Monday, June 9, 2008

MIRTHE gears up for Beijing Olympics

by Yan Zhang-Princeton University

The 2008 Olympic Games in Beijing have focused attention on the problems of air quality in urban environments and will serve as an important platform for developing and testing new technologies and procedures for analysis and management of air quality problems. Regional decisions concerning industrial development, agricultural practice and urban policy can play important roles in air quality problems linked to fine particulate matter. The Olympic Games will provide an important research venue for addressing these issues and unique opportunities for advancing novel environmental sensor systems and atmospheric models. In our work, we will deploy two environmental sensor systems at the Institute of Atmospheric Physics, Chinese Academy of Sciences near the Olympic Stadium in Beijing from June to August 2008 for continuous monitoring of trace gases, before, during, and after the Olympic Games. Data from these sensors will be incorporated into analyses using the Weather Research and Forecasting model, a state-of-the-art meteorological model which is coupled with an atmospheric chemistry (WRF-Chem) module. These analyses will be used to examine air quality problems in the Beijing metropolitan region and regional climatology problems linked to trends of decreasing precipitation in the Beijing metropolitan region associated with increased aerosol loadings.

The environmental sensor systems deployed in Beijing use Quantum Cascade Lasers (QCLs) as the core technology for measuring trace gases from "remote sensing" and "point" sensors. QCLs are tiny, tunable mid-Infrared (mid-IR) semiconductor laser sources that have extremely broad wavelength coverage (3-20 μm), which includes the wavelength range where trace gases have their strongest absorption features. The lasers are designed to emit at aparticular wavelength; thus, by knowing where a gas absorbs best, a laser can be designed for detection of that specific gas. As a result of new developments of QCLs, laser absorption spectroscopy is becoming aviable alternative to other analytical methods for trace gas sensing.

QCLOPS (Quantum Cascade Laser Open Path System) is an "open path" remote sensing system that uses two QCLs for monitoring multiple trace gases. The principal target gases for QCLOPS are ozone, ammonia, and carbon dioxide. Elevated ozone levels in urban regions around the world present one of the greatest air quality and public health challenges associated with industrial and automobile emissions. Ammonia plays an important and complex role in aerosol chemistry in urban environments and development of sensor systems for ammonia has proven especially challenging. Carbon dioxide is broadly recognized as an important greenhouse gas and its measurement in urban environments is an important goal of QCLOPS. The laser radiation is transmitted through the air and reflected back by a retro-reflector to a detector. The detector is connected to a data acquisition system and a computer. The computer runs a custom algorithm to calculate concentrations.

NO and NO2 are important ozone precursors and their presence in urban environments is strongly connected to automobile emissions. Detection of NO and NO2 is of great interest for air quality problems linked to elevated ozone concentrations. Fast and sensitive detection of NO can be realized by Faraday rotation spectroscopy. The best NO detection limit (sub-ppbV;parts per billion by volume) can be obtained at approximately 5.3 μm. An "externalcavity" (EC) QCL source that precisely coincides withth isoptimum absorption wavelength was developed and a Faraday rotation spectrometer based on the EC-QCL was developed for detection of NO. The measurement technique will allow for sensitive and selective measurements of NO even in the presence of strongly interfering gases (especially water vapor). A fully automaticand autonomous EC-QCL Faraday rotation spectroscopic sensorsystem will be deployed at the Beijing test site for contiuous atnospheric NO monitoring.

The Weather Research and Forecasting model, coupled with the WRF-Chem atmospheric chemistry module (WRF-Chem), provides a powerful platform for meteorological and air quality forecasting, as well as regional analyses of the impact of anthropogenic emissions on air quality and regional climate. WRF-Chem has been used at Princeton for analyses of aerosol impacts on regional precipitation climatology in the Baltimore and New York City metropolitan region. With the collaboration of the Nansen-Zhu International Center of the Institute of Atmospheric Physics (IAP), the Chinese Academy of Sciences, WRF-Chem will be implemented as a forecasting tool for the Beijing Olympics. An important element of the forecasting system will be integrating observations from sensor systems like QCLOPS in to the forecasting process. The Princeton group will also work closely with IAP in studying and understanding how the urban aerosols influence local weather and public health through coupled modeling and monitoring analyses.

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