In the previous version of the TD-inlet (Friedrich et al., 2020), which used a commercial oven that heated a ∼ 8 cm section of a quartz tube, the complete thermal dissociation of HNO₃ was achieved only at temperatures above 800 °C. Note that HNO₃ is the NO_Y trace-gas that requires the highest temperatures for thermal dissociation to NO₂, with all others, especially organic nitrates, dissociating at temperatures of 200–400 °C. The fractional thermal dissociation in the TD-inlet depends on both the residence time and temperature of the heated section. As some (unwanted) conversion of ammonia to NO_X can occur at temperatures > 700 °C if ozone is present (Friedrich et al., 2020) a reduction in temperature is beneficial in avoiding a positive bias to NO_Y measurements if high mixing ratios of NH₃ or ammonium nitrate particles are present. We note that the reduction in temperature will also make the instrument even less sensitive to sodium nitrate than the ∼ 15 % conversion efficiency reported by Friedrich et al. (2020).

In order to reduce the temperature required for complete dissociation of HNO₃ we constructed new inlets in which the heated section was longer. This was achieved by tightly wrapping 0.9 mm NiCr resistance wire around a ∼ 40 cm long section (close to one end) of a 70 cm long quartz tube (id = 10 mm). The temperature at the outer surface of the quartz tube was measured and regulated using a type-K thermocouple (RS PRO 407-1393). At 700 °C set-temperature, the temperature inside the quartz tubing (at approximately the centre of the heated section) under normal flow conditions was measured using a long temperature probe (RS PRO 255-588) as 733 °C. The tip of the temperature probe had contact with the inner surface of the quartz tube, so this does not necessarily represent the temperature of the gas only. 10 cm thick insulating material was wrapped around the quartz tube, heating wire and thermocouple before being encased in a welded aluminium housing.

The temperature required to fully dissociate HNO₃ when a 2 L (STD) min⁻¹ (SLM) flow of air passed through the inlet was determined by measuring thermal dissociation curves (thermograms) for HNO₃ and ammonium nitrate particles (NH₄NO₃). For the former we used a constant flow of HNO₃ in air (≈ 8 ppbv) from a permeation source (Friedrich et al., 2020). A flow of ammonium nitrate particles was generated from a 0.01 M solution of ammonium nitrate using an atomizer (TSI, model 3076) and a diffusion aerosol drier (TSI, model 3062) filled with a drying agent. In both cases, the total flow through the TD-inlets was 2.1 SLM at an absolute pressure close to 1000 mbar, resulting in mean residence times in the inlets between ∼ 0.9 s at room temperature and ∼ 0.3 s at 850 °C. The results are displayed in Fig. 2, in which a plateau in the thermogram between ∼ 600 and 850 °C was observed for both HNO₃ and ammonium nitrate particles. Similar thermograms for HNO₃ and NH₄NO₃ have been reported by Garner et al. (2020) and Keehan et al. (2020). The solid line is the expected thermogram based on the literature rate constant (k) for the thermal dissociation of HNO₃ in the gas-phase (Glänzer and Troe, 1974), accounting for the change in residence time (t) with temperature (Eq. 1): 2[HNO3]t=[HNO3]01-exp⁡-kt Note that the initial HNO₃ concentration ([HNO3]0) is consistent (±10 %) with the known permeation rate and dilution factor, indicating that the recombination of OH with NO₂ to reform HNO₃ does not compete with OH losses to the walls.

The calculated thermogram displays a plateau at temperatures above ∼ 600 °C, which is consistent with that measured. Complete dissociation is also achieved at temperatures that are ∼ 200 °C cooler than with our previous set up (Friedrich et al., 2020). The deviation between the calculated thermogram and that measured at temperatures < 600 °C is not unexpected and is related to e.g. surface-activated, thermal dissociation of HNO₃ which occurs even at room temperature (Crowley et al., 1993), differences in the gas temperature compared to that measured on the outside of the quartz tubing, axial and radial gradients in the temperature in the TD-inlet, radial gradient in the gas velocity and the resulting uncertainty in the reaction time (t). The slightly higher temperature required by ammonium nitrate particles to reach the plateau may be related to the extra time required to fully thermally decompose particles of 100–200 nm diameter to HNO₃ (and NH₃). Based on this thermogram, a TD-inlet temperature of 700 °C was chosen to ensure that all HNO₃ molecules and ammonium nitrate particles are detected, but avoiding detection of NH₃ which only becomes significant at TD-inlet temperatures of > 800 °C in the absence of trace gases that can scavenge oxygen atoms (Friedrich et al., 2020; Womack et al., 2017; Wild et al., 2016).

In the previous version of the instrument, a three-way valve was used to direct the 2 SLM flow from either of the heated inlets (one with and one without denuder) into the cavity. This meant that heated air in one of the TD-inlets was static for a period of several minutes, which lead to an increase in the gas-temperature. In order to avoid this, extra 3-way, solenoid valves (NR 225K032) were built in which switched between the 2 SLM flow to the cavity and a 2 SLM flow to a pump for each TD-inlet, ensuring that the gas flow was not interrupted and that the TD-inlet temperature was constant. In addition, we found that, when working with long inlet lines, overflowing zero-air close to the front-end of the inlet tubing (as opposed to close to the CRDS) avoided creating pressure differences when sampling ambient air or when zeroing. As pressure differences result (via changes in Rayleigh scattering) in different ring-down times, zeroing at the end of the inlet lines avoids the need to correct the data using Rayleigh scattering coefficients (Thieser et al., 2016).

The denuder we use is a 15 cm long, 4 cm diameter active-carbon tube (Dynamic AQS) with internal honeycomb structure (∼ 450 square holes of 1 mm × 1 mm) to enhance the trapping of trace-gases. The denuder, a slightly larger version than the one described by Friedrich et al. (2020), is housed in an aluminium casing with 1/2 in. connectors on conical end-sections (total length 33 cm). As discussed in detail by Friedrich et al. (2020) the efficiency of the active-carbon surface in removing various gas-phase NO_Y components (including NO, NO₂, N₂O₅ HNO₃, PAN) depends on the relative humidity of the air sampled. This can readily be understood in terms of competitive adsorption between H₂O and the NO_Y gas to be denuded (scavenging efficiency) or in terms of H₂O induced desorption of previous adsorbed NO_Y trace gases (memory effect). Both effects depend on the duration and intensity of exposure of the denuder to NO_Y components of ambient air samples and the relative strength of interaction of H₂O or the trace gas with the active carbon surface as defined by a gas-surface partition coefficient such as the Langmuir equililibrium coefficient (KLang). The latter, using NO as example trace-gas, is given by KLang(H2O)[H2O]/KLang(NO)[NO]. As H₂O mixing ratios in the lower atmosphere are generally many orders of magnitude larger than those of NO (or any other NO_Y trace-gas), it is important to protect the denuder from interaction with ambient water-vapour, requiring drying of the air to be sampled. Friedrich et al. (2020) showed that, for NO, the denuding efficiency (at room temperature) decreased dramatically at a relative humidity above ∼ 20 %. The effect on the denuding efficiency of other NO_Y traces gases such as 2-propyl-nitrate or HNO₃ was much weaker, reflecting the stronger interaction of more polar NO_Y trace gases with the active-carbon surface than found for NO.

Two goals of the present study were to develop a system with high particle transmission that could dry the ambient air to a RH of < 5 % for long periods and to explore ways to reactivate denuder surfaces that had reached full capacity to adsorb NO_Y. Initial experiments with commercial drying systems (e.g. a silica-gel based diffusion aerosol drier, TSI, model 3062) revealed efficient drying to RH < 5 % for only short periods of time (a few hours) when exposed to air of 90 %–100 % RH. As replacement of the drying medium at such short intervals is generally not an option for implementation in instruments that operate continuously under field conditions, we placed a Nafion-based diffusion dryer serially in flow in front of the diffusion aerosol drier. The Nafion drier (PermaPure model 274) is a single Nafion tube of length 120 cm and diameter 1 cm in a stainless-steel shroud which creates an annular space through which a dry counter-flow is passed. A counter-flow of 4–5 L of dry air (provided by a compressor with removal of condensed water) was required to reduce the RH from ∼ 100 % to ∼ 10 %–15 % when sampling 2.1 SLM of humid air through the Nafion tube.

Figure 3 shows the results of two experiments in which 2 SLM of air initially at RH of 80 %–90 % (continuously monitored using a RH sensor) was passed through the Nafion drier and then through the TSI-3062 which was loaded with ≈ 0.9 kg of either dry silica-gel (AppliChem A7220, 1000 grained orange) or molecular sieve (Roth, type 564, 0.3 nm) before the RH was measured with a second RH sensor. Under these operating conditions, the Nafion drier alone reduced the RH from ∼ 80 %–90 % to ∼ 10 %–15 %. When combined with the silica gel drier, the RH was initially reduced to < 2 % which however had increased to 5 % (our threshold) after ∼ 8 h. In contrast, the combination of Nafion drier with molecular sieve drier initially reduced the RH to < 1 %, and the 5 % threshold was reached only after 45 h of exposure. Clearly, use of molecular sieve is preferred over that of silica-gel in terms of capacity to dry air at an input RH of ≈ 10 %–15 %. One potential disadvantage of using molecular sieve is the higher temperature needed to reactivate it (i.e. to drive the strongly bound H₂O from the surface). While silica-gel can be reactivated at temperatures close to 100 °C, molecular sieve requires temperatures > 300 °C. In order to reactivate the molecular sieve under field-conditions, we deployed a heat-gun attached to a vertically mounted metal tube with a steel mesh at the lower end into which the “wet” molecular sieve was transferred. When operating the heat gun at 350 °C the molecular sieve (≈ 1 kg) was dried in ≈ 30 min.

During operation of the D-TD-CRDS, ∼ 20 cm³ (STD) min⁻¹ (sccm) of the air sampled by the P-Nit cavity and NO_X cavity was passed through RH -sensors (see Fig. 1). When the sensor attached to the P-Nit channel indicated that the RH was 5 %, the “wet” molecular sieve was exchanged for a reactivated sample. This is a conservative approach to avoid even low levels of denuder breakthrough and implied that, for ambient measurements in the mid-latitude boundary layer (with 30 % < RH < 90 %), the molecular sieve was replaced every 4–5 d. In principal this interval could be lengthened by using a larger molecular sieve drier.

The transmission of particles of various size through the drying system and denuder was tested in a similar manner to that described by Friedrich et al. (2020) who however only tested the denuder transmission (i.e. without the Nafion drier and molecular sieve drier components and their connectors). For these tests we generated polydisperse ammonium sulphate aerosol using an atomizer (TSI model 3076) with two different concentrations of ammonium sulphate solution, dried the aerosol using a diffusion drier (TSI model 3062) and detected particles using a scanning mobility particle sizer (SMPS, consisting of TSI model 3080/3081 classifier/differential mobility analyser, DMA, coupled to an Ultrafine Condensation Particle Counter; TSI Model 2025). The particle size distributions and number concentrations measured after the aerosol passed either through the Nafion drier or through conductive tubing of approximately the same length and volume are shown in Fig. S1 in the Supplement. Using the “dilute” ammonium sulphate solution, dN/dlog⁡dp at the peak of the log-normal distribution (38.5 nm) was ∼ 1.0 × 10⁶ cm⁻³ with a factor ∼ 10 more (1.1 × 10⁷ cm⁻³ at 63.8 nm) obtained using the “concentrated” ammonium sulphate solution.

We first tested the transmission of the Nafion drier by switching the flow of polydispersed aerosol back-and-forth between the conductive tubing and the Nafion drier. In Fig. S1 we plot both the particle number distribution and particle volume distribution for the two scenarios outlined above (conductive tubing versus Nafion dryer) and also for the two different ammonium sulphate concentrations. The diameter dependent transmission from two experiments (derived by dividing the particle number concentration in each size bin when flowing through the Nafion tubing by that obtained when flowing through the steel tube) is displayed as the solid red lines in Fig. 4. While the transmission for larger particles is indistinguishable from 100 %, there is a clear loss of particles of diameter < ∼ 100 nm, with the 50 % cut-off (d50) at ∼ 25 nm. We note that the atomizer used to physically generate the test-aerosol produces multiply charged particles and the loss of the small particles is likely a combination of their high diffusivity and larger losses through electrostatic deposition to the Nafion tube.

Despite the loss of some particles, the total particle volume (and thus mass) barely changed between using the conductive tubing and the Nafion drier with less than 1 % losses for both “diluted” and “concentrated” ammonium sulphate aerosol. This reflects the fact that the vast majority of the particle mass is in the larger particles, in this case with a maximum contribution from particles of diameter ∼ 270 nm, for which electrostatic loss is negligible.

In a separate set of experiments, we removed the majority of multiply charged particles by passing the polydisperse aerosol first through the X-Ray driven charge-neutralizer (TSI Model 308801) and DMA to pre-select particles of different diameters before flowing them either through the Nafion drier or the conductive tubing (as reference) and then to the CPC. Alternatively, the size selected particles were passed through the entire inlet assembly in which the Nafion drier, molecular sieve drier, denuder and TD-inlet are mounted vertically in line (see Fig. 1). A reference system consisting of a vertically mounted steel tube of 10 mm o.d. in which particle losses are negligible except for very small particles (von der Weiden et al., 2009) was used to define the relative transmission at the same flow rate. The steel tubing had the same length (∼ 2 m) and approximately the same residence time as the combined Nafion tube, molecular sieve drier and denuder (also vertically mounted). The results, expressed as the percentage transmission of particles through the Nafion drier are plotted in Fig. 4 (black data points) which indicates particle transmission is 96.9 ± 6.6 % (2σ) for particles of diameter ≥ 50 nm.

In terms of the transmission of small particles this represents an improvement on the results using polydisperse (multiply charged) aerosol. While the X-ray neutralizer removes multiple charges, all particles that are transmitted through the DMA have a charge. As mentioned above, this will have the effect of increasing electrostatic deposition of small particles and reduce the transmission compared to neutral (i.e. ambient) aerosol.

In summary, the particle transmission test showed that transmission losses (for charged particles) are only significant for particles of diameter < 50 nm. For small particles in charge equilibrium (i.e. chemically generated or ambient particles) the transmission could be even higher. However, in many situations, the mass (and thus nitrate content) of atmospheric particles is dominated by particles of diameter > 60 nm, in which case no correction for particle losses needs to be applied. The loss of very small particles does however imply a significant, negative bias when sampling nucleation mode particles. However, the nitrate mass found in ambient, nucleation mode particles (e.g., of diameter 10 nm) is in any case orders of magnitude below the detection limit of the D-TD-CRDS set up and indeed that of other presently available instruments (see below).

At a flow of 2.1 SLM, the residence time in the drying system and denuder is ∼ 3 s and thus compareble to that found between the differential mobility analyser (DMA) and and AMS-inlet in AMS calibration systems for which loss of ammonium nitrate through thermal decomposition to HNO₃ and NH₃ is known to be insignificant.

Even under dry conditions, the capacity of the denuder to remove NO_Y from the gas-phase is not infinite. In some laboratory experiments after flowing several hundred ppbv of HNO₃ for several hours through the denuder we observed breakthrough, i.e. a slow increase in the P-Nit cavity signal in the absence of (nitrate containing) particles. We therefore explored means to “reactivate” the denuder after such experiments. Considering that the addition of gas-phase humidity > 20 % leads to denuder breakthrough of NO and also results in the release of previously adsorbed NO_Y from the denuder, our approach is to use distilled water to “wash” NO_Y species from the denuder surface. This is achieved by completely immersing the denuder in distilled water and agitating using a sonic bath for several minutes before replacing the distilled water and repeating the procedure three or four times. Following this, the denuder was rinsed by flowing 2–3 L of distilled water through it. Finally, warm (∼ 40–50 °C), dry air was passed through the denuder and the relative humidity of the exhaust was monitored. The cleaning procedure was considered completed when the exhaust RH was below 1 %. Following such a reactivation procedure the denuder could be operated continuously for > 1 month (during which > 80 000 L of air of RH < 5 % passed through the denuder) with average ambient NO_Y levels of 1–2 ppbv without any observation of breakthrough. The lack of breakthrough could be established by analysing datasets in which large spikes in NO and NO₂ (caused by very close vehicular emissions) were observed in the NO_X channel but not in the P-Nit channel of the instrument (see Fig. S2) even after weeks of exposure.

The total uncertainty of the P-Nit measurements by the D-TD-CRDS is a product of uncertainties in the parameters listed in Eq. (1) related to NO₂ detection, as well as the uncertainty in the fractional conversion by thermal dissociation of P-Nit to NO_X and in the denuder efficiency for removal of gas-phase NO_Y before the particle nitrate is thermally dissociated to NO₂, and in the particle transmission through the drying system and denuder.

As discussed by Thieser et al. (2016), the total uncertainty for NO₂ detection is a product of uncertainty in the NO₂ cross-section (4 %), uncertainty in the l/d ratio (1 %) and Rayleigh scattering corrections for differences in pressure δP and humidity δRH between zeroing and sampling. In the present configuration, with zeroing at the front-end of the sampling line and with a drying system for the P-Nit channel, both δP and δRH are negligible. As we work on the plateau of thermograms defining the fractional conversion of HNO₃ and ammonium nitrate to NO_xthe conversion of P-Nit to NO₂ is close to stoichiometric and we have previously shown (Friedrich et al., 2020) that HNO₃ is expected to be detected with 100 % efficiency. In Fig. 4, we have shown that the average particle transmission is 96.9 ± 6.6 % for particles of diameter ≥ 50 nm. Propagating these individual uncertainties results in an overall uncertainty for P-Nit detection of 13 %.

The LOD of the P-Nit channel is determined by the minimum value of (k-k0) that can be measured (see Eq. 1), which depends on the reproducibility of the zero measurement (k0). The integration time dependent LOD was thus derived from an Allan deviation analysis of the signal from the P-Nit channel when zeroing as displayed in Fig. S3. Under ideal, laboratory conditions (i.e. in stable temperature, draft and vibration-free location) the standard deviation (1σ) of k0 at 5 s sampling interval is 24 pptv NO₂. Taking 2σ to define the LOD, this corresponds to 0.12 µgm-3 of particulate nitrate (conversion factor of pptv NO₂ to µgm-3 nitrate is 2.50 × 10⁻³ for 298 K and 1 bar and using a molecular weight of 62 g mol⁻¹ for nitrate).

Under laboratory conditions, extending the averaging time to 1 min (2σ = 14 pptv), would decrease the LOD to 0.035 µgm-3. One-minute averages of a P-Nit dataset obtained under field conditions (air conditioned container in a forest) are characterised by a factor 2.4 worse standard deviation of k0 (2σ = 34 pptv NO₂) and thus a LOD for detection of P-Nit of 0.085 µgm-3. This is largely a result of drifts in k0 (see Eq. 1) resulting e.g. from temperature changes at the instrument induced by drafts or by air-conditioning in the container.

To make quantitative comparison of the measurement of inorganic nitrate particles by D-TD-CRDS and HR-ToF-AMS, dry ammonium nitrate (NH₄NO₃) particles (ca. 10–400 nm) were generated from aqueous NH₄NO₃ solution using an atomizer (TSI, model 3076), and a diffusion drier (TSI, model 3062) filled with silica gel as drying agent. The aerosol flow (≈ 2 SLM for 30–100 s during each “injection”) was directed into the SCHARK where it was dynamically diluted with a continuous flow of 6 SLPM dry zero air. The mixing time in the SCHARK is ≈ 1 min. Figure 5a shows a 24 h-time series in which polydisperse ammonium nitrate particles were added to the SCHARK three times. Both the D-TD-CRDS and the HR-ToF-AMS observe co-varying increases in nitrate mass concentrations up to 25 µgm-3 over extended time-periods. Note that data between 18:45 UTC on 12 June and 06:50 UTC on 13 June has been excluded from the correlation as only background signal was collected. Figure 5b reveals excellent correlation (Pearson correlation coefficient r = 0.99) between the two datasets with an unweighted linear regression yielding a slope of IP-Nit (HR-ToF-AMS) / IP-Nit (D-TD-CRDS) = 0.95 ± 0.02 and an intercept of (-0.11 ± 0.04) µgm-3. The deviation of the slope from unity is well within the combined uncertainty of the two instruments. At the lowest particulate nitrate masses the plot deviates slightly from a purely linear correlation. These low nitrate masses are associated with the data obtained at long times after injection into the SCHARK when (1) less particles are available because of flow out from the SCHARK and (2) substantial shrinkage of the ammonium nitrate particles (resulting from thermal decomposition and loss of HNO₃ and NH₃ to the chamber walls) will have occurred so that the particle diameter was reduced compared to the larger masses. The change in slope at the lowest nitrate mass could thus potentially be associated with a lower AMS inlet transmission and detection efficiency for the smallest particles, compared to the D-TD-CRDS collection and detection efficiency, which is less affected by very small particle sizes.

Figure 6a shows a time series of D-TD-CRDS and HR-ToF-AMS measurement of OP-Nit derived from the reaction between limonene and NO₃ in the SCHARK chamber. Details of the experiment are given in the Supplement (Figs. S5–S7). The slope of the correlation plot (Fig. 6b) is OP-Nit (HR-ToF-AMS) / OP-Nit (D-TD-CRDS) = 1.08 ± 0.09 with an intercept of 0.22 ± 0.19 µgm-3. The deviation of the slope from unity is well within the combined uncertainty of the two instruments.

Note that in our chamber experiments we generate a large number of different organic nitrate species (C₇ to C₁₀) and the OP-Nit is thus not a single molecule. In this case the contribution of organic fragments to m/z 30 might vary from one organic nitrate to the next and the IE (and CE) of these organic nitrate species might also vary. However, as our SMPS-based calibration of IE was carried out with the same organic nitrate system (i.e. formed in the NO₃ + limonene system), such effects will cancel when comparing OP-Nit (HR-ToF-AMS) and OP-Nit (D-TD-CRDS).

When compared to each other, a number of advantages and disadvantages of the methods outlined in the introduction for measuring particle nitrate (AMS, wet-chemical, D-TD-LIF/CRDS) become apparent. For the AMS, the advantages include widespread use and very large user community, good sensitivity and high time resolution. In principle, the AMS can differentiate between inorganic and organic nitrates although most accurate evaluation of the HR-ToF-AMS data may necessitate prior knowledge of the aerosol composition. The disadvantages of the HR-ToF-AMS include high weight/large size, high instrument costs and complex calibration procedures.

For the wet-chemical systems the main advantages are good sensitivity and accuracy and simple calibration and the simultaneous detection of soluble trace gases such as HNO₃, HONO and N₂O₅. The major disadvantage includes low time resolution (∼ 1 h, which may limit deployment to non-mobile platforms), non-quantitative detection of organic nitrates, high instrument costs and large weight/size.

The advantages of the TD-based systems are their high time-resolution (apart from instruments that collect particles for several minutes on a filter prior to flash volatilisation), reasonable sensitivity, comparably low costs and simple calibration schemes (or no calibration at all for CRDS based detection schemes).

The TD-CRDS instruments described by Garner et al. (2020) and Keehan et al. (2020) have many similarities to that described here and also to its previous version (Friedrich et al., 2020). There are however some significant differences: In order to separate contributions from particluate and gas-phase nitrate to NO_Y Garner et al. (2020) used a filter to remove particles rather than a denuder to remove the gas-phase. This approach has the potential disadvantage that filters will not only collect particles but also to a variable extent (depending e.g. on ambient humidity, filter age and level of contamination) also trap HNO₃ thus potentially biasing the measurement of gas phase NO_Y to low values and resulting in large memory effects that may hinder measurements at high frequency. While Garner et al. (2020) focussed on inorganic particulate nitrate, Keehan et al. (2020) also examined the response of their TD-CRDS to organic particulate nitrate fromed from reaction of NO₃ with various VOCs (including Δ-carene, limonene, α-pinene and β-pinene) in a 400 L chamber and compared results to those obtained using an AMS. Their correlation of TD-CRDS with AMS data shows significantly more scatter that observed in the present experiments and a slope close to 0.9. Keehan et al. (2020) suggest that some of the scatter (i.e. variability in relative sensitivity) may be related to differences in the alkyl-nitrate structures that my alter the sensitivity e.g. of the AMS. As in the present study, Keehan et al. (2020) used an activated carbon denuder to remove gas-phase components of NO_Y and report time dependent changes in transmission of the denuder. They did not explore the effects of changing relative humidity on their denuder efficiency.

The better transmission of our D-TD-CRDS (compared e.g. to the AMS) for particles of diameter < 70 nm might represent an advantage in situations where a significant fraction of particulate nitrate is associated with small particles. Multi-channel instruments, such as the one described here and by others (Brownwood et al., 2021; Rollins et al., 2010; Lee et al., 2015; Womack et al., 2017) also measure nitrogen containing trace gases either as NO_X and NO_Y or as organic nitrates that enables direct calculation of the gas-to-particulate partitioning of reactive nitrogen in one instrument. The main disadvantage of the D-TD-LIF/CRDS systems is non-separation of organic and inorganic particulate nitrate and (unless the air is dried as described here) the problem of denuder-breakthrough.

Finally, we note that the very good agreement between the HR-ToF-AMS and the D-TD-CRDS observed in this study strongly suggest that the D-TD-CRDS is not only a useful instrument for measuring ambient total particulate nitrate but also represents a further possibility to calibrate a HR-ToF-AMS for both organic and inorganic nitrate aerosol.