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A primary aim of the development of the Q-AMS was to provide quantitative species specific information on aerosol particles, particularly in the ambient environment. The techniques and developments regarding this aim have been discussed in Jayne et al.

(2000), Jimenez et al. (2003) and Allan et al. (2003b,a). Central to this aim was the development of an AMS analysis suite by James Allan (described in Allan et al., 2003b) using the Igor Pro data analysis software (Wavemetrics, Inc., P.O. Box 2088, Lake Oswego, Oregon 97035, USA).

Mass concentrations derived from AMS data use the detected ion signals from the mass spectrometer. This is achieved using the following formula:

C = MW

IE.Q.NAI × 10¹² (2.1)

where C is the mass loading inµgm⁻³ and MW is the relative molecular weight of the parent species in g mol⁻¹. Q is the volumetric flow rate entering the instrument in cm³s⁻¹and NA is Avogadro’s number, which is equal to 6.022×10²³mol⁻¹. IE is the ionisation efficiency of the parent species and I is the detected ion rate in Hertz (Hz). The factor 10¹²is used to convert from g cm⁻³toµgm⁻³. Equation 2.1 is used to calculate the mass concentration at a particular m/z. Consequently, to calculate the mass concentration for a specific species, the summation of the ion signals for each mass fragment is required. Thus equation 2.1 becomes:

CS= MWS

IES.Q.NA

∑

alli

IS,i

!

× 10¹² (2.2)

where the subscript S refers to a particular species and i refers to the mass fragments of species S.

The Ionisation Efficiency (IE) of a particle is a species specific dimensionless quan-tity. It is defined as the number of ions detected per molecule of the parent species (e.g.

Canagaratna et al., 2007). Essentially, it is the probability of a sample particle being ionised, extracted through the mass spectrometer and subsequently being detected. Ex-plicit categorisation of this quantity proves impossible because of the species specific nature of the IE. This is particularly the case for the vast array of organic molecules present in the atmosphere. Consequently, a reference framework is used to derive an appropriate IE, where the ion signals at m/z 30 and 46 are used. These m/z signals cor-respond to the strongest signal intensity for nitrate aerosol, which volatises efficiently upon contact with the vaporiser. This value is calculated regularly during AMS opera-tion using a well documented experimental methodology (e.g. Jayne et al., 2000; Allan et al., 2003b; Canagaratna et al., 2007). Principally, a dry mono-disperse sample of ammonium nitrate requires generation. This is accomplished by generating an aqueous

ammonium nitrate solution, which is dried prior to size selection by a DMA system.

The sample is then introduced to the AMS. The ion signals at m/z 30 and 46 are then counted in PToF mode based on single particle vaporisation events. Via normalising this value to an estimation of the number of ammonium nitrate molecules present in the calibration sample, the ionisation efficiency of nitrate may be calculated. The es-timation of the number of ammonium nitrate molecules is accomplished as the size of the calibration particles has been pre-selected using the DMA system (usually 300 nm or 350 nm) whilst the bulk density (1.725 gcm⁻³) and Jayne shape factor (0.8, Jayne et al., 2000) are known for ammonium nitrate. The ionisation efficiency of nitrate may then be used to deduce the ionisation efficiency of other chemical species based on the assumption that the ionisation cross section of the parent molecules is proportional to the number of molecules present (Jimenez et al., 2003). Furthermore, assuming that the ionisation cross section is proportional to the molecular weight of the species, the following equation may be used to deduce the ionisation efficiency of a particular chemical species:

MWS

IES =RIESMWNO₃

IENO₃ (2.3)

where subscript NO3refers to quantities pertaining to nitrate. RIESis a species specific constant known as the Relative Ionisation Efficiency (Alfarra et al., 2004). This value is calculated for different chemical species via laboratory experiments or during the routine ionisation efficiency calibration described above. The importance of equation 2.3 is that a generalised method for the calculation of mass loadings may be derived via substitution of equation 2.3 into equation 2.2:

CS= 1 RIES

MWNO₃

IENO₃.Q.NA

∑

alli

IS,i

!

× 10¹² (2.4)

A major development regarding the analysis of AMS data was the introduction of the concept of ‘fragmentation tables’ by Allan et al. (2004b). This procedure allows extraction of species specific mass spectra by breaking up the ensemble mass spectra according to the chemical species. This technique is possible as a result of the re-producible dependencies between the relative sizes of the mass spectrum fragments.

Such relationships are definable in laboratory conditions resulting in the generation of standard AMS fragmentation tables which can be used with the AMS analysis toolkit software (Allan et al., 2003b). Consequently, this technique has established a more systematic approach for the analysis of ambient data, which is consistent across differ-ent AMS users (Allan et al., 2003b). A major enhancemdiffer-ent delivered by this approach has been the possibility to probe chemical types such as polycyclic aromatic hydrocar-bons, oxygenated and non-oxygenated hydrocarbons (Allan et al., 2004b; Zhang et al., 2005a). Chemical markers from identifiable sources such as combustion have also been identified (e.g. Schneider et al., 2006; Alfarra et al., 2007). These tables are amendable

Table 2.1: Main inorganic and organic ion fragments used to distinguish different aerosol species in AMS spectra. The bold text refers to the most useful fragments when identifying species. The table is adapted from Canagaratna et al. (2007).

Group Molecule Ion Fragments Mass Fragments

Water H2O H2O⁺, HO⁺, O⁺ 18, 17, 16

Ammonium NH3 NH3+,NH2+, NH⁺ 17,16, 15

Nitrate NO3 HNO3+,NO2+,NO⁺ 63,46, 30

Sulphate H2SO4 H2SO4+, HSO3+, SO3+, 98, 81, 80,

SO2+,SO⁺ 64, 48

Organic (Oxygenated) CnHmOy H2O⁺, CO⁺,CO⁺₂, 18, 28,44, H3C2O⁺, HCO⁺₂, CnHm 43, 45, ...

Organic (Hydrocarbon) CⁿHm CnHm+ 27, 29,41, 43, 55, 57, 69, 71, ...

and are continually updated based on new laboratory studies emanating from the AMS community (Allan et al., 2004b). A summary from Canagaratna et al. (2007) of the key mass fragments used in the identification of aerosol species is shown in Table 2.1.

The general degradation of the performance of the mass spectrometer with time re-quires correction in order to perform quantitative measurements (Allan et al., 2003b).

This degradation in performance results in the reduction of the signal magnitude de-tected during ion events. Such degradation will be uniform for all parent species present in the aerosol sample. A correction factor is applied based upon the relative strength of the ion signal at m/z 28 and 32, the mass spectrum peaks for nitrogen and oxygen respectively. These gas phase constituents, referred to as the ‘air beam’, are assumed to be constant in the ambient environment. Consequently, any reduction in signal strength with time at these two signals is a consequence of instrumental performance. Further-more, the volumetric flow rate into the instrument is also included in the correction, as this can vary with time; especially when the AMS is located on board an aircraft.

Consequently, a time resolved correction factor is applied across all recorded signals.

A further complicating factor regarding quantitative measurement of aerosol mass loadings detected by the AMS is the Collection Efficiency (CE) of the instrument. This quantity is the detected fractional mass compared to the mass which impacts upon the volatising heater (Huffman et al., 2005). It has been shown to be dependent upon the species of the aerosol sample, its mixing state and also the ambient environmental conditions, specifically the relative humidity. Internally mixed aerosol products will be subject to the same CE. Thus in such circumstances, the same CE would be pre-scribed to aerosol species such as sulphate, nitrate, ammonium and organics (Alfarra et al., 2004). For the opposite case of an external mixture, a species and size specific CE is required (Weimer et al., 2006). Currently, losses due to the CE of the AMS are

prescribed to particle bounce off the vaporiser (Quinn et al., 2006). Laboratory stud-ies have indicated that the phase of a particle is the major controlling factor, which determines the particle bounce off the vaporiser (Matthew et al., 2008). Particles com-posed primarily of ammonium nitrate or acidic sulphates, existing in a liquid state, were shown to be sampled with a CE close to 100%. Conversely, dry and solid am-monium sulphate particles were sampled with a CE of 24±3%. Crosier et al. (2007a) developed a linear CE correction based on observed changes in collection efficiency due to changes in ambient nitrate content. The dependence of a particle’s CE upon the ambient relative humidity has also been shown (Allan et al., 2004a). This study showed that the mass concentration of sulphate approximately doubled when the inlet temperature approached the dew point temperature i.e. at high relative humidity. Of-ten the CE is either deduced via comparison with collocated instruments (Canagaratna et al., 2007, and references therein) or using the parameterised treatments presented by Crosier et al. (2007a) or Matthew et al. (2008).