100 Years of Progress in Gas-Phase Atmospheric Chemistry Research

Remarkable progress has occurred over the last 100 years in our understanding of atmospheric chemical composition, stratospheric and tropospheric chemistry, urban air pollution, acid rain, and the formation of airborne particles from gas-phase chemistry. Much of this progress was associated with the developing understanding of the formation and role of ozone and of the oxides of nitrogen, NO and NO2, in the stratosphere and troposphere. The chemistry of the stratosphere, emerging from the pioneering work of Chapman in 1931, was followed by the discovery of catalytic ozone cycles, ozone destruction by chlorofluorocarbons, and the polar ozone holes, work honored by the 1995 Nobel Prize in Chemistry awarded to Crutzen, Rowland, and Molina. Foundations for the modern understanding of tropospheric chemistry were laid in the 1950s and 1960s, stimulated by the eye-stinging smog in Los Angeles. The importance of the hydroxyl (OH) radical and its relationship to the oxides of nitrogen (NO and NO2) emerged. The chemical processes leading to acid rain were elucidated. The atmosphere contains an immense number of gas-phase organic compounds, a result of emissions from plants and animals, natural and anthropogenic combustion processes, emissions from oceans, and from the atmospheric oxidation of organics emitted into the atmosphere. Organic atmospheric particulate matter arises largely as gas-phase organic compounds undergo oxidation to yield low-volatility products that condense into the particle phase. A hundred years ago, quantitative theories of chemical reaction rates were nonexistent. Today, comprehensive computer codes are available for performing detailed calculations of chemical reaction rates and mechanisms for atmospheric reactions. Understanding the future role of atmospheric chemistry in climate change and, in turn, the impact of climate change on atmospheric chemistry, will be critical to developing effective policies to protect the planet.

Laser-induced photoacoustic detection of ozone at 266 nm using resonant cells of different configuration

A highly sensitive pulsed photoacoustic (PA) spectrometer with different PA cell geometries was designed and fabricated in our laboratory to determine ozone detection at ppb level. The comparative performance and merits of these custom made cells were studied. The excitation source of PA spectrometer is a nanosecond pulsed laser at 266 nm (fourth harmonic of Nd:YAG laser) and a sensitive electret microphone as a photoacoustic detector. The sensitivity optimization of the PA system with different experimental parameters including the resonant acoustic modes of the 3 PA cells was carried out for the detection of ozone. The minimum detection limit for ozone achieved under our experimental conditions, with 3 PA cells were 10, 31 and 26 ppbV for cells designated as cell # 1, cell # 2 and cell # 3, respectively. This limit of ozone detection achieved in our work is quite appreciable to be able to detect ozone under safe permissible limits and the sensitivity achieved in our case is an order of magnitude better than earlier reports using sophisticated laser system like quantum cascade laser.

Using13N as tracer in heterogeneous atmospheric chemistry experiments

Current techniques aiming to study heterogeneous atmospheric chemistry under realistic conditions are often subject to restrictions caused by the low amount of processed material, the complex composition of gas and condensed phases and interference issues. The use of the short-lived tracer13N enables laboratory experiments pertaining to the atmospheric chemistry of nitrogen oxides to be performed at extremely low concentrations, ambient humidity and pressure.13N was produced through16O(p,α)13N reaction in a gas target in continuous mode, and transported as13NO in a carrier gas to the chemistry laboratory through a 70 m long capillary. Several gas-phase and surface chemical routines allowed converting13NO into other forms of oxidized nitrogen. Chemical separation and detection was achieved through chemically specific absorption reactions on the coatings of denuders, which additionally allowed separating gas-phase species from their particulate counterparts. In a first application, the reversible adsorption of NO2on a solid NaCl surface was investigated, from which an adsorption enthalpy of 28 kJ/mol was derived. As an example of a reaction with aerosol particles in gas suspension, the reaction of13N-labelled HNO3with sea-salt particles was studied using an aerosol flow reactor.