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 measured cabin temperatures. Particles lose water in the aerodynamic lens, thus we typically 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 AMS users follow this procedure.

Should instrument response time be taken into account in intercomparisons?

• In aircraft 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.

Is there a glossary of AMS jargon I don't understand?

• Yes! The AMS is a complex instrument, and as a result we have created a large set of AMS-specific words and acronyms. This glossary is a work in progress, so please edit and/or send us questions on it!

Concentration FAQs

Are AMS concentrations always reported under STP or ambient conditions?

• This depends on the particular study and group. All papers reporting μg 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.

• Note that sometimes an AMS User may not pay attention to this detail, so that the group generating the date doesn't even know how they are reporting. In that case it boils down to what the flowmeter used to calibrate the AMS flow reports, which can be either standard (with more than one definition, see below) or ambient at the location where the calibration was performed (which often is not the same location as where the measurements were performed).

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

What are μg sm-3?

• μg sm^-3 are μg 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 μg sm^-3 ppb^-1, but not if μg 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.

Is it normal that there are some negative concentrations?

• YES, THIS IS NORMAL. Any instrument has a certain amount of noise, and when measuring a zero concentration, equal numbers of positive and negative measurements should cancel out to provide an average of zero. When the concentrations are near detection limit, a significant fraction of the measurements may be negative. This fraction should be 50% in filtered air or extremely clean air, otherwise the instrument is not working properly. It is very important to NOT REMOVE the negative measurements from the analysis, as this would bias the averages.

• Unfortunately this has proven confusing for many people, especially those familiar with chromatographic techniques where the convention is to report small numbers as "below detection limit." However that practice can result in a bias, and the quantification of averages at low concentrations is better if the negative numbers are kept.

Are the AMS concentrations of sulfate (nitrate) reported as μg SO₄ m^-3, or μg S m^-3 (or NO₃ vs N; or OC vs OM)?

• 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.

Why do you write SO₄ & NO₃ and not SO₄^2- & NO₃^-?

• In the aerosol community people often don't add the charges because it is understood that those symbols refer to aerosol nitrate and sulfate.

• For the AMS, we also detect some signal as part of the SO₄ & NO₃ "species" which may arise from organonitrates and organosulfates, which are not ions (see e.g. Farmer et al., PNAS 2010), thus it may not always be proper to use the charges.

Organics FAQs

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. ES&T 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. E.g. Docherty et al., ACPD 2011 estimate that the N/C of OA in Riverside, California was underestimated by a factor of 2, due to the presence of organic nitrates.

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

• The AMS does detect some organic ions that contain S and thus an S/C atomic ratio can be calculated. This is thought to be accurate for species with C-S bonds like sulfonic acids, but to be a large underestimate for organosulfates (C-O-S bonding) due to the decomposition of those species into fragments that are normally lumped into the "sulfate" AMS species (see Farmer et al., PNAS, 2010). Docherty et al., ACPD 2011 estimate that the S/C of OA in Riverside, California was underestimated by a factor of 30, due to the presence of organosulfates.

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).

• Note that detection limits scale with the inverse of the square root of the averaging time. So if e.g. 10 min detection limits are provided, but you are interested in the 2 hr detection limits, those will be sqrt(10/120) ~ 0.29 x the 10-min detection limits.

• 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., JGR, 2009. The authors determined mass concentration accuracies of order 35% for an aircraft dataset collected in the polluted Texas environment. For ground based studies the accuracy is often somewhat better.

• 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 for the definition). It is important to note the relationship and approach to the determination of E_b that is taken for any given AMS data set. Generally CE ~ 0.5 works well for most ambient particles, and deviations from that rule can be estimated from the composition of the aerosol (see Middlebrook et al., AS&T, 2011 for further details).

• 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?

• The AMS measures vacuum aerodynamic diameters (d_va, see DeCarlo et al. AS&T 2004), which is proportional to particle density, and decreases with increasing dynamic shape factor for non-spherical particles. Note that d_va is DIFFERENT from the traditional definition of aerodynamic diameter (d_a) 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 B 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 compensates the decrease in particle density. This also assumes that the particles stay spherical after drying.

• For other situations, the theoretical formulations of DeCarlo et al. (2004) can be used to predict the changes of d_va due to water uptake or drying.

What is the size cut of AMS measurements?

• The transmission of particles through the AMS aerodynamic lens changes as a function of size, and is dependent both on the geometry of the critical orifice upstream of the lens, the lens in question, and the lens pressure.

• Transmission through the standard AMS lens into a collimated beam that reaches the vapouriser falls to zero at around 35 nm d_va going to smaller sizes (Zhang et al., ES&T 2004), although it depends on the particular instrument and lens pressure.

• 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., AS&T 2007; Takegawa et al., AS&T 2008; 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 d_va) (e.g. Canagaratna et al., 2007). This size cut corresponds to slightly smaller particles than the 1 μm cut in transition-regime aerodynamic diameter (d_ta) 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 g cm⁻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 d_va corresponds to a PM_0.9 cut in d_ta. 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 d_va corresponds to 0.7 μm physical diameter [for spherical particles] 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 used to compare with the AMS measurements."