The mass resolution of a time‐of‐flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi‐turn or multi‐passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi‐turn TOF mass spectrometer with electrostatic sectors. The concept of ‘perfect’ focusing conditions is introduced. Furthermore, a new type of multi‐turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 × 70 × 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi‐turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi‐turn TOF mass spectrometers is also discussed. Copyright © 2003 John Wiley & Sons, Ltd.

Severe winter haze accompanied by high concentrations of fine particulate matter (PM_2.5) occurs frequently in the North China Plain and threatens public health. Organic matter (OM) and sulfate are recognized as major components of PM_2.5, while atmospheric models often fail to predict their high concentrations during severe winter haze due to incomplete understanding of secondary aerosol formation mechanisms. By using a novel combination of single-particle mass spectrometry and an optimized ion chromatography method, here we show that hydroxymethanesulfonate (HMS), formed by the reaction between formaldehyde (HCHO) and dissolved SO₂ in aerosol water, is ubiquitous in Beijing during winter. The HMS concentration and the molar ratio of HMS to sulfate increased with the deterioration of winter haze. High concentrations of precursors (SO₂ and HCHO) coupled with low oxidant levels, low temperature, high relative humidity, and moderately acidic pH facilitate the heterogeneous formation of HMS, which could account for up to 15 % of OM in winter haze and lead to up to 36 % overestimates of sulfate when using traditional ion chromatography. Despite the clean air actions having substantially reduced SO₂ emissions, the HMS concentration and molar ratio of HMS to sulfate during severe winter haze increased from 2015 to 2016 with the growth in HCHO concentration. Our findings illustrate the significant contribution of heterogeneous HMS chemistry to severe winter haze in Beijing, which helps to improve the prediction of OM and sulfate and suggests that the reduction in HCHO can help to mitigate haze pollution.