Thermal dissociation cavity ring-down spectrometer (TD-CRDS) for detection of organic nitrates in gas and particle phase

. A thermal dissociation – cavity ring-down spectrometer (TD-CRDS) was built to measure NO 2 , peroxy nitrates (PNs), alkyl nitrates (ANs), and HNO 3 in the gas and particle phase. The detection limit of the TD-CRDS is 0.66 ppb for ANs, PNs, and HNO 3 and 0.48 ppb for NO 2 . For all four classes of NO y , the time resolution for separate gas and particle measurements is 8 minutes and for total gas + particle measurements is 3 minutes. The accuracy of the TD-CRDS was tested by comparison of NO 2 measurements with a chemiluminescent NOx monitor, and aerosol-phase ANs with an Aerosol Mass 15 Spectrometer (AMS). N 2 O 5 causes significant interference in the PNs and ANs channel under high oxidant concentration chamber conditions, and ozone pyrolysis causes a negative interference in the HNO 3 channel. Both interferences can be quantified and corrected for, but must be considered when using TD techniques for measurements of organic nitrates. This instrument has been successfully deployed for chamber measurements at widely varying concentrations, as well as ambient measurements of NO y . three ovens. Temperature ramp results were used to identify the correct setpoints for each oven to achieve 185 complete dissociation for each species. Both HNO 3 and AN measurements were performed by flowing zero air over a pure liquid analyte sample. The AN standard used was isobutyl nitrate (Aldrich 96% purity). A pure liquid sample of PN could not be obtained, so a NO 3 + Δ-carene mixture containing PNs was synthesized in the chamber as described above. Because concentrations were very stable, ramps were performed at 5 ºC/min. Normalized measured NO 2 concentrations are plotted against temperature in Figure 4 because absolute concentrations were different for each class of nitrate. 190 Complete dissociation of PNs occurred at a thermocouple temperature reading of 130 ºC, ANs at 385 ºC, and HNO 3 at 600ºC. The HNO 3 oven setpoint was chosen to be 700°C to allow the quantification of interference from NO 3 dissociated from N 2 O 5 in that channel. At 600°C, HNO 3 is completely dissociated, but there is only partial conversion of NO 3 to NO 2 , creating an interference in the hot channel in the TD-CRDS. At 700°C, HNO 3 is completely dissociated and NO 3 is completely converted to NO 2 . Note that the dissociation plateaus do not overlap with the beginning of the adjacent curve, confirming the 195 ability to quantitatively separate nitrate species by temperature.


Introduction
Nitrogen oxide based functional groups are an area of significant interest in atmospheric oxidative chemistry. Organic nitrates are formed through reactions between (oxidized) volatile organic compounds (VOCs), of which the global majority are biogenic in origin (Seinfeld and Pankow 2003;Perring, Pusede, and Cohen 2013) and NOx (=NO+NO2) or NO3 (Ng et al. 2017), which is predominantly anthropogenic in origin (Seinfeld and Pandis 2006). The two major organic nitrate products of 25 these reactions are alkyl nitrates (ANs) of the form RONO2 and peroxy nitrates (PNs) of the form ROONO2. These organic nitrates play an important role in regulating ozone in the troposphere by serving as temporary reservoirs of NO2 (Buhr et al. 1990; Thornton et al. 2002). Equilibrium partitioning of high molecular weight, low volatility organic molecules occurs, causing some organics to condense onto existing particles (Jimenez et al. 2009). These secondary organic aerosols (SOA) consist primarily of the highly oxidized products of VOC + oxidant reactions, because of their increased molecular weight and 30 higher polarity. Lower night-time temperatures decrease volatility even further, leading to increased partitioning into the particle phase (Fry et al. 2013). Warmer temperatures, deposition, and chemistry within the particles change the equilibrium, resulting in the release of NO2. Because of long residence times of SOA, significant quantities of NO2 can be transported away 1 shows that the thermal dissociation of each class of organic nitrates results in one NO2 and a hydrocarbon-containing X group.

XNO2 + ∆ → X + NO2
(R1) where X = RO2, RC(O)OO, RO,or OH. 45 PNs serve as a temporary reservoir of NO2 in the atmosphere, because the equilibrium between formation and dissociation is rapid. For example: has a Keq of 2.2 x 10 -12 cm 3 molecules -1 , resulting in a PN lifetime at 20 ppb NO2 of 0.56 seconds at 298K and 1 bar (Atkinson et al. 2006;JPL Data Evaluation 2015). In contrast, ANs and HNO3 predominantly serve as sinks of NO2, with spatial transport 50 scales that depend on their meteorology-dependent deposition lifetimes (Horowitz et al. 2007).
Previous studies of organic nitrates have been done by measuring specific nitrates (Wolfe et al. 2007;Horowitz et al. 2007;Parrish and Fehsenfeld 2000;Surratt et al. 2006;Lee et al. 2016) or by looking at the sum of nitrates using thermal dissociation NO2 measurements (Zellweger et al. 1999;Day et al. 2002;Hargrove and Zhang 2008;Paul, Furgeson, and Osthoff 2009;Rollins et al. 2010;Sobanski et al. 2016). The instrument described in this paper has drawn on aspects of three different thermal 55 dissociation nitrate measurement strategies in the literature. The general oven and flow plan was based on the thermal dissociation-laser induced fluorescence (TD-LIF) instrument built by the Cohen group at UC Berkeley (Day et al. 2002).
Instead of LIF, the NO2 detection device in the instrument described here is a commercial cavity ring-down spectrometer (CRDS). Once interferences are characterized and absorption cross-sections are known, CRDS does not require in-line calibration by an authentic standard gas cylinder during sample measurement, as discussed in Paul et al. (Paul,Furgeson,and 60 Osthoff 2009). Gas-particle partitioning measurements using a switchable charcoal denuder was incorporated from Rollins et al. (Rollins et al. 2010).
The benefit of using CRDS over chemiluminescence (CL) detection of NO2 is its selectivity. The (partial) thermal dissociation of multiple unstable nitrate compounds like ANs, PNs, and N2O5 into NO2 by the CL heating process and molybdenum catalyst has been well documented (Wooldridge et al. 2010). CRDS can make direct measurements of NO2, unlike CL, which uses a 65 metal catalyst to turn NO2 into NO and back-calculates NO2 concentration by subtraction. CRDS does not require heating or a catalyst, and is therefore more selective. LIF can be tuned to a specific spectroscopic transition like CRDS, but cavity length https://doi.org/10.5194/amt-2020-280 Preprint. Discussion started: 20 July 2020 c Author(s) 2020. CC BY 4.0 License.
becomes limiting for measurement of low concentrations, and requires delicately aligned multipass optical cells to achieve low limits of detection for NO2. The downsides of CRDS come from the expense and delicateness of the instrument.
Since high molecular weight oxidation products can condense into the particle phase, it is valuable to be able to make both gas 70 and particle phase measurements. Denuders work by using diffusion to separate gases from liquid-or solid-phase particles.
Higher diffusion rates for gases means that they are more readily absorbed into the walls of a charcoal denuder, leaving behind the particle phase. The fraction of gas removed depends on residence time in the denuder and the surface area available to diffusing gas molecules. The diffusion coefficient of NO2 is reported to be 0.154 cm 2 s -1 (Williams et al. 2012) and 0.070 cm 2 s -1 for n-propyl nitrate (Paul, Furgeson, and Osthoff 2009). According to previous studies using charcoal denuders, the denuder 75 removed the majority of particles with diameters <0.1 μm (Glasius et al. 1999) as well as all semivolatile organic gases.

Instrument design
Nitrogen In order to measure concentrations of organic nitrates by thermal dissociation, a multi-channel, switchable, controllable heating inlet system was constructed. This heating unit was then attached to a cavity ring-down NO2 detector (CRDS, Los Gatos Research Inc. Model #907-0009) to complete the instrument. An overall instrument schematic is shown in 80 Figure 1a.
The three quartz tube ovens were constructed out of 55 cm long, 3.8 mm inner diameter (ID), 7.0 mm outer diameter (OD) quartz tube wrapped in 15 cm nichrome wire (2mm wide ribbon with resistivity of 11 Ω/m) located 5 cm from one end. An 18 cm long, 8 mm ID, 10 mm OD quartz tube was slipped around the nichrome section to hold it in place. Over the 8mm ID tubing, two 8 cm long, 10.5 mm ID, 13 mm OD quartz tubes were placed with a thermocouple in between to hold the end of 85 the thermocouple in place. The whole heated section was wrapped in ½" thick ceramic insulation (McMaster-Carr # 9379K92) with foil coating, as shown in Figure 1b. It is important to note that the heat capacity of the oven is determined by the effectiveness of the insulation. Insufficient insulation can result in unstable oven temperatures or increase the time required for the gas to reach the required dissociation temperature, leading to increased sampling times on each oven and a degradation of the time resolution of the instrument. 90 The thermocouple was placed so it was the same distance and glass thickness from the nichrome as the nichrome was from the gas flow, so it was hypothesized that the thermocouple temperature reading would be representative of the internal temperature of the oven. Experiments comparing this external thermocouple to a thermocouple placed at the same position inside the gas flow showed that the average internal oven temperature was between 25 and 30 o C hotter than the external thermocouple reading. Because of the oven design, the temperature inside the heated portion of the oven is not uniform, but is hottest closest 95 to the end of the nichrome section, nearer to the exhaust.
The unheated portion of the quartz tubing used in this instrument is significantly shorter in length than the length originally calculated in Day et al. (2002) due to additional testing reported in Paul et al. (2009) (Paul, Furgeson, and Osthoff 2009). The shorter length was chosen to suppress the recombination reaction of NO2 radical with the organic sister product upon cooling. https://doi.org/10.5194/amt-2020-280 Preprint. Discussion started: 20 July 2020 c Author(s) 2020. CC BY 4.0 License.
The shorter ovens were shown to effectively reduce residence time, and therefore recombination, but still allowed adequate 100 time for gas cooling before entering the sampling chamber. A length of 55 cm was calculated from Equation 1 using the flow rate determined by the LGR CRDS (q = 2.5 lpm) and length of the shorter oven (h = 64 cm) reported in Paul et al., in order to give our instrument equal residence times (τ, see Eq. 1). Since the flow rate of the LGR CRDS is significantly smaller (1.2 lpm), the required tube length is shorter.
These ovens were attached to ¼" Teflon tubing with Teflon Swagelok tees and unions. Teflon connectors were chosen over stainless steel to reduce destruction of NO2 by heated steel (Hargrove and Zhang 2008). An oven-length piece of ¼" Teflon is used as the ambient temperature background NO2 channel, which has a typical temperature of 22 -24º C inside the inlet box.
The three ovens and background channel connect to a six-port solenoid valve with Teflon wetted surfaces. The outlet of the solenoid valve runs to the inlet of the LGR CRDS. 110 The inlet of the instrument has two possible pre-oven pathways: denuded and undenuded. The denuder is a 45 cm long cylinder of activated charcoal with a ¼" channel through the center. A three-way, Teflon-wetted solenoid directs the inlet air either through the denuder or through an equivalent length of Teflon tubing before the air sample enters the ovens.
An Omega CN616TC1 Temperature Controller was used to regulate the temperature of the ovens. The inlet end of the oven nichrome wire was attached to the positive terminal of the Mouser 24VDC power supply and the exhaust end was wired to a 115 Mouser DR06D12 solid state relay. These relays received signals from the temperature controller, either allowing or prohibiting current flow through the nichrome wire by completing the circuit loop. The temperature controller was able to detect the temperature of the ovens using K-type thermocouples. The desired temperatures were set using the CN616 Software provided with the temperature controller. Experiments showed that a single 24V power supply did not provide enough current to heat the Channel 1 oven to an appropriate temperature, so a second 24V power supply was used to supply power to Channel 120 1. This succeeded in getting the oven as high as 820 o C; the typical temperature setpoint was 700 o C.
Valve switching was controlled by a Measurement Computing (MCC) USB-ERB08 relay module. Each solenoid was soldered to a diode to prevent damage from voltage spikes generated by switching. These leads were then connected to the normally closed (NC) ports of the MCC relay unit, which completed the circuit to open the specified valve.
One limitation of the TDCRDS instrument is its reliance on a single detector. This necessitates sequential measurements of 125 each relevant species, creating a minimum time resolution for the instrument. This minimum time resolution can be large compared to the rate of change of the measured species in the atmosphere or in a chamber experiment. Concentration changes are accounted for by assuming a linear change in each channel between two consecutive samplings of that channel. This simplifying assumption only holds if the time between samplings is relatively short. The goal is to minimize the instrument time resolution by minimizing the sampling time of each oven without introducing error caused by mixing analyte in the tubing 130 between the switching valve and the CRDS sample cell. Plausible channel switching rates between 30 and 90 seconds were tested to measure the stabilization time of each channel. This testing was conducted by flowing 10 sccm of zero air through a three-necked round bottom flask containing 0.2 ml of isobutyl nitrate (IBN) chilled to -21 o C. This 10 sccm flow was diluted with 7.25 lpm of zero air to achieve a concentration of ~ 700 ppb. Figure S2 shows the NO2 vs time curve for the TD-CRDS when gas was sampled with various lengths of time in each oven. 135 The high concentration peaks are when the IBN dilution is flowing through Channels 1 and 2 (650 o C and 385 o C, respectively) and the troughs are IBN flowing through Channels 3 and 4 (120 o C and ambient 23 o C, respectively). For this application, 45second channel time yielded the best trade-off between channel stabilization and time resolution.
Channel timing with the denuder was determined in a similar manner, and it was determined that 1 minute per channel was necessary to achieve stabilization with the charcoal denuder. This leads to an 8 minute complete cycle time, since there is 1 140 minute for denuded and 1 minutes undenuded on each of the four species channels. The last 3 measured points in each channel period are averaged to obtain the concentrations that are used for each channel. A full cycle in this gas / aerosol mode is shown in Figure 2.
For each full cycle of concentration measurements from the eight channels, the concentrations of the individual classes of NOy The "Total" concentrations in Eqs. 2 refer to gas + aerosol phase, to obtain gas-phase only concentrations, the aerosol can be 150 subtracted from the total for each channel.

Determination of NO2 sensitivity
Two sets of tests were performed to verify the sensitivity of the LGR CRDS to NO2. Response at high concentrations was verified at concentrations of 250 to 1000 ppb using dilutions of NO2 in zero air. A 514.5 ppm calibrated mixture of NO2 in N2 155 (Airgas) was diluted with a zero air source to generate the required mixing ratios. Response at low concentrations was compared to a Thermo chemiluminescent NOx detector between 1.5 to 11.5 ppb. Low concentration NO2 was obtained using ambient lab air diluted using zero air. The results of these experiments are shown in Figure 3.
The fit line has a slope of close to 1 over both measured ranges, indicating good agreement under both high and low concentration conditions. These experiments suggest an upper limit error due to the NO2 detection of 10%. The intercept offset 160 of the low concentration experiment is 0.64 ppb, which may be attributable to the interference of organic nitrates in the chemiluminescent measurement, since this is an ambient measurement. Thus, the CRDS is accurate under both atmospherically relevant and elevated laboratory experiment conditions, but regular calibration against a known source or comparison with another NO2 measurement is nevertheless recommended.

Production of alkyl nitrates and peroxy nitrates using the Reed Environmental Chamber (REC) for TD-CRDS 165 instrument characterization
The 400 L Teflon bag Reed Environmental Chamber (REC, Draper et al. 2015) was used to generate VOC + NO3 reaction products that could be analyzed using the TD-CRDS. The REC chamber was operated with steady inlet flows to the top of the chamber of zero air (4.3 lpm), O3 (200 sccm), NO2 (4.4 sccm of 515 ppm), and VOC (14.2 sccm zero air through chilled liquid source containing gas-phase VOC of ~100 ppm), which mix and react (average residence time ~ 90 minutes) and are sampled 170 for analysis from the bottom of the chamber. Zero air was generated using a Sabio Model 1001 zero air generator, which removes water, particulates, and reactive gases. Ozone was generated using a UV light source (Pen-Ray Hg lamp at 254 nm) inside the middle neck of a three-necked round bottom flask, and the concentration was altered by adjusting the depth of the light source in the flask. The constant NO2 source was a gas cylinder (Airgas, concentration analyzed 4/17/2013) with a concentration of 514.5 (± 2%) ppm NO2 in N2. Approximately 300 ppb VOCs (typical VOCs used are ∆-carene, limonene, α-175 pinene or β-pinene) were generated by flowing zero air over a chilled liquid sample of VOC in a three-necked round bottom flask.
Ozone, zero air, and NO2 flows were allowed to stabilize inside the chamber prior to introducing VOC flow to initiate the experiment. All flows were then continuous until the completion of the experiment. Particle number and size data was collected using a Scanning Electron Mobility Sizing (SEMS, Brechtel Manufacturing, Inc.), connected via conductive silicone tubing to 180 minimize particle losses. Ozone concentration was measured using a Dasibi Model 1003-AH ozone monitor or Teledyne Model T400.

Determination of oven temperature setpoints
Temperature ramps were performed on different mixtures of known gases to determine the appropriate setpoint temperatures for each of the three ovens. Temperature ramp results were used to identify the correct setpoints for each oven to achieve 185 complete dissociation for each species. Both HNO3 and AN measurements were performed by flowing zero air over a pure liquid analyte sample. The AN standard used was isobutyl nitrate (Aldrich 96% purity). A pure liquid sample of PN could not be obtained, so a NO3 + Δ-carene mixture containing PNs was synthesized in the chamber as described above. Because concentrations were very stable, ramps were performed at 5 ºC/min. Normalized measured NO2 concentrations are plotted against temperature in Figure 4 because absolute concentrations were different for each class of nitrate. 190 Complete dissociation of PNs occurred at a thermocouple temperature reading of 130 ºC, ANs at 385 ºC, and HNO3 at 600ºC. The HNO3 oven setpoint was chosen to be 700°C to allow the quantification of interference from NO3 dissociated from N2O5 in that channel. At 600°C, HNO3 is completely dissociated, but there is only partial conversion of NO3 to NO2, creating an interference in the hot channel in the TD-CRDS. At 700°C, HNO3 is completely dissociated and NO3 is completely converted to NO2. Note that the dissociation plateaus do not overlap with the beginning of the adjacent curve, confirming the 195 ability to quantitatively separate nitrate species by temperature.

Quantification and treatment of N2O5 interference
High concentration Δ-carene nitrate oxidation experiments in the REC chamber typically had 650 ppb O3 and 400 ppb NO2.
When high concentrations of O3 and NO2 are present, they react in the chamber to form N2O5. This was verified by performing a temperature ramp from a chamber at low and high oxidant concentrations ( Figure 5). N2O5 dissociates across a broad 200 temperature range, in contrast to the sharp dissociation curves for peroxy-or alkyl-nitrates, such that the presence of N2O5 removes the clear plateau between PNs dissociation and ANs and give an interference in both PNs and ANs channels. A chamber with low NOx conditions (335 ppb O3 and 3.2 ppb NO2) that was left to equilibrate for 56 minutes after addition of Δ-carene gave a maximum N2O5 concentration of 1.6 ppb. The resulting temperature ramp gives the expected dissociation curve, showing both PNs and ANs plateaus ( Figure 5). In this case, there is good separation between PNs and ANs, because 205 N2O5 is lower in concentration. This N2O5 interference has been previously observed by Womack et al. 2017. Note that given the gradual dissociation of N2O5 across this full temperature range, the extent of the interference depends on the exact temperature setpoints, so any similar TD-based organonitrate instrument that may be operated in high-N2O5 conditions should characterize its individual N2O5 interference.
To measure the N2O5 interference such that it can be corrected for, we ran an experiment with only oxidants in the chamber 210 ( Figure 6). The TD-CRDS detects one NO2 molecule from the first dissociation of NO2 from N2O5 (Reaction 3) either in Oven 3 (the PNs channel) or in Oven 2 (the ANs channel), and another NO2 is observed when the released NO3 fragment further dissociates in the HNO3 channel (Reaction 4). We note that due to its high reactivity and wall losses (especially the NO3 fragment), the total N2O5 detection is substantially less than 100% of the N2O5 concentration present at the inlet. A kinetic model paired with measurements of NO2 and O3 in order to predict N2O5 can be used to quantify the interferences in each 215 channel for a given setup.
The result of N2O5 is an elevated baseline in each of the PNs, ANs, and HNO3 channels before the VOC is added. If an accurate N2O5 measurement is available, the interference from N2O5 (and NO3) can be subtracted out of each channel, and these pre-220 VOC injection signals can be used to assess the likely lower inlet transmission of N2O5 and NO3 vs. the more stable PNs, ANs, and HNO3. In the absence of a separate N2O5 measurement, kinetic modeling can be used to predict how much N2O5 will be formed in each experiment, which can then be subtracted out. For example, in chamber experiments, comparing modeled N2O5 amounts to the amounts of signal in the ANs, PANs, and HNO3 channels before VOC is added can quantify what fraction of N2O5 appears in each channel. If Reaction 3 happens across the PNs and ANs temperature range (130-385 °C), and Reaction 225 4 between the ANs and HNO3 range (385-700 °C), the sum of the signals from the ANs and PANs channels before addition of VOC should be equivalent to the N2O5 signal from the HNO3 channel (from the NO3 fragment of the N2O5 dissociating to NO2). For the instrument application shown here, operating at UC Irvine in September 2019, 7% of the modeled N2O5 produced https://doi.org/10.5194/amt-2020-280 Preprint. Discussion started: 20 July 2020 c Author(s) 2020. CC BY 4.0 License. based on a model constrained to measured NO2 and O3 is detected in the PNs channel at 150 °C, and 28% in the ANs channel at 385 °C. 230

Determination of denuder efficiency
The activated carbon denuder was tested for efficient removal of gas phase molecules by flowing gas mixtures of single molecules diluted in zero air through the denuder. Gas mixtures were tested at several concentrations to determine if efficiency was concentration-dependent. Transmission of the denuder is defined to be the percentage of gas-phase molecules that passed through the denuder and were detected downstream when all should have been removed. 235 NO2 transmission was tested in 2016 at two relatively low concentrations, to mimic atmospheric conditions, and one higher concentration to mimic chamber conditions. In all cases at this time, greater than 96% of the NO2 was absorbed (Table 1)  The same process was used to determine the transmission of isobutyl nitrate (an alkyl nitrate) in 2016 ( Table 2). The outlet of the denuder was connected to both Channel 1 (temporarily at 520°C) and Channel 2 (385°C). The transmission of the denuder 245 was not dependent on the concentration of gas in the original gas mixture, or on which oven was used. This ANs transmission was also re-tested in 2019, and in this case, no significant change in breakthrough was observed. These measured fractions of gas-phase breakthrough can be used to correct the aerosol measurements made in the denuded channels of the instrument cycle (see corrections discussion below). First, the chamber was hooked directly to the SEMS in order to get a background measure for the number of particles in the 255 bag. Then the chamber mixture was pulled through the TD-CRDS inlet tubing while bypassing the denuder in order to quantify particle losses to the tubing. Finally, the chamber mixture was sampled while flowing through the tubing and the denuder to get the total particle loss through the instrument. The time series of this experiment is shown in Figure 7. In order to quantify the efficiency of the denuder and tubing inlet of the TD-CRDS, the particle volume was averaged over the sampling time under each condition. This assumes particle concentration in the chamber was constant over the course of the entire experiment. 260 Since the chamber had been running for 24 hours prior to measurements, it is reasonable to assume that all concentrations had reached equilibrium.
A total of 28% of the aerosol particles (assessed by volume) that flow into the instrument were lost to the tubing and the denuder before detection. There does not appear to be any bias toward removing smaller or larger particles. The denuder is responsible for only 10% of total particle loss. This suggests that every deployment of this instrument should carefully consider 265 and if possible quantify inlet line losses.

Determination of detection limits
The CRDS can be set to zero automatically at regular intervals, which is accomplished by diverting inlet air through an NO2 scrubber. The re-zeroing procedure results in small changes to the baseline before and after zeroing events. On ambient measurements, these changes are typically less than 0.5 ppb (see Figure S3), and on zero air, typically less than 0.2 ppb, and 270 are sometimes positive and sometimes negative. We determine the standard deviation of four hours of zero measurements (0.16 ppb) to estimate our blank error, σzero.
From this observed blank error, the detection limit of the instrument (LOD = 3σ) can be calculated for each channel. The error for the NO2 channel is based only on σzero alone, since no subtraction is required (3σzero = 3 × 0.16 ppb = 0.48 ppb). For all other channels, the error in the subtracted value A -B is calculated as: 275 where σA = σB = 0.16 ppb are the errors in the pre-subtraction NO2 concentration measurements. Thus, the estimated detection limit for the subtracted channels (ANs, PNs, and HNO3), 3σA-B = 3 × 0.22 ppb = 0.66 ppb.

Kinetic modelling of thermal dissociation ovens
Modelling of the ovens can be employed to simulate the dissociation and recombination of the detected species in any oven 280 design. Pressure-and temperature-dependent rate constants for dissociation (Day et al. 2002) and recombination (JPL Data Evaluation 2015) reactions of PNs, ANs, and HNO3 were used (see Table S1), alongside an assumed (a) step function or (b) linear rate of cooling from the heated to the unheated portions of the oven (see Figure S4). We also included the IUPAC rate constant for a representative RO + O2 (7.2x10 -14 e -1080/T , IUPAC), and OH wall loss rate (46 s -1 ) from Knopf, Pöschl, and Shiraiwa 2015. Based on these rate constants and the assumption that recombination or wall losses are the only fates for 285 dissociated radicals, we found that the PNs measurement would be the most affected by recombination. We found an expected 10% difference in the amount of PNs recombined by the end of the PNs oven between assuming linear cooling and step function, so the more conservative step function assumption can be used to provide a lower limit concentration.
All ovens were modelled at their setpoint temperature, which is maintained by the thermocouple relay. However, each oven surely has gradients in temperature along its length, resulting in this average oven temperature measured at its midpoint (see 290 Figure 1b). As an example, for HNO3 there was very little difference in dissociation based on small changes in oven temperature. HNO3 is 100% dissociated at the end of the oven, so modelling at 30°C hotter than the setpoint temperature just extends the cooling region slightly. This leads to approximately 0.3% less recombination than the setpoint temperature model.
The same concept applies to the PNs oven, but the recombination difference is larger (1-2% depending on model molecule) due to a 30°C increase in temperature being a larger percentage of the total temperature. 295 The PN dissociation and recombination oven model results in Figure S5 show a predicted 55.9% dissociation in the step function model, and 65.4% dissociation in the linear function model with no background NO2 and 10 ppb initial PNs concentration.
In the PNs oven, the only important reactions modelled were the dissociation and recombination of PNs. The background concentration of NO2 was considered and was found to have a significant impact on the recombination rate, especially at high 300 concentrations. Figure S6 shows the percent PNs that remain dissociated at the detector as a function of initial concentrations of PNs and NO2. Two separate types of PN were considered in the modelling, due to their slightly different rates of recombination. Methyl PN gave 58.1% detection at 10 ppb initial concentration and no background NO2, while ethyl PN gave 55.9% detection under the same conditions. Given the relatively small difference in recombination percentages, no effort was made to incorporate both species into the model. Ethyl PN was chosen as the representative species because it was assumed 305 that most PNs being encountered would be two carbons or larger. Including reasonable atmospheric concentrations of OH (4x10 6 molecules /cm 3 ) in the model made no difference to the percent recombination of ethyl PNs, and was therefore left out.
We note that the PNs measurement will be most affected by recombination, but that this recombination can in principle be corrected for.
In the ANs oven, in addition to the major reactions of ANs dissociation and recombination, the reaction RO + O2 is also 310 important. The RO + O2 reaction is extremely fast at high temperatures (see Table S1) like those found in the heated portion of the AN oven, and we assume the reaction to be irreversible. Because O2 is abundant, the reaction negligibly affects O2 concentration. As a result, these assumptions give a model prediction of 100% detection of alkyl nitrates at all initial AN and NO2 concentrations.
In the HNO3 oven, in addition to the dissociation and recombination of HNO3, the loss of OH radical to the walls is significant, 315 competing with recombination. The model assumes that any OH that hits the walls after the heating part of the oven is lost due to reactions with the walls; as a result, recombination is generally less of an effect on the HNO3 measurement. Figure S7 shows example model outputs for the HNO3 oven, predicting the percent dissociation of HNO3 at the point of detection over a large range of initial concentrations for both HNO3 and NO2. As expected, recombination is most important at larger NO2 and HNO3 concentrations; below 50 ppb of each, for this instrument configuration the detection efficiency is above 80%. 320

Ozone pyrolysis at high temperatures interferes with HNO3 measurement
One additional reaction that can affect the HNO3 measurement is the pyrolysis of O3. At high temperatures, some fraction of O3 dissociates, releasing atomic O which reacts with NO2 to form NO + O2, which results in NO2 being removed from the final measurement. Therefore, in background conditions of high O3 concentration, the NO2 concentrations measured after the HNO3 oven are biased low and can even cause the [HNO3] to appear negative upon subtraction. Day et al. 2002 noted 325 that at or above 530 °C, all O3 will separate into O2 and O molecules, which will then react with NO2. This suggests that for this instrument, the pyrolysis of O3 will result in a lower signal only in Oven 1 (700 °C) due to the high temperatures.
In some experiments from the 2018 SAPHIR NO3ISOP campaign, HNO3 measurements appeared negative due to lower signals from the hottest channel. Using other available instruments' measurements of O3 and HNO3, we determined that approximately 4% of the O3 signal was converted to this apparent negative HNO3 signal during one experiment (on 8-Aug-330 2018). However, this fraction did not appear consistent across experiments, perhaps due to substantial HNO3 inlet losses, and we did not determine a robust and consistent correction factor for this effect. Given that this does not affect alkyl nitrate measurements, and that there were other measurements of HNO3 available, we did not pursue this further. But in principle, this is a relatively modest effect that can be corrected for after experimentally determining the efficiency of ozone pyrolysis for a particular inlet oven build. 335

Data corrections
The above modeled 100% efficiency in detecting ANs is fortunate, since the ANs measurement has thus far been the output of greatest interest from this instrument. Should one wish to use such an instrument for accurate measurements of PNs and HNO3, this too is possible, but requires the determination of correction factors to account for the recombination in those ovens.
Beyond the correction factors for radical recombination in the cooling region after each oven (1), additional corrections that 340 can be applied are: (2) oven-specific denuder breakthrough, based on data such as that shown in Tables 1 and 2, (3) background corrections, to account for any background signal detected in each channel while sampling zero air (this could account for inlet and/or denuder offgassing), (4) subtraction of N2O5 interference, as described in section 3.4 above, and (5) correction of the https://doi.org/10.5194/amt-2020-280 Preprint. Discussion started: 20 July 2020 c Author(s) 2020. CC BY 4.0 License.
HNO3 channel for O3 pyrolysis loss of NO2, as described in section 3.8 above. The importance of each of these corrections will depend on the nature of the experiments conducted; some example applications are shown below to illustrate this. 345 We have implemented each of these corrections as optional to apply to any raw data collected in our Igor-based data workup routine, which also sorts the data from the various ovens, averages, and subtracts the relevant signals.

AMS / TD-CRDS aerosol terpene nitrate comparison at CU Boulder chamber 350
The TD-CRDS was compared to the CU Boulder Jimenez group aerosol mass spectrometer (AMS) during collaborative chamber experiments in Summer 2015, using the data from the denuded ANs channel of the TD-CRDS and the high-resolution AMS organic nitrate (pRONO2) measurement to assess the correlation of these two aerosol-phase organic nitrate measurements. The experiments plotted here are those which showed substantial aerosol nitrate formation using Δ-carene or ɑ-pinene as a VOC precursor and NO3 from an N2O5 trap, both spanning the nominal range of 10-100 ppb, at varying relative 355 humidity. The comparison of individual measurements across two weeks of experiments show significant scatter, but an orthogonal distance regression (ODR) fit to the scatterplot of TD-CRDS data vs. AMS data shows a slope of about 0.88-0.94 (depending on intercept treatment), and R 2 =0.73 (Figure 8).
The AMS organic nitrate concentrations in Figure 9 were calculated by apportioning the total nitrate concentration using the NOx + ion ratio (NO2 + /NO + ) method (Farmer et al. 2010), where the relative ratios of organic to inorganic NOx + ratios ("ratio-360 of-ratios"; Fry et al. 2013) were determined by the average of several dry, unseeded experiments and ammonium nitrate ratios from offline calibrations (3.12 for Δ-carene, 3.78 for ɑ-pinene). The organic-inorganic separation was conducted in order to account for possible NH4NO3 or particle HNO3 formation as was suggested by substantial shifts in NOx + ratios observed during wet, seeded experiments, as has been reported previously (Takeuchi and Ng 2019). Figure S8 shows a comparison of the Figure   8 results to a plot of the TD-CRDS measurements against the AMS total nitrate (unapportioned), the latter resulting in slightly 365 lower slopes and correlation coefficients.
Previous comparisons between AMS and thermal dissociation-based aerosol organic nitrate instruments have found varying agreement for ambient measurements (Ng et al. 2017). Some of these differences could be due to the fact that the ambient atmosphere contains a mix of diverse products from the oxidation of monoterpenes and isoprene in the presence of other gases; the resulting differing mixes of alkyl nitrate structures could alter the sensitivity of one or both instruments. 370

Ambient measurements of organonitrates in Portland, OR
During one week in November 2014, the TD-CRDS inlet was situated outside the south end of the Reed College Chemistry building. Simultaneous measurements of NO2, PNs, ANs, and HNO3 were made and one representative day is shown in Figure   9, illustrating typical measurable ambient variability and diurnal cycle. 375

Chamber measurements of isoprene nitrates at SAPHIR chamber (Jülich, Germany)
The TD-CRDS was also used in the month-long SAPHIR NO3 + isoprene campaign in the summer of 2018. The Simulation of Atmospheric Photochemistry in a Large Reaction Chamber (SAPHIR) is a 270 m 3 double-walled Teflon chamber with movable shutters allowing for simulation of both daytime and nighttime chemistry. The experiments were run in batch mode with periodic injections of oxidants and reactants. The reactant concentrations were comparable to real atmospheric 380 concentrations of NO2, O3, and isoprene. Some experiments were run under humid conditions and some had seed aerosol added to facilitate condensations of gas products into the particle phase.
The low, near-ambient concentrations of reactants used, the small degree of partitioning of isoprene nitrates to the aerosol phase, and the relatively long inlet line required resulted in the aerosol organonitrate products being lower than the limit of detection of the TD-CRDS for the particle-phase ANs monitoring. The gas-phase ANs measurements from the TD-CRDS 385 ranged from sub-ppb up to 16 ppb of organic nitrates, with an observed alkyl nitrate molar yield for NO3 + isoprene of ~ 100% under all explored reaction conditions. In order to determine gas/aerosol partitioning of nitrates, the gas-phase ANs measured by TD-CRDS were compared to AMS organic nitrate aerosol measurement. These results are the subject of a forthcoming paper (Brownwood et al., in preparation, 2020).

Chamber measurements of isoprene nitrates at REC (Portland, OR) 390
The TD-CRDS was also used for chamber experiments throughout the 2018-2019 academic year at the Reed Environmental Chamber (REC), running experiments similar to those from SAPHIR, but at substantially higher concentrations. These experiments aimed to determine whether gas-particle partitioning coefficients (Kp) for the NO3-initiated oxidation of isoprene would be similar in a 0.4 m 3 chamber at much higher concentrations to those measured in the 270 m 3 SAPHIR chamber at much lower, near-ambient concentrations. 395 The gas-particle partitioning coefficients calculated in these experiments used the aerosol and total gas + aerosol measurements from the TD-CRDS and a total mass measurement from a Brecthel SEMS (BMI Model 2002). The partitioning coefficients derived from these experiments were 5 × 10 -4 and 4.4 × 10 -3 m 3 µg -1 , for background aerosol loadings of 230 and 20 μg m -3 , respectively. One of these experiments is shown in Figure 10. The aerosol (caer) and total ANs concentrations, and background aerosol loading (Mtot) were averaged over the shaded period, aerosol-phase was subtracted from total to obtain cgas, from which 400 Kp was determined via Equation 4: The fact that the two experiments at different background aerosol mass loadings (Mt) did not give exactly the same Kp value could reflect the uncertainty of these measurements, or that the aerosol partitioning is not perfectly described as absorptive partitioning, or that wall losses change as aerosol loadings change. Most important, the range of Kp measured here fall exactly 405 within the range of values observed over a month of NO3 + isoprene experiments conducted under much lower concentration conditions at the SAPHIR chamber (5 × 10 -4 -6 × 10 -3 m 3 µg -1 , Brownwood et al., in preparation, 2020) These Kp values were compared to theoretical calculations of Kp predicted by the simplified p o L prediction (SIMPOL.1) group contribution method (Pankow and Asher 2008), and we find this average volatility consistent with a tri-functional isoprene nitrates, such as isoprene hydroperoxy nitrate, which has a SIMPOL.1 predicted Kp value of 2.38 × 10 -3 m 3 µg -1 . This shows a 410 promising consistency of equilibrium gas-aerosol partitioning of isoprene nitrate products measured in two dramatically different chambers, and suggests the robustness of the TD-CRDS over a wide range of concentrations.

Conclusions
Using three custom home-built oven channels, a charcoal denuder, and an automated valve control system, a thermal dissociation cavity ringdown spectrometer (TD-CRDS) was constructed for the speciated measurement of gas-and aerosol-415 phase organic nitrates, split into the classes NO2, PNs, ANs, and HNO3. This instrument has been successfully demonstrated for measurements on atmospheric simulation chambers operating at a wide range of concentrations and ambient measurements.
Users or developers of similar such instruments are encouraged to consider the several data corrections described herein, which will be more or less important depending on the details of the instrument deployment.

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experiments contain 300 ppb Δ-carene. The 'high oxidant' experiment was performed with 650 ppb O3 and 400 ppb NO2, which yields substantial N2O5 formation. The 'low oxidant' experiment was performed with 335 ppb O3 and 3.2 ppb NO2, and reveals the clean separation of PNs and ANs by plateaus. There are no distinct plateaus for PNs and ANs in the high oxidant experiment, because they are washed out by the more gradually temperature-dependent dissociation of N2O5.
https://doi.org/10.5194/amt-2020-280 Preprint. Discussion started: 20 July 2020 c Author(s) 2020. CC BY 4.0 License. Figure 6: N2O5 contribution to TD-CRDS channels assessed by an oxidant-only chamber experiment. Allowing (measured) NO2 and O3 to stabilize sequentially enables prediction of N2O5 concentration (red trace) using a kinetics box model, such as KinSim (Peng and Jimenez 2019). Then, the signal in the PNs and ANs channel of the TD-CRDS can be examined during the N2O5 rise time, and percentages can be applied to assess the fraction of N2O5 that is detected in each channel. For our TD-CRDS, this analysis reveals these percentages are 7% in the PNs channel and 28% in the ANs channel. 565 https://doi.org/10.5194/amt-2020-280 Preprint. Discussion started: 20 July 2020 c Author(s) 2020. CC BY 4.0 License. Figure 7. SEMS-measured particle volume versus time to test denuder efficiency. From 2:15 to 3:00 the SEMS was measuring directly from the chamber. From 3:00 to 4:00 the SEMS was measuring particles from the TD-CRDS tubing (only internal tubing to the inlet system) without the denuder. From 4:00 to 5:00 the SEMS measured through the TD-CRDS tubing and the denuder. The horizontal lines represent the average particle volume over the sampling period. The missing data was due to room air entering 570 the lines while the SEMS was detached from the chamber and reattached to the TD-CRDS inlet.

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Environmental Chamber, at 20 μg m -3 ammonium sulfate background aerosol. Note that these traces are not corrected for N2O5 interferences in the ANs and PANs channel, but N2O5 was fully consumed in the period of Kp determination.