FAQ for AMS Data Users

The purpose of this page is to serve as a repository of answers to frequently asked questions from users of AMS data products. Contributions are most welcome, and should be directed to the curators of the page (JL Jimenez and M Cubison as of June 2009).

General FAQs

Are particles dry or wet when sampled in the AMS?

On aircraft the airflow into the AMS is typically not dried, but RH was typically low because of the heating from ram and heat transfer in the cabin. An estimate of the actual humidity just before the AMS can be derived from the outside RH and temperature and the reported cabin temperatures. Particles lose water in the aerodynamic lens, thus we do not report concentrations of particle phase water.

On ground-based studies it is highly recommended to dry the particles using a steel-encased nafion dryer to reduce variations in collection efficiency arising from particles that retain significant water in the AMS (e.g. Allan et al., JGR 2004). However not all users follow this procedure.

Should instrument response time be taken into account in intercomparisons?

In comparisons with instruments that have a longer tail in response time (such as those based on ion-chromatography), the AMS data may need to be degraded with a one-sided function empirically-determined from the analysis of the response of both instruments to rapid and large changes in concentration.

Concentration FAQs

Are AMS concentrations always reported under STP or ambient conditions?

This depends on the particular study and group. All papers reporting ug m-3 of aerosol should state the basis of the m-3 in the experimental section. Unfortunately this is not always the case, and the generators of the data should then be contacted.

To avoid confusion, we recommend the use of the unit ug sm-3 (see below) for concentrations reported under STP conditions, and of ug am-3 for concentrations reported under ambient conditions.

What are ug sm-3?

ug sm-3 are ug m-3 converted to standard pressure and temperature (STP) conditions of 1013 mbar, 273K without any evaporation/condensation. These units are very useful when comparing data taken over a range of pressure and temperatures, such as aircraft data. They are also very useful when comparing aircraft data to data from ground sites, or data from different ground sites which have different ambient pressures and/or temperatures.

Ratios to gas-phase species (e.g. OOA/Ox) are constant when expressed in ug sm-3 ppb-1, but not if ug m-3 are used.

Note that unfortunately there are many other definitions of STP, many of which are compiled in this Wikipedia article. Some of them are also used in the aerosol field, so be aware of these possible differences.

Are the AMS concentrations of sulfate (nitrate, organic) reported as ug SO4 m-3, or ug S m-3 (NO3 vs N, OA, vs C)?

The AMS measures the concentrations of SO4 (not only S), NO3 (not only N) and the organic mass (not only the OC or C). For SO4 and NO3 the conversion is straightforward, while to calculate OC one needs to know the OM/OC ratio.

Organics Questions

Is the AMS "organics" the same as "OC" or "organic carbon" measured with other instruments and reported by some models?

"Organics" correspond to organic aerosol mass (often referred to as OM or OA). It includes OC (organic carbon), as well other atoms (H,O,N) in organic species. See Aiken et al. (Environ. Sci. Technol. 2008) for details.

How come you report both "organics" and "OC" in some datasets?

The high resolution AMS can measure the OM/OC which is needed for this conversion (OC = organics / (OM/OC) ), as described in Aiken et al. (Environ. Sci. Technol. 2008). Thus in some cases we report both "organics" (which is the same as "OM" or "OA") and OC.

Does the AMS underestimate organic nitrates?

An instrument can't underestimate something that it doesn't report! The response of the AMS to organic nitrates is being studied in detail by several groups, but it appears that at least some of the nitrogen in these species appears in the NO+ and NO2+ ions that are grouped as part of "nitrate" by the "ambient" version of the fragmentation table (as described by Allan et al., J. Aerosol Sci. 2004). It is the AMS Users' responsibility to use additional information from their experiments to do better than the default frag table, if possible.

Can the AMS measure the N/C of ambient organic aerosol?

This can only be done with the W-mode of the HR-ToF-AMS, as the other instruments / modes do not have enough resolution to separate these ions, and their signals are low for ambient aerosols (see Aiken et al. 2008)). Even with W-mode data, this is quite difficult to do and requires very good instrument tuning and utmost care in the high-resolution fitting and data analysis. An analysis may produce good O/C and H/C ratios but bad N/C ratios, as O and H are much more abundant and not sensitive to fitting errors to the same degree as N-containing fragments are.

For lab or chamber experiments, fog water, etc. in which the N/C is much higher than for ambient aerosols, this problem may not be as important.

For the time being the N/C will be somewhat underestimated in the presence of organic nitrates. On-going work aims to calibrate for this effect.

Detection Limits, Accuracy, and Precision FAQs

How are the detection limits estimated?

Detection limits (DL) are typically estimated from measurements with the normal AMS modes while sampling through a total particle filter. Typical detection limits are given in Table 2 of DeCarlo et al. (Anal. Chem., 78: 8281-8289, 2006).

Recently, Drewnick et al. (Atmos. Meas. Tech., 2, 33–46, 2009) detailed a continuous method of estimating DLs from the variability in the background signals for different species. This is particularly useful where the time-series data has been collected before the instrument has had chance to reach equilibrium in terms of both chamber pressure and electronic stability. Typically these influences are most prevalent in airbourne data, where the AMS is typically only spun-up a few hours before take-off. In addition, the influence of background ions is shown by Drewnick et al. to increase significantly when sampling large mass concentrations, and it according takes some time for these concentrations, and the associated rises in DLs, to fall back to their previous levels.

Do the detection limits change with the averaging time?

Yes, detection limits change a lot with the averaging time used, and they are not meaningful unless the averaging time is also reported. From previous results, the DLs scale as the inverse sqrt of the averaging time.

Continuous DL estimates can be derived with the method of Drewnick et al. (Atmos. Meas. Tech., 2, 33-46, 2009).

What are the accuracy and precision of AMS concentration measurements?

The overall accuracy of an AMS mass concentration measurement for a given species is a convolution of many different uncertainties. However, many of these are statistically-independent and thus, when added in quadrature the larger errors dominate the overall uncertainty. A useful table summarising the principal sources of uncertainty is given in the supplementary information for Bahreini et al., J. Geophys. Res., doi:10.1029/2008JD011493, in press. The authors determined mass concentration accuracies of order 35% for a dataset collected in the polluted Texas environment.

Without light-scattering information, the AMS mass accuracy is generally dominated by uncertainity in the particle collection efficiency due to particle bounce (E_b, see Huffman et al., Aerosol Sci Technol. 39, 1143-1163, 2005). It is important to note the relationship and approach to the determination of E_b that is taken for any given AMS data set.

The AMS mass concentration precision at low concentrations is the same as the detection limits. These have historically been defined by considering the variability in measurements during filter periods, and more recently using the continuous approach detailed by Drewnick et al., Atmos. Meas. Tech., 2, 33–46, 2009. Precision at higher concentrations is much better than the accuracy.

Sizing FAQs

What is the diameter measured by the AMS?

These are vacuum aerodynamic diameters (d_va, see DeCarlo et al. Aerosol Sci. Technol. 38, 1185-1205, 2004), which are proportional to particle density, and decrease with increasing shape factor for non-spherical particles. Note that this is DIFFERENT from the traditional definition of aerodynamic diameter which is proportional to the square root of the particle density and also has a different dependence on particle shape factor (see DeCarlo et al. 2004 for details).

Does the measured diameter change if the particles are dry or wet?

This depends on the particle density. As shown in Appendix 2 of DeCarlo et al. (2004) for mixed organic/inorganic ambient particles with densities around 1.5 g cm-3, the effect of adding water on the particle dva is surprisingly small. This is by chance, as the increase in particle diameter closely compensate the decrease in particle density.

What is the size cut of AMS measurements?

The transmission of particles through the AMS aerodynamic lens changes greatly as a function of size, and is dependent both on the geometry of the lens in question and the upstream pressure. Transmission through the standard AMS lens into a collimated beam that reaches the vapouriser falls to zero at around 35 nm (dva) going to smaller sizes. At the large end, the decline in transmission is not as sharp, falling off at around a micron, depending on operating pressure. Hence the AMS is often referred to as a "near"-PM1 instrument. Details on the aerodynamic lens and/or transmission curves can be found in Jayne et al., AS&T 2000; Zhang et al., AS&T 2004; Liu et al., Aerosol Sci. Technol. 71, 721-733, 2007; Bahreini et al. AS&T 2008; Bahreini et al. JGR 2009. There is some variation between specific aerodynamic lenses in different instruments as evidenced by comparing those papers.

If a particular AMS is operated at too low lens pressure the transmission of larger particles can be seriously degraded (see Zhang et al. AS&T 2004 and Bahreini et al. AS&T 2008). Another reason for lower than normal transmission is poor alignment of the aerodynamic lens, which has been a problem for some groups and campaigns.

What size range should be used to compare models or other measurements to the AMS?

The following paragraph from Fast et al. (ACPD 2009) summarizes this issue well:

"The size cut of the particles that can be measured by the AMS is reported to be 1 μm in vacuum aerodynamic diameter (PM1 in dva) (e.g. Canagaratna et al., 2007). This size cut corresponds to slightly smaller particles than the 1 μm cut in transition-regime aerodynamic diameter (dta) that is typically used to define PM1 ambient measurements using cyclone or impactor inlets operated at ambient pressure, with the exact correspondence being dependent on ambient pressure and on particle density and shape and thus composition (DeCarlo et al., 2004). For example, for the average density of 1.4 gcm−3 calculated from the chemical composition measurements at T0 or CENICA [in Mexico City] (Aiken et al., 2008a; Salcedo et al., 2006) and the pressure of Mexico City, a PM1 cut in dva corresponds to a PM0.9 cut in dta. There can be some variation in individual aerodynamic lenses as well, which in some cases lead to smaller size cuts (Liu et al.,2007). The PM1 cut in dva corresponds to 0.7 μm physical diameter under the average conditions in Mexico City. Therefore, only predicted organic aerosols from the four size bins [in the model] below 0.7 μm were to compare with the AMS measurements."