A. R. Ravishankara

Bio: A. R. Ravishankara is an academic researcher from National Oceanic and Atmospheric Administration. The author has contributed to research in topics: Reaction rate constant & Kinetics. The author has an hindex of 41, co-authored 88 publications receiving 5129 citations. Previous affiliations of A. R. Ravishankara include Cooperative Institute for Research in Environmental Sciences & University of Colorado Boulder.

Reactive uptake of NO3 on pure water and ionic solutions

Abstract: The reactive uptake coefficients (γ) of NO 3 onto pure water and dilute solutions of NaCl, NaBr, and NaNO 2 were measured using a wetted-wall flow-tube setup combined with a long-path absorption cell for the detection of NO 3 . The measured γ values were in the range 1.5 × 10 -4 - 6 × 10 -3 , depending on the salt concentration in the water. By measuring γ as a function of salt concentration, HD l 0.5 for NO 3 in water was determined to be (1.9±0.4)×10 -3 M atm -1 cm s -0.5 at 273 K, assuming that the rate coefficient for the reaction of NO 3 with Cl - is 2.76×10 6 M -1 s -1 at 273 K. The Henry's law coefficient for NO 3 in water is estimated to be 0.6±0.3 M atm -1 , assuming that the diffusion coefficient of NO 3 in water is D l = (1.0±0.5)×10 -5 cm 2 s -1 . Uptake of NO 3 on pure water is interpreted as due to reaction of NO 3(aq) with H 2 O (l) to produce HNO 3 and OH in the liquid phase. Implications of these findings to the chemistry of NO 3 in the troposphere are also discussed.

N2O5 oxidizes chloride to Cl2 in acidic atmospheric aerosol.

Abstract: Molecular chlorine (Cl2) is an important yet poorly understood trace constituent of the lower atmosphere. Although a number of mechanisms have been proposed for the conversion of particle-bound chloride (Cl‐) to gas-phase Cl2, the detailed processes involved remain uncertain. Here, we show that reaction of dinitrogen pentoxide (N2O5) with aerosol-phase chloride yields Cl2 at low pH (

Role of adducts in the atmospheric oxidation of dimethyl sulfide

Abstract: Based on the results from recent laboratory studies and field observations, we suggest that adduct formation may play a key role in the atmospheric oxidation of dimethyl sulfide (DMS). Experimental evidence for the formation of the DMS · OH and CH3SOx· O2 adducts are described and used in a simple ‘box model’ to calculate the ratio of [MSA]/[SO42–] observed in the atmosphere. These calculated ratios are compared with those inferred from field measurements. Based on these comparisons, we suggest that the changes with temperature in the mechanism of the OH + DMS reaction, one of the primary initiation steps in atmospheric DMS oxidation, are unlikely to be the source of the variation in the [MSA]/[SO42–] ratio with temperature. We suggest that the competition between the thermal decomposition of adducts such as CH3SO2 and their bimolecular reactions with atmospheric species is the likely cause of the variation. Finally, a few observations needed to better our understanding of the DMS oxidation are identified.

CF3CFCH2 and (Z)-CF3CFCHF: temperature dependent OH rate coefficients and global warming potentials

Abstract: Rate coefficients over the temperature range 206–380 K are reported for the gas-phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF3CFCH2), k1(T), and 1,2,3,3,3-pentafluoropropene ((Z)-CF3CFCHF), k2(T), which are major components in proposed substitutes for HFC-134a (CF3CFH2) in mobile air-conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed-laser photolysis to produce OH and laser-induced fluorescence to detect it. Rate coefficients were found to be independent of pressure between 25 and 600 Torr (He, N2). For CF3CFCH2, the rate coefficients, within the measurement uncertainty, are given by the Arrhenius expression k1(T) = (1.26 ± 0.11) × 10−12 exp[(−35 ± 10)/T] cm3 molecule−1 s−1 where k1(296 K) = (1.12 ± 0.09) × 10−12 cm3 molecule−1 s−1. For (Z)-CF3CFCHF, the rate coefficients are given by the non-Arrhenius expression k2(T) = (1.6 ± 0.2) × 10−18T2 exp[(655 ± 50)/T] cm3 molecule−1 s−1 where k2(296 K) = (1.29 ± 0.06) × 10−12 cm3 molecule−1 s−1. Over the temperature range most relevant to the atmosphere, 200–300 K, the Arrhenius expression k2(T) = (7.30 ± 0.7) × 10−13 exp[(165 ± 20)/T] cm3 molecule−1 s−1 reproduces the measured rate coefficients very well and can be used in atmospheric model calculations. The quoted uncertainties in the rate coefficients are 2σ (95% confidence interval) and include estimated systematic errors. The global warming potentials for CF3CFCH2 and (Z)-CF3CFCHF were calculated to be <4.4 and <3.6, respectively, for the 100 year time horizon using infrared absorption cross sections measured in this work, and atmospheric lifetimes of 12 and 10 days that are based solely on OH reactive loss.

The HITRAN 2008 molecular spectroscopic database

Abstract: This paper describes the contents of the 2016 edition of the HITRAN molecular spectroscopic compilation. The new edition replaces the previous HITRAN edition of 2012 and its updates during the intervening years. The HITRAN molecular absorption compilation is composed of five major components: the traditional line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, infrared absorption cross-sections for molecules not yet amenable to representation in a line-by-line form, collision-induced absorption data, aerosol indices of refraction, and general tables such as partition sums that apply globally to the data. The new HITRAN is greatly extended in terms of accuracy, spectral coverage, additional absorption phenomena, added line-shape formalisms, and validity. Moreover, molecules, isotopologues, and perturbing gases have been added that address the issues of atmospheres beyond the Earth. Of considerable note, experimental IR cross-sections for almost 300 additional molecules important in different areas of atmospheric science have been added to the database. The compilation can be accessed through www.hitran.org. Most of the HITRAN data have now been cast into an underlying relational database structure that offers many advantages over the long-standing sequential text-based structure. The new structure empowers the user in many ways. It enables the incorporation of an extended set of fundamental parameters per transition, sophisticated line-shape formalisms, easy user-defined output formats, and very convenient searching, filtering, and plotting of data. A powerful application programming interface making use of structured query language (SQL) features for higher-level applications of HITRAN is also provided.

Climate Change 2007: The Physical Science Basis.

Abstract: Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

Anthropogenic and Natural Radiative Forcing

Abstract: This chapter should be cited as: Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Coordinating Lead Authors: Gunnar Myhre (Norway), Drew Shindell (USA)