Laboratory evaluation of organic aerosol relative ionization efficiencies in the aerodyne aerosol mass spectrometer and aerosol chemical speciation monitor

Abstract Organic aerosol (OA) mass concentrations measured by the Aerodyne Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Species Monitor (ACSM) depend on particle relative ionization efficiency (RIEOA). Here, a series of laboratory experiments were conducted to investigate how RIEOA differs for different classes of OA and between AMS and ACSM instruments. For OA surrogates with high oxidation state ( ), the measured RIEOA is 1.60 ± 0.56 for all instruments. However, for OA surrogates with lower the absolute magnitude and the variability of the measured RIEOA increases. The increase in RIEOA is different between the AMS and ACSMs due to differences in m/z-dependent ion transmission/detection efficiencies in the mass spectrometers. A new metric is introduced to explore RIEOA—the fractional contribution of ions at less than or equal to m/z 50 (fΣm/z50 ). When fΣm/z50 is high (>64%, similar to ambient OA), the average RIEOA is 1.23 ± 0.62. This supports the use of a default RIEOA of 1.4 for typical ambient conditions, where secondary OA dominates the OA budget. When fΣm/z50 is less than 64%, the RIEOA increases as higher m/z ions contribute more to the total signal. These observations reflect the fact that the observed RIEOA is a combination of multiple processes. The fΣm/z50 may be used as a metric to determine if primary-like OA is contributing significantly to total OA. Overall, the results indicate that changes in RIEOA are most important for studies measuring primary-like OA, and the magnitude of the changes will depend on the instrument being used to measure the OA.


Introduction
Atmospheric particles, or aerosol, play important roles in the earth system, impacting human health, climate, and air quality (Cohen et al. 2017;Monks et al. 2009;Myhre et al. 2013). An important component of aerosol is organic aerosol, or OA (Chen et al. 2022;Jimenez et al. 2009;Zhang et al. 2007). The exact chemical structure and physicochemical properties of OA vary depending on source and whether it was primarily emitted or produced by secondary processes such as gas-phase photochemistry or condensed-phase aqueous chemistry (Hallquist et al. 2009;Moise, Flores, and Rudich 2015;Rudich, Donahue, and Mentel 2007). Thus, measurements to determine the OA sources and chemical transformations are important to improve our understanding of aerosol.
The Aerodyne Aerosol Mass Spectrometer (AMS; Fr€ ohlich et al. 2013;Ng et al. 2011b). However, there are some key differences between the AMS and ACSM. The main differences between the AMS and ACSM include differences in how air background is subtracted (use of particle time-of-flight with AMS and use of particle filter with ACSM as there is no particle time-of-flight in ACSM), detector, and simplified electronics which reduces the overall complexity of the ACSM compared to the AMS (Fr€ ohlich et al. 2013;Ng et al. 2011b). However, the general technique for sampling aerosol is similar with the AMS and ACSM. Briefly, ambient aerosol is sampled through an aerodynamic lens (Liu et al. 1995a(Liu et al. , 1995b and focused into a narrow beam. Then, the aerosol is evaporated using a heated surface ($600 C to vaporize non-refractory aerosol) and ionized by 70 eV electrons Jimenez et al. 2003;Jayne et al. 2000). The ions are then extracted into a mass spectrometer to be measured. The aerosol mass concentration of any given species, C s (lg m À3 ), can be obtained from the measured mass spectrum signal using the following equation: Here, MW NO3 is the molecular weight of species nitrate as nitrate (g mol À1 ) is used as the main calibrant for AMS and ACSM (Jayne et al. 2000), Q is the volumetric flow rate (cm 3 s À1 ), N A is Avogadro's number (molecules mol À1 ), I s,i is the ion rate of each ion, i, for species, s (ions s À1 ), 10 12 is for unit conversion (g to lg and cm À3 to m À3 ), IE NO3 is the nitratedependent ionization efficiency (ion molec À1 ), RIE s is the species-dependent relative ionization efficiency (unitless), and CE s is the collection efficiency, a unitless factor that accounts decreased focusing efficiency due to non-spherical shape, the shape factor of the aerosol (E S ), the transmission of the aerosol through the lens (E L ), and the bounce of the aerosol on the vaporizer (E B ) (Huffman et al. 2005;Middlebrook et al. 2012). The IE used for conversion to mass concentration is based on using ammonium nitrate particles, as it is a well-characterized and well-behaved aerosol (Jayne et al. 2000). To convert from ammonium nitrate equivalent mass concentration to the species specific mass concentration, a relative ionization efficiency for each species (RIE s ) is used and defined as the ratio of the IE for each species, s, to that of nitrate (Alfrarra 2004;Canagaratna et al. 2007;Jimenez et al. 2003). For inorganic species, including ammonium, sulfate, and chloride, their individual RIE can be calibrated with lab-generated aerosol with no matrix effects (Nault et al. 2018;Xu et al. 2017). However, because OA found in the atmosphere is composed of 100s-1000s of different compounds with different physicochemical properties (Hallquist et al. 2009;Moise, Flores, and Rudich 2015;Rudich, Donahue, and Mentel 2007), a calibration for the OA RIE (RIE OA ) from a single OA standard is not possible. Instead, several standards would be necessary to explore the potential range of values for RIE OA .
There are many processes controlling RIE OA (Jimenez et al. 2003(Jimenez et al. , 2016Murphy 2016). In addition to the ionization process itself, the measured value of RIE accounts for multiple processes leading up to detection of the OA ions. Thermal decomposition of oxidized OA via dehydration and decarboxylation prior to ionization of the gases is one well known process Moldoveanu 2018): This leads to detection of CO 2 , H 2 O, CO, and R 0 . Carbon-Carbon (C-C) bond cleavage to form methyl or ethyl radical compounds plus an unsaturated hydrocarbon may also occur at these temperatures (Rice 1931(Rice , 1933Rice, Johnston, and Evering 1932). For example, the following products from thermal decomposition of n-butane have been observed and calculated (Rice 1931(Rice , 1933: CH 3 CH 2 CH 2 CH 3 þR À !RHþCH 3 CH 2 CH 2 CH À 2 (R6) RHþCH 3 CH 2 CH 2 CH À 2 !RHþCH 2 ¼CH 2 þCH 3 CH À 2 (R7) CH 3 CH 2 CH 2 CH 3 þR À !RHþCH 3 CHðCH 2 CH 3 Þ À (R8) Here, R can be CH 3 , CH 3 CH 2 , or H. For larger saturated hydrocarbons, including branched hydrocarbons, even more products were observed (Rice 1933). Note, though possible for the vaporization conditions of the AMS and ACSM, the thermal decomposition of saturated hydrocarbons (R3-R9) has not been investigated for the AMS and ACSM. After thermal decomposition, the molecules undergo ionization. As the ionization cross section is approximately proportional to the molecule's molecular weight (Jimenez et al. 2003), the ionization efficiency will depend on the molecular weight of the thermally decomposed OA. Further, the ionization cross section is also related to the oxidation state (OSc ¼ H/C À 2 Â O/C; Kroll et al. 2011) of the thermally decomposed OA, with more reduced molecules having larger electron ionization cross sections than more oxidized molecules (Jimenez et al. 2003). However, as shown in Xu et al. (2018), there is not a clear trend in the RIE OA with OSc : It has been hypothesized that the molecular weight of the thermally decomposed OA compounds will dictate how long the molecule will remain in the ionization region and thus affect RIE OA (Murphy 2016). However, ambient observations of OA have shown minimal impact for high molecular weight OA impacting the quantification of total OA mass concentration (Jimenez et al. 2016). Finally, after ionization, other processes that may impact RIE include ion transmission and detection Jimenez et al. 2003). As demonstrated here, the complexity of processes that lead up to detection of OA species makes a priori predictions of RIE OA difficult. Thus, direct measurements provide the best method to determine RIE OA . Laboratory measurements of commercially available oxidized OA surrogates indicate an average RIE OA of 1.4 Jimenez et al. 2003Jimenez et al. , 2016Xu et al. 2018). This RIE OA leads to generally good agreement between the AMS or ACSM and other measurements sampling ambient aerosol (e.g., DeCarlo et al. 2008;Hayes et al. 2013;Nault et al. 2018). Recent observations of directly emitted OA, or primary OA (POA), have indicated higher RIE OA than the default value of 1.4 (Katz et al. 2021;Reyes-Villegas et al. 2018). Xu et al. (2018) investigated OA surrogates that ranged in molecular weight and OSc and found that RIE OA remained relatively constant for OSc representative of secondary OA (SOA) at 1.6 ± 0.5 and increased with decreasing OSc : This is in line with the studies that found higher RIE OA was needed for POA (Katz et al. 2021;Reyes-Villegas et al. 2018). However, Xu et al. (2018) observed high variability in RIE OA versus OSc , reducing the ability to provide guidance in what RIE OA to use with observations collected by AMS and ACSM. Further, Xu et al. (2018) only investigated RIE OA with an AMS and did not determine if what was observed on the AMS is applicable to the ACSM, and what role the other processes that can impact IE and/or RIE may have. Field measurements, e.g., Katz et al. (2021), suggest that RIE OA for POA may be quite different between the two types of instruments.
This study expands upon the work from Xu et al. (2018) to investigate RIE OA measurements for both AMS and ACSM instruments and to provide guidance on how to calibrate or estimate RIE OA . An important aspect of this study is the investigation of the ion transmission efficiency for different versions of AMS and ACSM that use different mass spectrometers, and how this impacts the measured RIE OA . We propose a simplified method to calibrate RIE OA that reduces the need for equipment used in Xu et al. (2018) while maintaining high reproducibility. We also investigate a new metric for estimating RIE OA self-consistently for all AMS and ACSM systems when calibration is not possible.

Instrumentation
In this study, seven instrument versions with different mass spectrometers were investigated. These included the Aerodyne Quadrupole Aerosol Mass Spectrometer . For all instruments, the ionization cage is nearly identical thus should not impact the interpretation of the RIE OA and ion transmission. Also, the standard vaporizer was used for all instruments (Joo et al. 2021). The temperature for the AMS vaporizer was verified using sodium nitrate, as discussed in Williams (2010). All were used to measure ion transmission, and the Q-AMS, Q-ACSM, and TOF-ACSM were used to measure RIE OA of single component OA surrogates and OA surrogates mixed with ammonium nitrate. All instruments were equipped with the standard vaporizer and PM 1 lens (Joo et al. 2021). A differential mobility analyzer (DMA, TSI 3080, Shoreview, MN, USA), DMA column (TSI 3081), (ultrafine) condensation particle counter (CPC TSI 3775 and UCPC TSI 3776), centrifugal particle mass analyzer (Cambustion CPMA; Olfert and Collings 2005), and an atomizer (TSI 3076) were used for the calibrations.
Besides, the measurements conducted in this study, measurements of RIE OA from other instruments from different years (Xu et al. 2018) and groups (Katz et al. 2021;Slowik et al. 2004) are also included. Comparisons of measurements in this study with those from prior studies highlight minimal role in potential slight differences in vaporizer temperature and instrument tuning in the measured RIE OA .

OA surrogates
A list of OA surrogates used for calibrations is given in Table 1. The surrogates were used without further purification, as they were !90% purity. All compounds except, anthracene and palmitic acid, are from Sigma-Aldrich; anthracene and palmitic acid are from Fluka. The solvents used for the solutions were either deionized water or acetonitrile (HPLC Plus, !99.9%, Sigma-Aldrich). Acetonitrile was selected as the organic solvent as it consistently showed minimal signal (<0.1 mg m À3 OA) when atomized compared to other solvents. No corrections were made for the OA standards as there was minimal change in the mass spectra for standards atomized in water versus atomized in acetonitrile ( Figure S1). Ammonium nitrate was used for IE calibrations and as an internal standard. Similar to recommendations for ammonium nitrate calibrations, for the atomizer used (TSI 3076), the solutions used here were at 0.01 M or less, or less than 1000 particles cm À3 at 300 nm. This ensures that there were minimal doubly-charged particles, which impacts the comparison in the mass calculated from the CPC and mass observed by the AMS. An example particle time-of-flight measurement for two of the solutions is shown in Figure S2 to show minimal doubly-charged particles.
Some of the solutions used here were mixtures of OA and ammonium nitrate. This provided a means to eliminate the need to measure CE, reducing the complexity of the experiment. Similar to the pure compounds, the total concentration of the solution should be 0.01 M or less, or less than 1000 particles cm À3 at 300 nm, for TSI 3076 atomizer. Approximately equal molar amount of OA standard and ammonium nitrate were dissolved in either water or acetonitrile, depending on OA standard's ability to dissolve in water. Note, only non-acidic compounds could be mixed with ammonium nitrate, as discussed in Xu et al. (2018).

Laboratory measurement of OA RIE
The procedure for the RIE calibration has been discussed in prior studies (e.g., Xu et al. 2018). The general premise is that a known concentration of the calibrant (M in ) is introduced into the AMS or ACSM. The known concentration can either be derived from a CPC, using known mobility diameter from a DMA and density from the calibrant, or by mass selection with a CPMA (Olfert and Collings 2005;Xu et al. 2018). For simplicity and reproducibility for most laboratory environments, only the CPC and DMA were used for these measurements. Furthermore, as discussed in SI (Section S1 and Figure S3), the use of the CPMA to quantify volatile aerosol mass can lead to uncertainty. The CPMA was only used to determine whether the mixture of ammonium nitrate with OA surrogates were internally or externally mixed. To derive M in (mg m À3 ), the following equation was used: where d m is the mobility diameter from the DMA (nm), 10 À7 is the conversion factor from nm to cm, q is the density of the OA standard (g cm À3 ), N is the particle concentration (cm À3 ), and 10 12 is the conversion factor between g to mg and cm À3 to m À3 . M in is then used as to determine the RIE for OA standard as follows: where all other variables are the same as Equation (1). RIE NO3 is 1.1 for AMS and 1.05 for ACSM Middlebrook, Bahreini, and Jimenez 2007). Here, we assume that the particles are spherical and that the particle density is equal to the material density. In Equation (3), it is not possible to separate RIE S from CE S without additional information. We measured CE directly, using the Q-AMS in particle timeof-flight operation mode. The single particles can be directly counted using a "single particle" detection mode. In this mode, the number of single particle events for which AMS ion signal exceeded a threshold signal were counted and ratioed to the number of particles observed by the CPC. This ratio corresponds to a direct measure of the collection efficiency due to the particle bounce: Here, N PToF and N CPC are the particle number concentrations observed from the AMS and CPC, respectively. Compounds found to have CE $1 for the Q-AMS were used as single components on other instruments that cannot measure single particles (ACSMs). Some OA surrogates (non-acidic ones; Xu et al. 2018) were mixed with ammonium nitrate to reduce particle bounce, as particles with high ammonium nitrate content have an E B $ 1. Ammonium nitrate also provides an internal standard to directly determine the RIE OA . Using the CPMA, it was confirmed that ammonium nitrate was internally mixed with most of the OA surrogates ( Figure  S4). These mixtures were used on ACSMs to minimize uncertainty in CE. To determine the RIE S for the OA standard in mixture, first, the ammonium nitrate mass concentration measured on the AMS/ACSM is converted to the particle concentration (cm À3 ) using Equation (2) and the known density (1.72 g cm À3 ) and ammonium nitrate correction factor (0.8) (Jayne et al. 2000). Then, the ammonium nitrate particle concentration is subtracted from the total CPC particle concentration to derive the OA standard particle concentration. Finally, Equations (2) and (3) are used with the OA standard particle concentration to derive the RIE S for the OA standard, with CE S ¼ 1.

Ion transmission measurements
To measure the dependency of ion transmission on m/z, an effusive source of naphthalene was introduced into the detection chamber, similar to the Q-ACSM (Ng et al. 2011b). The naphthalene was contained in a stainless-steel bulb with a 1-2 mm pin hole leak. The naphthalene source was located in the ionization chamber for all instruments. The naphthalene spectra were obtained on all the instruments with the inlet closed, ensuring that no external aerosol was influencing the signals at the naphthalene ion fragments. The instruments were operated with inlet closed for at least 24 h prior to sampling naphthalene to ensure OA background had been reduced.
The spectra were analyzed by normalizing the signal at each m/z to the signal at m/z 51. Then, the normalized signal was compared to the normalized NIST spectra for naphthalene (Wallace 2022). Finally, any signal that was equal to less than 1% of the parent ion (m/z 128) from the NIST spectra was removed, and groups of closely spaced ions within 5 amu were averaged together (Ng et al. 2011b).

RIE OA versus oxidation state
The analysis from Xu et al. (2018) was replicated and expanded with a range of laboratory-generated OA standards (Figure 1 and S5). This included examining standards in different solvents (water versus acetonitrile), mixtures (pure versus with ammonium nitrate), instruments, and lower OSc compounds. As in Xu et al. (2018), for compounds with OSc similar to more oxidized organic aerosol (OOA) and more background-like OA (rural and remote), the RIE OA shows less variability (mean and one standard deviation is 1.60 ± 0.56 and median is 1.73). However, as OSc decreases to values less than À0.75, similar to more urban-like OOA and hydrocarbon-like primary OA (cooking OA and hydrocarbonlike OA, COA and HOA, respectively), there is large variability with RIE OA versus OSc : This strongly suggests that though OSc explains some of the variability in RIE OA , there are numerous other processes impacting RIE OA . Even for the same compound, there are large differences in measured RIE OA , indicating the difficulty and potentially higher uncertainty associated with measuring the RIE OA for the most reduced OA. Further, measurements of the OSc of ambient aerosol require high-resolution mass spectrometers, which the Q-ACSM, TOF-ACSM, and older AMS measurements (e.g., Q-AMS and CTOF-AMS) lack. Thus, the use of other metrics is desirable to determine the RIE OA .

RIE OA comparison between different methods and instruments
Xu et al. (2018) showed that measurements of internally mixed ammonium nitrate/OA particles were useful for accurately determining the RIE OA of nonacidic OA standards. In these measurements, mixtures were used to explore simpler methods of measuring RIE OA that do not require the use of a CPMA. Ammonium nitrate is used as an internal standard and the measured OA loading is compared to the ratio of OA/NO 3 expected from the calibration solution. This method eliminates the need for the CPMA as the mass does not need to be known a priori to measure the RIE OA. Moreover, internal mixing with ammonium nitrate leads to the aerosol having minimal bounce, reducing the uncertainty in CE and need to measure CE. Figure 2 compares the RIE OA for the five SOA-like standards-levoglucosan, xylitol, 1,2,6hexanetriol, mannitol, and threitol-both pure and mixed with ammonium nitrate. The generally low variability (2-13%, Figure 2) observed between these measurements confirms the utility of using internally mixed non-acidic ammonium nitrate/OA particles for these measurements.
To further investigate the utility of this method, a mass scan with a CPMA at a constant DMA diameter was used to evaluate whether the aerosol particles generated from the mixed OA/ammonium nitrate solution were internally mixed or not. Four different OA standards mixed with ammonium nitrate were measured by CPC and LTOF-AMS. As shown in   Figure S4, for SOA-like standards (mannitol and 1,2,6-hexanetriol), the scans show that the OA and ammonium nitrate are internally mixed. This provides further evidence that ammonium nitrate can be mixed with SOA-like, non-acidic, OA standards to measure their RIE without a CPMA. However, for POA-like standards (tetradecane and anthracene), the results are less clear as tetradecane is internally mixed, but anthracene is not internally mixed. Thus, the use of internally mixed particles containing ammonium nitrate does not reduce uncertainties in the measurements of RIE OA for reduced OA. The scans with POA-like standards and ammonium nitrate may partially be impacted by partial evaporation of either component that may occur in the CPMA due to their volatility (e.g., Huffman et al. 2009) (see Section S1 and Figure S3).
The effects of different solvents were also investigated since water does not dissolve many OA standards. Using a high purity solvent, e.g., acetonitrile, that fully dissolves different OA standards reduces the need for complex systems, such as custom-built evaporation-condensation generators (Sinclair and La Mer 1949;Xu et al. 2018). High purity acetonitrile was used as the solvent for standards that are immiscible in water. Figure 2 shows that there was minimal variability across the pure compounds dissolved in water versus compounds dissolved in acetonitrile (8-10% variability). This indicates that there were minimal differences in the interactions between the very polar water and somewhat polar acetonitrile as solvents. Thus, it is expected that there would be similarly minimal interactions with the POA-like OA standards with acetonitrile. Furthermore, Figure 2 shows that there was minimal variability between OA standards mixed with ammonium nitrate, no matter solvent (9-15% variability). This provides more evidence that there was minimal solvent interaction with the OA standard. Taken together, these observations indicate that internal mixtures with ammonium nitrate and different solvents (acetonitrile or H 2 O) can be used as alternative and simple methods for accurately measuring RIE OA across a wide range of SOA-like standards.
Comparisons of the RIE OA measured using the Q-AMS versus the Q-ACSM and TOF-ACSM show two different trends that depend on OA composition (Table 1 and Figure 2). First, for the SOA-like standards (levoglucosan, xylitol, 1,2,6-hexantriol, mannitol, and threitol), the three instruments (Q-AMS, Q-ACSM, and TOF-ACSM) agreed within 25%. However, for POA-like standards, such as oleic acid, squalane, and pentadecanoic acid, the Q-ACSM and TOF-ACSM show large differences in RIE OA when compared with the AMS. The largest differences in RIE OA between both versions ACSM and Q-AMS instruments occur for low OSc , which is generally associated with measurements of direct and/or fresh emissions of primary particles, and not background OA (e.g., urban background, rural, and remote), where the OA has higher OSc :

Differences in ion transmission across AMSs and ACSMs impacts RIE OA
The mass spectra of POA-like standards generally have a larger fraction of the total signal at higher m/z than those of SOA-like standards. Thus, how sensitive the instruments are to the higher m/z may play an important role in the RIE OA . This can be explicitly seen in Equation (3), as lower sum of total ions would lead to lower RIE s . Since the largest differences between AMS and ACSM RIE OA occur with OA standards that have higher fractional total ion signals at higher m/z, the ion transmission, T m/z , was investigated across instruments. The T m/z has been measured for the Q-ACSM (Ng et al. 2011b); however, it has not been for other ACSMs and AMSs. As discussed in Jimenez et al. (2003), T m/z affects RIE OA , which makes a priori calculations of RIE OA difficult. However, T m/z was not considered once the HTOF was introduced, as discussed in DeCarlo et al. (2006). In Figure 3, the ion transmission of naphthalene fragments is shown for the Q-ACSM, TOF-ACSM, TOF-ACSM-X, CTOF-AMS/m-AMS, HTOF-AMS, and LTOF-AMS. For this analysis, a linear fit is currently assumed for T m/z and that the ion transmission goes to 0 at the x-intercept for each individual instrument, as shown in Figure 3. As there is no data for m/z > 128 (parent ion of naphthalene), it cannot be ruled out that the T m/z is non-linear. Further, there is plenty of evidence from previous publications that non-zero intensities for ions greater than the x-intercept for the HTOF-and LTOF-AMS have been observed (e.g., Dzepina et al. 2007;Malmborg et al. 2017;Ulbrich et al. 2022a). Thus, the results presented for the remainder should be considered upper limits in T m/z corrections. The m/z value at which the transmission starts decreasing is lowest for the Q-ACSM (starts at $ m/z 51) and highest for the HTOF-and LTOF-AMSs (starts at $ m/z 80). Further, how steeply the ion transmission decays show similar values for the ACSMs and CTOF-AMS but a slower decay for the HTOF-and LTOF-AMSs.
One potential complication interpreting the observed differences in ion transmissions measured across the different instrument versions is the ion extractor and the ion detector. The ion extractor, which is $46 mm long, is used in Tofwerk TOFs and is used to transfer the ions from the ionization region into the TOF (Drewnick et al. 2005). The ions are guided and made into a parallel beam into the extractor. The ions drift through the extractor at $50 eV before the ions are orthogonally extracted into the TOF by a pulsed voltage. Both the TOF-ACSM-X and HTOF-AMS have similar ion extractor and TOF technology; however, the detector is different (multichannel plate, MCP, for AMS and ETP MagneTOF detector for TOF-ACSM-X). The different ion detectors used in the range of mass spectrometers in this study can exhibit different mass dependent ion detection efficiencies. The apparent lower ion transmission may be associated with the ETP MagneTOF detector detecting high m/z ions less efficiently than the MCP. This, however, does not explain the differences between the CTOF-and HTOF-AMS, as both use MCPs. Though the ion extractor is similar between the CTOF-and HTOF-AMS, the size of the drift tube is different and will affect ion transmission. Thus, a combination of factors that are associated with the TOF and detector impact the apparent T m/z and complicate the ability to predict RIE OA . The following discussion about T m/z results should be interpreted as an effective ion transmission, which convolves all potential processes leading to ion detection efficiency.
The effect of differences in T m/z on measured mass is explored with example POA spectra found in the AMS database (Ulbrich et al. , 2022a(Ulbrich et al. , 2022b. POA mass spectra generally have more ions at higher m/z (Figure 4) than SOA mass spectra ( Figure 5) due to the presence of molecular species that have not yet been photochemically processed (Donahue et al. 2012;Jimenez et al. 2009) and reduced by thermal decomposition . The cumulative distributions of observed ions, using the T m/z shown in Figure 3, show that 60-70% of the total ion signal are observed in the Q-ACSM while 80-90% of the ions are observed in the TOF-ACSM, TOF-ACSM-X, and CTOF-AMS (Figure 4). Since the Q-ACSM T m/z is routinely measured with an internal naphthalene standard, reported Q-ACSM mass concentrations typically include a correction for T m/z (Ng et al. 2011b). Thus, for cases where most of the ion signal is < m/z 150, this correction results in Q-ACSM mass concentration are nearly identical to the AMS. For POA dominated cases, however, Q-ACSM mass concentrations are systematically lower than AMS because unmeasured Q-ACSM ion signal above m/z 150 (due to near-zero ion transmission) cannot be corrected for with T m/z. This is further explored in Figure S6. The mass spectra of oleic acid from the Q-AMS and Q-ACSM are compared, as well as the uncorrected and corrected T m/z for the Q-ACSM is shown. There is less ion signal in the uncorrected than the corrected (e.g., uncorrected has 6% less ion signal without including T m/z ). However, both uncorrected and corrected T m/z do not fully account for all the ion signal of oleic acid. The uncorrected signal of oleic acid from the Q-ACSM is 73% of the signal observed on the Q-AMS, and the T m/z corrected signal from the Q-ACMS is 78% of the signal observed on the Q-AMS. Thus, the RIE OA of oleic acid measured on the Q-ACSM is lower than measured on the Q-AMS Yet, the difference in oleic acid RIE OA is larger between the Q-AMS and Q-ACSM than the 22% difference in T m/z (Table  1), indicating other unknown processes are impacting RIE OA for POA-like OA surrogates measured on the Q-ACSM.
For the SOA, all instruments generally observe a larger fraction of the total ions (>75%, Figure 5). Note, the number of ions differs between fresher SOA (less oxidized, oxidized OA, LO-OOA) and more aged SOA (more oxidized, oxidized OA, MO-OOA). A  Wallace (2022). Each m/z was normalized to the signal at m/z 51 and clusters of ions were averaged. Lines shown are the fit through the points. Note, only signals greater than 1% of the parent ion signal are shown (Ng et al. 2011b).
combination of factors could be influencing the larger number of high m/z ions in the LO-OOA spectrum, including the fact that the data is from the average of 15 urban studies (Ng et al. 2011a) in which LO-OOA may not be fully separated from POA in the positive matrix factorization (PMF), and/or the fact that the extent of aging for the LO-OOA may be different between campaigns.
For studies that occur in sites away from direct emissions (e.g., urban background sites, rural sites, and remote sites), the differences in ion transmission are expected to have minimal impact on the mass measured between different instrument versions, leading to good agreement between AMS and ACSM results Fr€ ohlich et al. 2015).
However, for studies investigating point sources (e.g., indoor emissions; Katz et al. 2021) or less aged SOA, differences in the apparent T m/z and how it impacts RIE OA may need to be accounted for in comparisons between different AMS and ACSM instruments.

Evaluation of laboratory measurements
Because a priori knowledge and/or measurements for molecular weight and OSc are not feasible for the AMSs and ACSMs, another way to constrain RIE OA would be of value. Further, as discussed above, OSc does not appear to fully explain the measured RIE OA .  (Ulbrich et al. 2022b(Ulbrich et al. , 2022a. The different lines in (b), (d), (f), and (h) represent the cumulative ions observed for Q-ACSM (short-dashed), TOF-ACSM, TOF-ACSM-X, and CTOF-AMS (long-dashed), and HTOF-and LTOF-AMS (solid line). Note, this is an extreme example as ambient aerosol is rarely composed mostly of POA. Further, this is calculated without Q-ACSM T m/z accounted for, which leads to lower total signal observed. Final processed observations from the Q-ACSM will have total cumulative signal similar to the HTOF and LTOF AMS with T m/z included.
Since, as discussed above, RIE OA can differ between AMS and ACSM instruments due to T m/z dependence, we investigated the use of the fractional contributions of ions at high m/z's compared to total ions observed to parameterize RIE OA . RIE OA as a function of the fractional contribution of ions less than m/z 50, hereafter called f Rm/z50 , is shown in Figure 6. We use m/z 50 because ion transmission is near 100% for all instruments (Figure 3). Further, as shown in Figure  S7, when parameterizing RIE OA as a function of the fractional contribution of ions, a cutoff of m/z 50 provides the best fit (maximum in R 2 and a minimum in chi squared) to observations. For the AMS data, there were two sets of populations. For standards with f Rm/z50 ! 64%, the mean (median) RIE OA was 1.23 ± 0.62 (1.18). The standards sampled with the ACSMs for these compounds fall within the spread in the RIE OA measured with the AMS, in agreement with Figure 2. As the standards studied here have similar functionalities to SOA, it is expected that they will undergo thermal decomposition to CO 2 , H 2 O, CO, and/or smaller molecules (R1-R2). This thermal decomposition prior to electron ionization appears to lead to the RIE OA of 1.23 ± 0.62 for high f Rm/z50 standards.
For OA standards measured on the Q-AMS with f Rm/z50 < 64%, there was an increasing trend between RIE OA and f Rm/z50 upwards. The trend indicates that for each 1% decrease in f Rm/z50 , the RIE OA increases by $0.07 (absolute value). Compounds in this region generally have long-chain saturated hydrocarbon backbones with lower thermal decomposition than the standards more similar to SOA. However, as discussed above, some thermal decomposition may occur for saturated hydrocarbon backbones (Rice 1931(Rice , 1933Rice, Johnston, and Evering 1932) during thermal decomposition of attached oxygenated functional groups (R1-R2). Measured RIE OA across different AMS instruments and for most standards fall within or very near the 95% interval of the fit, with important exceptions of PAHs and saturated hydrocarbons. Q-ACSM measurements also fall outside this range as discussed below.
PAHs are generally resistant to fragmentation after electron ionization due to the stabilizing effects of the delocalized p-electrons (Dzepina et al. 2007; McLafferty and Ture cek 2021); thus, it is not surprising that the PAHs show the lowest f Rm/z50 values. Also, unlike non-aromatic hydrocarbon that may undergo thermal decomposition at $600 C, PAHs undergo thermal decomposition at much higher temperatures (>1000 C) (e.g., Tsai et al. 2002). Similar to other studies (Dzepina et al. 2007;Jimenez et al. 2016;Slowik et al. 2004), a low RIE OA was observed for the PAH standards, similar to the low RIE OA for lubricating oil with PAHs. PAHs generally have lower cross sections than compounds of similar carbon number (Bose and Westmoreland 2020; Harrison et al. 1966), Figure 5. Example mass spectra of (a) LO-OOA and (c) MO-OOA from PMF analysis of ambient datasets. Spectra taken from the AMS Database (Ulbrich et al. 2022b). Similar to Figure 4, for (b) and (d), the different lines represent the cumulative ions observed for Q-ACSM (short-dashed), TOF-ACSM, TOF-ACSM-X, and CTOF-AMS (long-dashed), and HTOF-and LTOF-AMS (solid line). Note, this is calculated without Q-ACSM T m/z accounted for, which leads to lower total signal observed. Final processed observations from the Q-ACSM will have total cumulative signal similar to the HTOF and LTOF AMS with T m/z included.
potentially suggesting a lower RIE OA compared to other compounds. However, unlike other compounds, the cross sections for PAHs can vary widely for the same number of carbon atoms or same number of fused rings (Bose and Westmoreland 2020), again indicating that predicting the RIE OA for these species is challenging. Furthermore, minimal fragmentation for the high molecular weight PAHs may lead to lower overall observation due to the T m/z being <100% at the m/z of the parent compounds ( Figure  3). Thus, the combination of these processes is thought to lead to the deviation of RIE OA versus f Rm/z50 . However, PAHs are generally a small fraction of the total OA signal (less than 1%) (e.g., Dzepina et al. 2007); so, this deviation from the observed trend of RIE OA versus f Rm/z50 will not impact interpretation of most ambient measurements. Finally, this deviation is also observed for aromatic-containing standards, as the one point from Xu et al. (2018) well below the uncertainty in the trend of RIE OA versus f Rm/z50 is phthalic acid. The contribution of pure aromatic backbone SOA compounds to ambient OA is not as well established, to date; thus, the impact of aromatic containing SOA compounds to RIE OA is unknown.
Lubricating oil was also investigated in this study because it may be another potentially important contribution to HOA (e.g., Worton et al. 2014). For non-PAH containing lubricating oil, RIE OA versus f Rm/z50 follows the same trend in RIE OA (point labeled Engine Oil in Figure 6). Slowik et al. (2004) and Jimenez et al. (2016) reported lubricating oil to have an RIE OA $1.4, but the lubricating oil studied in Slowik et al. (2004) was composed of PAHs, which generally has lower RIE OA ( Figure 6). Thus, while different from the non-PAH lubricating oil measurements, the RIE OA reported in Slowik et al. (2004) is consistent with the PAH standard results reported here and do not follow the trend of RIE OA versus f Rm/z50 . Saturated hydrocarbons RIE OA show the largest differences between studies and measurements (Table 1). This may reflect a combination of processes, including how the standards were atomized (including partial evaporation with CPMA), the amount of thermal decomposition prior to ionization (R3-R9), and T m/z . RIE OA is highly variable for these standards. Since the volatility of saturated hydrocarbons is high (e.g., Lu, Zhao, and Robinson 2018), the contribution of saturated hydrocarbons to the HOA PMF factor is expected to be low. As prior studies have indicated that HOA and other POA contains oxygen and other functional groups, and the compounds measured in (Middle) f Rm/z50 for typical urban and rural/remote observations (Ulbrich et al. 2022a). (Bottom) Measured RIE of OA (RIE OA ) from this study for different instruments versus f Rm/z50 . The fit for the observations with f Rm/z50 < 0.64, does not include PAHs and the saturated hydrocarbons, and is only for the AMS. The shaded area is the 95% confidence interval to the fit. Data from Katz et al. (2021) is included to show that the derived fit explains the measured RIE observed in that study while the lower RIE OA from Slowik et al. (2004) is due to the PAHs in the lubricating oil. The slope for the fit is -14.46, intercept is 10.55, and R 2 is 0.53. this study that fall within the trend in Figure 6 also have oxygen functional groups, the deviation from the saturated hydrocarbons will have minimal influence on the interpretation of ambient measurements.
Similar to the AMS, the ACSMs measured the RIE OA for standards similar to SOA of $1.4 (e.g., f Rm/z50 > 0.64). However, for f Rm/z50 < 0.64, two different populations occur for the Q-ACSM and TOF-ACSM; Q-ACSM RIE OA remains constant between 1 and 2 while the TOF-ACSM shows a slower increase in RIE OA versus decreasing f Rm/z50 . As has been discussed throughout, the higher m/z ions appear to be contributing to the higher RIE OA . However, as discussed in Section 3.3 and shown in Figure 3, the ACSMs have lower T m/z than the AMS. Note, the Q-ACSM corrects for the fraction of ions not observed above m/z 50 with the measured T m/z (Ng et al. 2011b) and thus leads to similar total ions as observed with the AMS for cases where all ion signal is observed at m/z < 150. As shown in Figure 4, for many types of POA, 4-13% of the ions are found at m/z > 150. Since the transmission efficiency for the Q-ACSM is near zero for these higher m/z ions, these ions are not measured and thus not easily corrected for ( Figure 2). However, as the T m/z correction should only lead to a smaller difference than observed, other unknown processes are likely also impacting the differences between the observed RIE OA measured on the Q-ACSM and AMS. This could lead to the relatively constant RIE OA for the Q-ACSM at low f Rm/z50 . Since the TOF-ACSM detects a higher fraction of ions with m/z > 150 (Figure 3), the trend between the RIE OA versus f Rm/z50 is more apparent. The TOF-ACSM RIE OA is generally lower than that of the AMS due to the lower transmission efficiency of the TOF-ACSM compared to the AMS (Figures 3 and 6).

Comparison of the Improved Constraints on RIE OA with external dataset
Consistency between these measurements and an external dataset from Katz et al. (2021) further verifies the relationship between RIE OA and f Rm/z50 , as shown in Figure 6. As Katz et al. (2021) measured the RIE OA on a different AMS instrument and a separate technique, this provides confidence in the relationship to better constrain the RIE OA . Using the f Rm/z50 observed by Katz et al. (2021) for the HOMEChem COA observations (34%), the derived relationship shown in Figure 6 would suggest an RIE OA of 5.63 ± 1.17. This agrees with the average RIE OA during HOMEChem of 5.57 ± 3.71 at the 95% confidence interval. Also, using the f Rm/z50 for HOA and COA, the predicted RIE OA for the ambient measurements from Philadelphia (Avery, Waring, and DeCarlo 2019), reanalyzed in Katz et al. (2021), would be between 4.44 ± 1.17 and 5.49 ± 1.70, which is in agreement with the RIE OA calculated for the data set of 4.16. Similar to the trends observed in the analysis of OA standards with Q-ACSM and TOF-ACSM (e.g., Figure 2), Katz et al. (2021) observed differences in RIE OA agreement between the Q-and TOF-ACSMs and HTOF-AMS. The ACSMs were about a factor 2 lower than the HTOF-AMS and SMPSs. This is a similar factor difference for the measured oleic acid RIE OA observed between the AMS and Q-ACSM ( Figure 2). Due to T m/z corrections, though, it is expected that the Q-ACSM would only be lower than the factor of 2 observed. At this point, it is not apparent what is leading to the higher difference between the Q-ACSM and HTOF-AMS. Therefore, caution is recommended when measuring direct reduced OA sources with the Q-ACSM.

4.3.
Atmospheric implications for f Rm/z50 for the AMS and ACSM Figure 6 also shows the range of f Rm/z50 observed for bulk OA measurements from urban and rural/remote (Ulbrich et al. , 2022a(Ulbrich et al. , 2022b sites. Various urban studies show f Rm/z50 ranges from 0.56 to 0.68. The f Rm/z50 for urban sites is higher than pure POA and between LO-OOA and MO-OOA because urban sites are typically dominated by SOA ). Though some urban sites fall below f Rm/z50 < 64%, the uncertainty in the fit of RIE OA versus f Rm/z50 is still consistent with the use of an RIE OA of 1.4 for bulk OA. Away from urban regions, except for biomass burning events, the contribution of POA is minimal ), leading to the rural and remote mass spectra being dominated by ions less than m/z 50 ( Figure 6). Thus, the results in Figure 6 indicate that the RIE OA of 1.4 is applicable for urban background measurements that are not strongly impacted by fresh/local emissions. The f Rm/z50 for different POA factors fall in the same region where RIE OA increases for the OA standards. As POA factors contribute a small fraction of the bulk OA in urban environments ), their contribution is minor for RIE OA . However, when conducting source apportionment with PMF and/or directly sampling source aerosol, higher RIE OA will be necessary (Katz et al. 2021;Reyes-Villegas et al. 2018). This indicates primary-like OA reported with PMF may be an upper-estimate due to the higher RIE OA ; however, due to the potential competing effects stated before (particle lens transmission, internal versus external mixing), there may be offsetting effects that may reduce the overall amount the primary-OA is over-estimated. As shown in Jimenez et al. (2016), within the stated uncertainty of the AMS and Sunset OC instrument, there was minimal systematic difference between the two instruments even at high primary OA fractional contribution to total OA in an urban environment. An important caveat for implementing the observed trend in RIE OA versus f Rm/z50 at this time, though, for ambient data, is that the trend was derived with labgenerated OA standards. This means that there are no competing effects with particle lens transmission and/or internally versus externally mixed aerosol. The aerosol was size-selected to ensure 100% transmission, which may not be possible for primary aerosol in ambient conditions (Zhang et al. 2005a(Zhang et al. , 2005b. Also, the E B was well characterized either with counting of single particles on the Q-AMS or mixtures of ammonium nitrate and OA surrogates. Fresh POA generally is not well-mixed with rest of the non-refractory aerosol, resulting in not well characterized E B (Canagaratna et al. 2004;Zhang et al. 2005aZhang et al. , 2005b. As shown in Equation (1), OA mass concentration is a function of the product CE and RIE OA . POA CE and RIE OA may have off-setting effects on mass concentration. Thus, the parameterization shown in Figure 6 may result in incorrect RIE OA as this parameterization does not take E L and E B into account. However, f Rm/z50 will provide a metric to determine if a higher RIE OA will be needed for the OA. If the observed f Rm/z50 is < 0.64, this indicates that the OA with RIE OA ¼ 1.4 would be an upper limit of the OA mass concentration.

Conclusions
As organic aerosol, OA, is an important fraction of particulate matter, accurate quantification of its mass concentration is important. Here, the organic aerosol relative ionization efficiency, RIE OA , was measured on the Aerodyne Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Speciation Monitor (ACSM) (involving a total of 7 different mass spectrometers) to further evaluate what controls the RIE OA and how the different instruments respond to OA standards. Although the OA oxidation state explains some of the differences in measured RIE OA for different standards, there is also large variability in the RIE OA for a given OA oxidation state. In this study, a new metric was developed to investigate RIE OA for the AMS and ACSM, and the results from this study are summarized below.
1. The RIE OA for different OA surrogates were measured on the Q-AMS, Q-ACSM, and TOF-ACSM. It was found that for SOA-like standards, the RIE OA agreed across the three instruments; however, for POA-like standards, there were differences in the measured RIE OA with the Q-AMS having higher RIE OA than the TOF-ACSM (intermediate) and Q-ACSM (lowest). 2. Further, there were minimal differences in the measured RIE OA when different solvents were used (water versus acetonitrile) and in comparing pure OA-standard to a mixture of an OA-standard with ammonium nitrate. The use of ammonium nitrate alleviates the need to measure the collection efficiency and to directly measure the RIE OA . 3. Some of the differences in the RIE OA can be partially explained due to differences in the dependency of the ion transmission on m/z (T m/z ). This dependency is most important for the ACSMs measuring fresh POA emissions. 4. The fractional contribution of the total ion signal for ions less than m/z 50, which is denoted f Rm/z50 , provides a more robust method to predict how RIE OA varies across OA standards. 5. The metric f Rm/z50 indicates that for bulk urban aerosol away from fresh, direct POA emissions, rural OA, and remote OA, the RIE OA is 1.23 ± 0.62, in agreement with the value of 1.4. This metric can be used to either determine if bulk OA or OA factors from positive matrix factorization may need a higher RIE OA and may be over-estimated. 6. The metric and measured RIE OA found in this study is applicable to prior measurements using different AMSs, providing overall confidence in this metric's sensitivity to the tuning and vaporizer temperature. 7. The metric also assists in explaining the differences in RIE OA observed between polycyclic aromatic hydrocarbons and other POA-like OA. 8. Finally, the metric shows how the ACSMs measured RIE OA behaves differently than the AMS measured RIE OA. 9. However, it is important to keep the caveat in mind that there may be competing factors impacting the quantification of POA aerosol, including particle lens transmission and/or internally versus externally mixed aerosol.
10. At this time, it is still encouraged to report POA mass concentration from PMF and source apportionment studies with RIE OA ¼ 1.4 as an upper-limit, the f Rm/z50 for the PMF or direct OA measurement, and the potentially lower limit of the OA mass concentration with the caveat about particle lens transmission and mixing state of OA.
As the newly proposed behavior of RIE OA versus f Rm/z50 has only been investigated for laboratory standards and a subset of published observations, further evaluation of this trend needs to be conducted on ambient measurements before it can be widely used to correct AMS and ACSM measurements.