Chemical ionization mass spectrometer for long-term measurements of atmospheric OH and H2SO4

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Abstract

An atmospheric pressure/chemical ionization mass spectrometer (AP/CIMS) has been developed for continuous long-term measurements of atmospheric OH and H2SO4. The corresponding methods both involve chemical ionization of H2SO4 by NO3 ions with OH being first titrated by excess SO2 to form equivalent concentrations of H2SO4 in the system. The chemical ionization mass spectrometry (CIMS) system has been operated since April 1998 at the Meteorological Observatory Hohenpeissenberg, a mountain research station of the German Weather Service in South Germany. A technical description of the apparatus is presented followed by a detailed estimate of uncertainties in calibration and ambient air measurements resulting from changes in instrumental and/or ambient parameters. Examples from both calibration runs and ambient air measurements are shown. For the present system and operating conditions accuracy, precision, and detection limit are estimated to be 39%, 30%, and 3 × 104 molecules cm−3 for H2SO4, and 54%, 48%, and 5 × 105 molecules cm−3 for OH measurements, respectively, based on 5 min signal integration.

Introduction

The hydroxyl radical, OH, is the most important oxidant in the gas phase chemistry of the lower atmosphere. It efficiently controls atmospheric levels of methane (a major greenhouse gas), carbon monoxide, compounds of sulfur, nitrogen, and a number of other compounds such as hydrocarbons. This central role of OH as a gas phase “detergent” defines to a large part the self-cleansing power or the so-called “oxidation capacity” of the atmosphere [1]. The dominant primary source for OH in the lower troposphere is photolysis of ozone at wavelengths below 320 nm: O3+hν→O(1D)+O2 O(1D)+H2O→2 OH Typically, 10% of the O(1D) reacts with water vapor according to Eq. (2) (assuming a H2O mixing ratio of 1%). Most of the O(1D) is energetically quenched to the ground state, O(3P), by collision with O2 or N2. OH has an average lifetime of a few seconds or less and maximum concentrations of only 106–107 molecules cm−3 (or 0.04–0.4 parts per trillion by volume, pptv) in the lower troposphere. Therefore, measuring OH in ambient air presents a major experimental challenge.

Gaseous sulfuric acid, H2SO4, is mainly produced from the oxidation of SO2 by OH: SO2+OH+M→HSO3+M HSO3+O2→SO3+HO2 SO3+H2O+M→H2SO4+M In the atmosphere H2SO4 is predominantly removed by deposition to aerosol particles and by absorption into droplets. The atmospheric lifetime of H2SO4 in the gas phase varies between a few minutes and about 1–2 hours. Similar to OH, atmospheric H2SO4 concentrations show a strong diurnal variation, typically from as low as 104 molecules cm−3 at night to daytime maxima in the 106–107 molecules cm−3 range. Because H2SO4 has a very low vapor pressure under atmospheric conditions (below 10−4 Pa) [2], [3], [4] special attention has been given in recent years to its role in gas-to-particle conversion, a process that includes homogeneous condensation of H2SO4, possibly in combination with other neutral compounds or atmospheric ions, forming molecular clusters that may subsequently grow to stable submicrometer size particles [e.g., [5], [6], [7]]. Once these particles have reached a few hundred nanometers in diameter they efficiently scatter sunlight and play a crucial role in the formation of clouds and the regulation of global climate. There is a strong need for reliable measurements of H2SO4 to better understand these important processes and their potential modification by man-made sulfur emissions [8].

Both OH and H2SO4 have for a long time eluded detection and measurement in the atmosphere. Using CIMS-based techniques, H2SO4 was for the first time measured in the stratosphere by Arnold and Fabian [9], and in the troposphere by Eisele and Tanner [10]. Previous attempts to measure atmospheric OH were only partially successful either because of relatively low sensitivities or measurement interferences associated with the corresponding techniques [11], [12], [13]. Only in recent years after substantial improvements and with the development of several new techniques have atmospheric OH measurements become more successful [e.g., [14], [15], [16], [17], [18]]. Among these new developments one of the most sensitive and versatile methods is atmospheric pressure chemical ionization mass spectrometry (AP/CIMS).

In the present work we describe a new AP/CIMS system that has been specifically designed and tested for long-term stationary field measurements but which, in principle, can also be used on mobile platforms or in laboratory studies. The present AP/CIMS system is an outgrowth of previous MS-based techniques that have been successfully applied for measuring atmospheric positive and negative ions, sulfur compounds, hydrocarbons, and OH [10], [19], [20], [21], [22], [23], [24], [25]. Major features distinguishing the present system from previous versions include the use of different vacuum pumps (cryopumps) and an improved OH calibration unit. However, what is even more distinguishing from previous work is the goal of applying the present system for long-term measurements. Therefore, an extensive evaluation of the present performance of the system and, in particular, of potential measurement interferences is given in this work based on preliminary experiences from two years of test runs and ambient air H2SO4 and OH measurements.

The present AP/CIMS system has been developed and installed at the Meteorological Observatory Hohenpeissenberg (MOHp), a mountain research station operated by the German Weather Service (DWD). The station is located approximately 50 km north of the Alps at an elevation of about 1000 m above sea level. There are no major industrial sources in the vicinity. The station is mainly surrounded by forests and agricultural pastures. As part of DWD’s atmospheric chemistry and climate research program the system has been operated since April 1998 measuring both H2SO4 and OH concentrations in the ambient atmosphere (on 151 days in 1998 and 186 days in 1999). It has also been successfully operated on the west coast of Ireland in June 1999 as part of the EU project PARFORCE (New Particle Formation and Fate in the Coastal Environment). The complete results of these measurements will be published separately in the near future.

Section snippets

Measurement principles

The techniques used in the present study to measure atmospheric H2SO4 and OH both employ negative chemical ionization converting H2SO4 molecules into HSO4 core ions by reaction with NO3 core ions at atmospheric pressure [9], [26], [27]: H2SO4+NO3→HSO4+HNO3

The majority of the NO3 reactant ions are clustered with neutral HNO3 and/or H2O molecules as NO3(HNO3)m(H2O)n (m = 0–2, n = 0–3). However, because all of these compounds have been shown to react with H2SO4 at similar rates ([26]; for a

Measurements, tests, and improvements

Over the past two years our CIMS instrument has mainly been operated in the negative ion mode, although occasionally we have also recorded spectra of positive ions resulting from H3O+ proton transfer to sample air molecules. In general, a much higher number of masses with significant ion signals are observed in the positive ion mode. When operating in the negative ion mode sufficient HNO3 vapor is added to the ion source region to produce almost exclusively NO3 reactant ions, primarily as NO3

Conclusions and future plans

We have described the present operating status of a new AP/CIMS system for long-term measurements of atmospheric OH and H2SO4 concentrations. A detailed error analysis has been conducted, including uncertainties from instrumental as well as ambient air parameters (wind and chemical interferences). With respect to H2SO4 measurements, CIMS methods originally developed by Eisele and Tanner [10] and by Arnold and co-workers [9], [32] are unique in their sensitivity and time resolution. For our

Acknowledgements

We would like to thank R. Weiner, P. Settele, and R. Ruf for helping with the installment of the CIMS system, improvements of the electronic components, contributions to various test and field measurements, and their assistance in data evaluation. We also thank S. Gilge for providing the NO, NO2, and CO data. The technical development of the CIMS system and manufacturing or purchase of system components were entirely funded by DWD/BMVBW. Field measurements presented in this work were in part

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