The origin of this material is atmospheric particulate matter collected on filters in a central ventilating system in a building in Beijing city center. The collection period was 10 years, from 1996 to 2005. The particulate matter was recovered from the filters by mechanical vibration; a total of 3 kg was collected. The recovered material was sieved using a 32 μm sieve, and then homogenized. The sieved material (2 kg) was subdivided into 1031 pre-washed amber glass bottles with each bottle containing 1.5 g material. Then material was stabilized by 60Co irradiation (2.5 Mrad). The CRM production was conducted in accordance with ISO Guide 34.

Twelve bottles were randomly selected from the entire batch and elemental analyses were carried out on the contents of each bottle using ICP-AES. The between-bottle standard deviation for each element was less than 3 %. It was adjudged that the prepared material was sufficiently homogeneous for its intended use as a reference material.

The results of stability tests demonstrated that any long-term variations in the values of all the measured elements in the material were insignificant.

Elemental analyses were reported by 17 organizations (see below). Certified values are given for element mass fractions that were determined by at least two analytical methods, and were decided in accordance with ISO Guide 35 (Table 1). The uncertainty attached to the certified values is the expanded uncertainty using a coverage factor k=2, corresponding to a confidence interval of approximately 95 %. Reference values are given for elements with relative standard deviations of the mean of the analytical laboratory means of 6-10 %, or with 3-9 laboratories contributing to the mean (Table 2). All certified and reference values were determined on an "as received" basis, that is, the values were determined without drying the material.

AAS: atomic absorption spectroscopy 
HG-AAS: hydride generation - atomic absorption spectroscopy 
HG-ICP-AES: hydride generation-inductively coupled plasma-atomic emission spectrometry 
ICP-AES: inductively coupled plasma-atomic emission spectrometry
ICP-MS: inductively coupled plasma-mass spectrometry
INAA: instrumental neutron activation analysis 
PIXE: proton induced X-ray emission spectrometry 
XRF: X-ray fluorescence spectroscopy

　HG-AAS: hydride generation - atomic absorption spectroscopy 
　HG-ICP-AES: hydride generation-inductively coupled plasma-atomic emission spectrometry 
　HR-ICP-MS: high resolution inductively coupled plasma-mass spectrometry 
　ICP-AES: inductively coupled plasma-atomic emission spectrometry
　ICP-MS: inductively coupled plasma-mass spectrometry
　INAA: instrumental neutron activation analysis 
　PIXE: proton induced X-ray emission spectrometry 
　XRF: X-ray fluorescence spectroscopy

1. This CRM should be kept tightly closed in its original bottle and stored in a desiccator at room temperature (≤30 ℃).
2. Prior to weighing aliquots for analysis, the contents of the bottle should be shaken gently.
3. A minimum sample intake of 0.02 g is recommended.
4. Precautions should be taken to avoid inhalation of the material.
5. Do not use for purposes other than research. When disposing of the material, adhere strictly to local laws concerning processing and disposal of waste materials.

The expiration date for the certified values of this CRM is January 2028 assuming that the above mentioned storage conditions are adhered to. NIES will notify via its website if any changes in the values are recognized within the term of validity.

The certified and reference values for this CRM were based on the analytical values from the following participating organizations:
National Institute for Environmental Studies; Chikyu Kagaku Kenkyusho Co., Ltd; China Institute of Atomic Energy; Environmental Control Center Co., Ltd.; Green Blue Corporation; Idea Consultants Inc.; Japan Chemical Analysis Center; Japan Environmental Sanitation Center; Murata Keisokuki Service Co. Ltd; Musashi Institute of Technology; Naitoh Environmental Science Co., Ltd.; Nittech Research Inc.; Norwegian University of Science and Technology; Shimadzu Techno-Research Inc.; Sumika Chemical Analysis Service, Ltd.; University of Sheffield; University of Shizuoka, Institute For Environmental Science.

Institute for Reference Materials of the State Environmental Protection Administration, P.R. China; Japan International Cooperation Agency (JICA), The Shino-Japan Friendship Center for Environmental Protection Project Phase III JICA Expert Team.

The Environmental Analytical Chemistry Research Laboratory within the Laboratory of Intellectual Fundamentals for Environmental Studies, NIES, takes entire responsibility for this CRM. Technical information and the latest research reports regarding this material can be obtained from the following address.

Information that could be useful for handling this material is provided, though the values are non-certified.

Appendix Fig.1 
Size distribution (numbers of particles) of NIES CRM No. 28 Urban Aerosols
This distribution was obtained by analyzing a microscopic image using image analysis software. 
The detection limit is 1 µm. The diameters of 99% of the particles were less than 10 µm.

Appendix Table 1
Mass fraction of carbon and nitrogen in NIES CRM No. 28 Urban Aerosols

Appendix Table 2
Mass fraction of PAHs in NIES CRM No. 28 Urban Aerosols