs and sinks for NO_x (NO + NO_2). Biogenic org. nitrates represent a significant fraction of the total NO_y budget in forested regions. During the 2013 Southern Oxidant and Aerosol Study (SOAS), speciated org. nitrates produced from the oxidn. of isoprene (C_5H_8) and monoterpenes (C_(10)H_(16)) were measured with high temporal resoln. and sensitivity by a time-of-flight chem. ionization mass spectrometer (ToF-CIMS) from a 20 m tower. Both dark and photochem. mechanisms were obsd. to be important for the prodn. of multifunctional org. nitrates at SOAS. The combined vol. mixing ratios of RONO_2 detd. by ToF-CIMS and GC-MS were compared to a thermal-dissocn. laser-induced fluorescence (TD-LIF) measurement of total org. nitrates in the gas and aerosol phase. The mass closure of org. nitrates in the gas-phase and their fractional contributions to the aerosol phase will be presented. The significance of org. nitrates to the overall nitrogen and oxidant budgets at SOAS will be discussed