Exploring the inorganic composition of the Asian Tropopause Aerosol Layer using medium-duration balloon flights

Satellite observations have revealed an enhanced aerosol layer near the tropopause over Asia during the summer monsoon, called the “Asian Tropopause Aerosol Layer” (ATAL). In this work, aerosol particles in the ATAL were collected with a balloon-borne impactor near the tropopause region over India, using extended-duration balloon flights, in summer 2017 and winter 2018. The chemical composition of these particles was further investigated by quantitative analysis using offline ion chromatography. Nitrate (NO${}_{\mathrm{3}}^{-})$ and nitrite (NO${}_{\mathrm{2}}^{-})$ were found to be the dominant ions in the collected aerosols with values ranging between 87 and 343 ng m⁻³ at STP (standard temperature and pressure) during the summer campaign. In contrast, sulfate (SO${}_{\mathrm{4}}^{\mathrm{2}-})$ levels were found to be above the detection limit (>10 ng m⁻³ at STP) only in winter. In addition, we determined the origin of the air masses sampled during the flights using the analysis of back trajectories as well as a convective proxy from cloud-top temperature fields derived from a geostationary satellite. The results obtained from this analysis were put into the context of large-scale transport and aerosol distribution using GEOS-Chem chemical transport model simulations. The first flight in summer 2017 which sampled an air mass within the Asian monsoon anticyclone (AMA), influenced by convection over Western China, was associated with particle size diameters from 0.05 to 0.15 µm. In contrast, the second flight sampled air masses at the edge of the AMA associated with a larger particle size radius (>2 µm) with a higher NO${}_{\mathrm{2}}^{-}$ concentration. The sampled air masses in winter 2018 were likely affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months before our campaign, associated with concentration enhancements of SO${}_{\mathrm{4}}^{\mathrm{2}-}$ and Ca²⁺. Overall, our results suggest that nitrogen-containing particles represent a large fraction of cloud-free and in-cloud aerosols populating the ATAL, which is partially in agreement with the results from aircraft measurements during the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) campaign. The exact nature of those particles is still unknown, but their coincidences with subvisible cirrus clouds and their sizes suggest nitric acid trihydrate (NAT) as a possible candidate, as NAT has already been observed in the tropical upper troposphere and lower stratosphere in other studies. Furthermore, GEOS-Chem model simulations indicate that lightning NO_x emissions could have significantly impacted the production of nitrate aerosols sampled during the summer of 2017.

1 Introduction

1.1 The Asian summer monsoon and the transport of pollution

Over the past 2 decades, rapid economic growth in Asia has led to serious environmental threats to water and air quality. Every winter, pollutants can be observed through satellites in the form of a grayish veil of particulate matter, referred to as the “Asian brown cloud” (Ramanathan and Crutzen, 2003). In summer, the southwest Asian monsoon (SAM) discharges polluted air over very long distances. According to trajectory calculations, about 20 % of air masses in the tropical lower stratosphere have been in contact with air in the boundary layer in Asia (Orbe et al., 2015). Polluted air masses transported from the boundary layer to higher altitudes are confined within the Asian monsoon anticyclone (AMA) (Ploeger et al., 2017). In the AMA, pollution is accumulated and is further dispersed over a large area of the Northern Hemisphere, reaching latitudes from 10 to 40^∘ N and longitudes from 10 to 140^∘ E (Park et al., 2007; Randel et al., 2010; Ungermann et al., 2016). The air exported from the AMA influences the composition of the entire lowermost stratosphere of the Northern Hemisphere (Ploeger et al., 2017; Santee et al., 2017; Yu et al., 2017). Deep convective clouds represent conduits for air pollution to reach the upper-troposphere–lower-stratosphere (UTLS) region. Aerosols in the UTLS have longer residence times than those in the lower troposphere, influencing the chemistry of the atmosphere and the Earth's climate (Rasch et al., 2008). In addition, they also affect the concentration of chemical species through changes in photolysis rates and heterogenous reactions (Pitari et al., 2014). It has been further reported that aerosols in the UTLS can impact climate by altering the properties of cirrus clouds via homogeneous or heterogeneous ice nucleation (Li et al., 2005, 2009; Yin et al., 2012; Fadnavis et al., 2013; Wagner et al., 2020).

A layer of aerosol enhancements observed by the Cloud-Aerosol Lidar and Pathfinder Satellite Observation (CALIPSO) satellite and the Stratospheric Aerosol and Gas Experiment (SAGE) II spaceborne instruments (Vernier et al., 2011; Thomason and Vernier, 2013), also known as the “Asian Tropopause Aerosol Layer” (ATAL), coincides with the presence of enhanced trace gas pollutants (e.g., carbon monoxide, CO, and hydrogen cyanide, HCN) in the UTLS region. Balloon-borne measurements (Vernier et al., 2015, 2018) confirmed the presence of the ATAL at altitudes of 14–18 km, connected to the AMA. The positive trend in UTLS aerosols inferred from satellite observations since the late 1990s may reflect the increasing influence of anthropogenic emissions on stratospheric aerosol levels. Indeed, global chemical transport model simulations suggest that sulfate, nitrate, and organic aerosols produced from gas-phase precursors populate the UTLS region over Asia in various relative fractions during the summer monsoon (Brabec et al., 2012; Gu et al., 2016; Fairlie et al., 2020).

1.2 What is the significance of the ATAL?

The ATAL constitutes one of the most important sources of UTLS aerosols in the absence of volcanic eruptions (Vernier et al., 2011). It has the potential to affect the Earth's radiative balance (Vernier et al., 2015), stratospheric ozone chemistry, and the properties of cirrus clouds. For example, an increase in the solid-particle concentration relative to the liquid background aerosol levels could trigger heterogeneous freezing and the formation of cirrus clouds at a lower relative humidity with respect to ice (Zawadowicz et al., 2015; Wang et al., 2020). Model simulations suggest that the ATAL represents 20 % of the total column surface area density in the stratosphere of the Northern Hemisphere (Yu et al., 2018) with potential halogen heterogeneous chemistry on aerosols that can affect ozone trends (Solomon et al., 2016). The types of aerosols populating the ATAL could affect those chemical processes. Finally, the presence of absorbing aerosols (e.g., soot) in the UTLS could shift the level of zero net radiative heating upward and enhance troposphere-to-stratosphere transport (Yu et al., 2015).

1.3 What is known about ATAL's composition?

The composition of the ATAL is a very active research topic. Energy-dispersive X-ray analysis (EDX) of aerosols sampled near 10–12 km aboard commercial aircraft as part of the Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) program, at the bottom part of the ATAL, suggests a ratio of carbon to sulfur in the range of 2–10 (Vernier et al., 2015). Aircraft limb infrared measurements carried out during the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) campaign in Nepal and India show the presence of ammonium nitrate in aerosol particles, validating satellite observations from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instruments (Höpfner et al., 2019). A combination of community models and aerosol–climate–chemistry models indicates that, along with surface-emitted and secondary organic aerosols, the ATAL could be comprised of a significant amount of mineral dust either as a major component (Fadnavis et al., 2013; Lau et al., 2018; Ma et al., 2019; Bossolasco et al., 2021) or minor component (Yu et al., 2015, 2017; Gu et al., 2016; Fairlie et al., 2020).

The aerosol particles in the ATAL are looked upon as an insignia of the presence of pollution in the monsoon circulation from large SO₂ and NO_x emissions in South and Southeastern Asia. Human-induced biomass burning (Van der A et al., 2008), fossil fuel combustion (Ghude et al., 2009), wildfires (Goode et al., 2000; Andrae and Merlet, 2001), and lightning (Martin et al., 2007; Yuan et al., 2019) are the significant anthropogenic and natural sources of NO_x. Soil biogenic emission of NO_x represents a large fraction of total NO_x (Jalié et al., 2004). Reactive nitrogen is emitted from the tropical soils by microbial processes as NO (Yienger and Levy, 1995; Conrad, 1996). Investigations of the composition of the aerosol particles in the ATAL are exiguous, although preliminary data from balloon-borne measurements indicate the presence of nitrate aerosol particles (Vernier et al., 2018). Recent in situ aerosol mass spectrometric measurements also reveal the presence of nitrate, ammonium, and sulfate within the ATAL (Höpfner et al., 2019; Yuan et al., 2019).

Here, we investigate the inorganic composition of the ATAL over India during the summer monsoon and in winter using a balloon-borne aerosol impactor system with offline ion chromatography (IC) analysis. Section 2 describes the concept of the balloon experiment and the impactor system. The results of the IC analysis of the samples collected on the ground (GRND), during the two balloon flights (ZF2 and ZF3) in the summer of 2017, and during the flight (ZFW) in winter 2018 are described in Sect. 3. Section 4 compares those results obtained from balloon-borne measurements and satellite observations. Section 5 describes the influence of the Canadian wildfire event on the Balloon-borne measurement campaigns of the Asian Tropopause Aerosol Layer (BATAL) winter flight. The origin of the air masses sampled during those flights is assessed in Sect. 6 using back-trajectory analysis combined with convective proxies. Section 7 addresses the formation of nitrite and its measurements. The GEOS-Chem model simulations are presented in Sect. 8 in order to put the measurements in the context of regional aerosol transport and distribution; this is followed by a summary and conclusions in Sect. 9.

2 Balloon flights, instrumentation, and chemical analysis approach

2.1 Rationale for the experiment

Contingent on measurements during the 2015 BATAL campaign, a concentration of about 20 particles cm⁻³ was found near the tropopause for aerosols with a radius greater than 75 nm (Vernier et al., 2018). This translates into a mass concentration of 40 ng m⁻³ at STP (standard temperature and pressure; hereafter STP is assumed when mass concentrations are given), assuming that the aerosols were liquid sulfate droplets. During that time, the lower detection limit for the IC instrument at NASA Langley Research Center was around 20 ng m⁻³. In order to reach the detection limit of sulfate aerosols, one would need to sample at least 0.5 m³, assuming the sulfate concentration above. Based on those results and weight limitations, we decided to use an impactor with a flow rate of 7 L min⁻¹ which would need to float in the UTLS region for several hours to sample a sufficient air volume (2 h of sampling results in 0.84 m³).

Figure 1 (a) Schematic diagram of the zero-pressure flight (ZP) concept. (b) Picture of the science payload, impactor preparation, and balloon flight launch. (c) Time–height curves of the GPS altitudes of the three zero-pressure flights during summer 2017, in comparison with that of winter 2018, launched from TIFR-BF, Hyderabad, India.

2.2 Balloon experiment

We used zero-pressure plastic balloons to achieve a float near the tropopause and sample enough aerosols to reach the detection limit of the IC. The Tata Institute of Fundamental Research Balloon Facility (TIFR-BF) in Hyderabad, India, provided the infrastructure to conduct the experiment. Polyethylene balloons with volumes between 300 and 500 m³ (manufactured by TIFR) were used for the zero-pressure flights (ZF) to carry a communication/control package (developed by TIFR), a science module (including a meteorological radiosonde), a Compact Optical Backscatter and Aerosol Detector (COBALD) system (Vernier et al., 2015; Yu et al., 2017), an aerosol impactor, and a ballast module at the end of the flight train. The schematic diagram shown in Fig. 1a describes a typical balloon flight. During the ascent, the atmospheric pressure decreases allowing gas inside the balloon to occupy a large space (stage 2 in Fig. 1a). The equilibrium point is reached when the hydrogen escapes from the side escape tubes attached at the bottom of the balloon, until the inside pressure equals the outside pressure (stage 3 in Fig. 1a), leading to a pressure differential of zero (zero-pressure balloon). The float altitude depends upon the volume of the balloon, the density of gas, and the total weight of the system following a simple Archimedes principle. Extreme cold temperatures near the tropopause affect the float due to radiative cooling, leading to a reduction in the buoyancy force, which entrains the descent of the system (stage 4 in Fig. 1a). To counterbalance this effect, ballast shots are released from a container to reduce the total weight (stage 5 in Fig. 1a), leading to the ascent of the balloon.

2.3 Balloon-borne aerosol impactor

We developed a balloon-borne aerosol impactor (BAI) for the ZF flights. This aerosol sampler is comprised of a four-stage impactor, a vacuum pump, a volumetric flow controller, and a Raspberry-Pi-based controller connected to a meteorological sonde. The mechanical part of the impactor was designed by California Measurements, Inc. and is based upon the principle of inertia, where the flow and the instrument dimension determine the size cutoff at different stages. The size cutoffs with respect to the particle radius for the impactor's four stages (S-1, S-2, S-3, and S-4) are 2, 0.5, 0.15, and 0.05 µm, respectively, at 7 L min⁻¹ . The pump is controlled electronically based on the pressure measurements from the meteorological sonde. Our objective is to sample aerosols within the ATAL region; to achieve this, the pump was switched on below 150 hPa (∼14 km) and switched off above 70 hPa (∼18 km). However, due to a reduction in the pump efficiency at those levels, the flow rates lay between 5 and 6 L min⁻¹, leading to a small shift in the size cutoff by up to 18 % (e.g., 2.36 µm instead of 2 µm for a flow of 5 L min⁻¹ for S-1).

In 2017, we conducted a series of balloon flights using the BAI and a COBALD sonde for aerosol backscatter measurements of cloud and aerosol layers encountered by the BAI. The time–height evolution of the three ZFs is shown in Fig. 1c, with flight ZF1 being a test flight to understand and maintain the float altitude using ballast. The maximum flight duration was obtained during ZF3, which had a float time of nearly 2 h 50 min above 150 hPa and below 70 hPa. The oscillation of the balloon trajectories is due to the cooling of the gas inside the balloon and the subsequent release of ballast to regain higher altitudes. The BAI was preserved in a foam box containing dry ice during transportation to TIFR; once there, the filters were immediately unloaded and stored in 47 mm petri dishes that were frozen at −24 ^∘C until further analysis at the Physical Research Laboratory, Ahmedabad, India. Figure 2 represents the time evolution of altitude, temperature, and relative humidity inside the box containing the impactor and also outlines the different phases of the experiment.

Figure 2 Time series of the altitude, temperature, and relative humidity profiles of the samples inside the foam box during the ZF2 flight.

2.4 Analysis of major ions in aerosol samples

Aerosol samples were extracted in deionized water (Milli-Q, specific resistance ≥18.2 M Ω cm) in sterile polypropylene vials for 30 min (three intervals of 10 min each) using ultrasonication. The extract was further analyzed for water-soluble inorganic species (WSIS, such as Na⁺, K⁺, Mg²⁺, Ca²⁺, NH${}_{\mathrm{4}}^{+}$, Cl⁻, NO${}_{\mathrm{2}}^{-}$, NO${}_{\mathrm{3}}^{-}$, and SO${}_{\mathrm{4}}^{\mathrm{2}-})$ using an ion chromatograph (Dionex ICS-5000 DC-5, Thermo Scientific). For calibration, 1000 mg L⁻¹ stock solution of each cation (using Merck high-purity analytical-grade NaNO₂, (NH₄)₂SO₄, KNO₃, CaCl₂ ⋅ 2H₂O, and Mg metal) were prepared. In addition, mixed standards were prepared by diluting stock solutions in polypropylene vials, thereby satisfying the primary requirement of instrument calibration for cations. Similarly, anion Multi-element standard II (1000 mg L⁻¹ in H₂O, reference no. HC 409399, Merck) was diluted subsequently as instrument calibration for anions. Post extraction, the extract of each sample was then separated and eluted in the cation column (Dionex IonPac™ CS16, 5 mm ×250 mm) and anion column (Dionex IonPac™ AS23, 4 mm ×250 mm) via the interaction with the mobile phases, i.e., 30 mM methyl sulfonic acid (MSA) for cations and a mixture of 4.5 mM carbonate +0.8 mM bicarbonate solutions for anions. The quantification of each ion was then performed using the conductivity detector. Several blanks were analyzed along with the samples. The ionic concentrations reported here are blank corrected. As the concentrations of different species were too low in UTLS aerosol samples, only those values which were at least 2 times higher than their respective blanks are reported. More than 50 % of samples were repeated for reproducibility, and they were found to vary by between 2 % and 20 % for all of the analyzed ions. To validate the analysis, Dionex Six Cation-I Standard (product code 040187) and Dionex Combined Seven Anion Standard II (product code 57590) were diluted and checked in the respective cation and anion calibration curves which were found within ±10 % relative standard deviation (RSD).

Figure 3 (a) Results from the analysis of inorganic aerosol for aerosol ionic composition of the filters collected (i) on the ground (ii) during ZF2, (iii) during ZF3 in summer 2017, and (iv) during ZFW in winter 2018. (b) The percentage distribution of individual ions. S-1 to S-4 indicate the four stages of the impactor. The size cutoffs are >2, 0.5, 0.15, and 0.05  µm for S-1, S-2, S-3, and S-4, respectively.

3 Results of the IC analysis

Figure 3 shows the ionic concentration of ground samples (GRND) as well as those of airborne samples (ZF2 and ZF3) collected aboard zero-pressure flights on 15 and 21 August 2017. Additionally, the figure also shows the ionic concentration of the flight held during the winter 2018 campaign denoted as (ZFW). In GRND samples, Na⁺ and Ca²⁺ cations are seen on S-1 and S-2 with corresponding anions (NO${}_{\mathrm{3}}^{-}$, SO${}_{\mathrm{4}}^{\mathrm{2}-}$, and NO${}_{\mathrm{2}}^{-})$ coexisting at the same stage. High NH${}_{\mathrm{4}}^{+}$ is observed only on S-3, with a concentration of 212 ng m⁻³ at STP. K⁺ was also seen on S-3, with a concentration of 26 ng m⁻³ at STP (fine mode), and could have originated from biomass burning. City pollution from Hyderabad is likely the source of those aerosols observed on the GRND filters. Flights ZF2 and ZF3 show significant amounts of NO${}_{\mathrm{3}}^{-}$ and NO${}_{\mathrm{2}}^{-}$ (87–343 ng m⁻³ at STP) with trace amounts of mineral dust proxies (Ca²⁺). Biomass burning (K⁺) was observed in the results of flight ZF2 only. The presence of non-sea-salt Ca²⁺ in aerosols is often used as a proxy for mineral dust (Schüpbach et al., 2013), whereas non-sea-salt K⁺ in aerosols is often used as a proxy for biomass burning (Li et al., 2003). Although their concentrations were too low (close to the detection limit), their presence indicates the possibility of trace contributions from mineral dust and biomass burning.

Other species were below 5 (for cations) to 10 (for anions) ng m⁻³ at STP, the detection limit of the IC instrument for our analytical setup. Charge balance was not achieved due to a higher negative charge, mainly from NO${}_{\mathrm{3}}^{-}$ and NO${}_{\mathrm{2}}^{-}$, than positive charge, mainly from NH${}_{\mathrm{4}}^{+}$, Ca²⁺, and K⁺ (Fig. 3), implying the existence of NO${}_{\mathrm{3}}^{-}$ and NO${}_{\mathrm{2}}^{-}$ in forms other than salt. For instance, nitric acid trihydrate (NAT, HNO₃ ⋅ 3H₂O) could be another aerosol cluster in which NO${}_{\mathrm{3}}^{-}$ may be present in the tropical UTLS (Voigt et al., 2000). We did not find a significant amount of ammonium in our ZF flight samples during the summer. Overall, the concentration of nitrate (80–100 ng m⁻³ at STP) found on both flights seems to be lower than the levels observed during StratoClim (Höpfner et al., 2019). In the only flight during the winter of 2018, Na⁺ and K⁺ were almost inexistent. In comparison, the mineral dust proxy (Ca²⁺) was present in trace amounts on all four impactor stages and could be associated with SO${}_{\mathrm{4}}^{\mathrm{2}-}$, which was also found on all four stages (Fig. 3b).

Balloon-borne and aircraft sampling techniques have been used since the early 1970s to study the composition of aerosols in the UTLS region (Lazarus et al., 1970). Höpfner et al. (2016) reported the presence of enhanced amounts of NO₃ between 12 and 15 km within the Asian summer monsoon region. While sulfate tends to be stable enough to be collected and further analyzed without major chemical transformation, other nitrate-containing particles can be more unstable. NO${}_{\mathrm{3}}^{-}$ salts apart from NH₄NO₃ are not significantly volatile after sampling (Newman, 1993). The dissociation of NH₄NO₃ into gas-phase HNO₃ and NH₃ increases sharply with increasing temperature and relative humidity (Stelson and Steinfield, 1982; Lightstone et al., 2000), leading to a significant loss of particulate nitrate (PN). The slight retention of HNO₃ (gas) on the PTFE (polytetrafluoroethylene) filter could represent a significant source of particulate nitrate on filters at low concentrations and has been used in the past to estimate stratospheric HNO₃ (Lazarus et al., 1970). Additional information available during ZF2 will be discussed to assess the presence of ice clouds.

Figure 4 (a) COBALD balloon in situ scattering ratio (SR) and color index (CI) profiles and (b) CALIOP satellite SR and volume depolarization profiles co-located in time and space (within 20 km and 1 h, respectively) on 15 August at 19:00 UT. (c) Time series along ZF2 of SRs at 940 and 470 nm from COBALD as well as the GPS altitude and measured pressure from the iMet radiosonde.

4 COBALD and CALIOP point to the presence of ice clouds during ZF2

We will now focus the discussion on ZF2, which included a COBALD backscatter sonde and was launched to be co-located in space and time (within 20 km and 1 h, respectively) with satellite observations from the CALIOP lidar aboard the CALIPSO satellite. Figure 4a and b show scattering ratio (SR) and color index (CI) profiles from COBALD (470 and 940 nm) as well as CALIOP SR and volume depolarization profiles at 532 nm. Both balloon and satellite observations show a layer between 13.5 and 16 km with high depolarization (CALIOP) and a high color ratio (COBALD), likely made of aspherical particles. The derived particulate depolarization ratio from CALIOP Level 2v4.1 within the layer was $\mathrm{0.47}+/-\mathrm{0.06}$ (Fig. S3 in the Supplement) and was associated with an optical depth of $\mathrm{0.03}+/-\mathrm{0.02}$, indicating the presence of a subvisible cirrus cloud. Flight ZF2 floated near 14.5–17 km for more than 2 h (Fig. 1c). The time series (Fig. 4c) indicate that the measurements took place within two different air masses: first within an ice cloud, as discussed above, and then in a cloud-free region. The pump connected to the impactor was switched on below 150 hPa and run for ∼16 min within the cloud and for ∼1 h 30 min in a cloud-free region.

4.1 In-cloud nitrate particles

The sampling within an ice cloud (Fig. 4) during ZF2 could, therefore, indicate the presence of in-cloud NO${}_{\mathrm{3}}^{-}$. HNO₃, (an oxidation product of NO_x) and NH₃ (released from agricultural sources) are said to be absorbed into cloud droplets which then aid in the conversion of HNO₃ to aerosol NO${}_{\mathrm{3}}^{-}$ (Hayden et al., 2007). HNO₃, being readily soluble, tends to completely dissolve in cloud water (Steinfeld and Pandis, 1998). Bela et al. (2016) found an almost 90 % scavenging efficiency for HNO₃ by sampling the inflow and outflow of convective storms over the United States during the Deep Convective Clouds and Chemistry (DC3) field campaign. However, we found that ZF2 sampled 90 ng m⁻³ at STP of NO${}_{\mathrm{3}}^{-}$ for particle sizes between 2 and 0.5 µm on stage 2 and 11 ng m⁻³ at STP of NO${}_{\mathrm{3}}^{-}$ on stage 3, corresponding to particle sizes between 0.5 and 0.15 µm; this indicates that other mechanisms might be taking place for HNO₃ and nitrate to survive convection. Our measurements took place at a much higher altitude (16–17 km instead of 10–12 km) and much colder temperature (−70 to −80 ^∘C) than DC3, which may explain this apparent contrast between DC3 and our measurements. Bela et al. (2016) found that ice retention had little impact on scavenging efficiency in the Weather Research and Forecasting (WRF) model coupled with Chemistry, suggesting that HNO₃ formation in ice clouds via lightning-induced NO_x could be a more efficient source than convective transport. The GEOS-Chem chemical transport model (CTM) showed the presence of inorganic nitrate aerosol to be dominant in the ATAL (Gu et al., 2016), with a significant fraction from lightning-induced NO_x (Fairlie et al., 2020). The authors concluded that gas–aerosol conversion of HNO₃ was the driving factor behind this dominance, via the processes discussed above.

4.2 NAT particles

Another candidate for the presence of nitrate on the filters could be NAT particles. They have been reported in tropical ice clouds by Voigt et al. (2008) with sizes (D<6 µm) consistent with their sampling on stages 1 (>2 µm) and 2 (0.5–2 µm) of our impactor. In addition, NAT nucleation seems to be more efficient in subvisible ice clouds at higher ambient temperature than the temperature associated with NAT formation at −78 ^∘C (Voigt et al., 2008). The sampling within the ice cloud at temperatures between −65 and −75 ^∘C would allow the presence of NAT. However, in the process of sampling, transport, and extraction, there is the strong possibility of NAT particle losses (if they were collected). In addition, if NO${}_{\mathrm{3}}^{-}$ was present in another form (refractory nitrate), it would remain relatively stable during the abovementioned processes. Observed cations were close to or below the detection limit compared with the significant concentrations of NO${}_{\mathrm{2}}^{-}$ and NO${}_{\mathrm{3}}^{-}$. This observation along with the higher abundance of NO${}_{\mathrm{2}}^{-}$ allowed us to suggest the presence of NAT particles. However, the reported NAT concentrations should be considered as the lower limit, presuming some losses (unquantifiable) during the sampling, transport, and extraction processes.

4.3 In-cloud calcium and its implication

The IC results of flight ZF2 showed the presence of particles of Ca²⁺ (9 ng m⁻³ at STP) on stage 2 of the impactor (0.5–2 µm). The erosion of calcareous soils followed by strong convective vertical transport during summer results in cloud water calcium (Issac et al., 1990). Cloud water experiments have shown the formation of Ca(NO₃)₂ in the presence of NH₃. Hill et al. (2007) and Leaitch et al. (1986) found a positive correlation between Ca²⁺ and NO${}_{\mathrm{3}}^{-}$. In addition to Ca²⁺, ZF2 also showed the presence of NO${}_{\mathrm{3}}^{-}$ (90 ng m⁻³ at STP) on the same stage of the impactor (large particles <2 µm), further implying the possibility of Ca(NO₃)₂ formation in the presence of the acid HNO₃. Lastly, a high concentration of nitrite (193 ng m⁻³ at STP) was also found on stage 2 of the impactor. The presence of nitrite in clouds is further discussed in Sect. 7.

5 The influence of Canadian wildfire plumes during the winter flight

For the only flight during the winter of 2018, Na⁺ and K⁺ were almost inexistent. In comparison, the mineral dust proxy (Ca²⁺) was present on all four impactor stages: 30 ng m⁻³ on stage 1 (particle sizes >2 µm), 46 ng m⁻³ on stage 2 (particle sizes between 0.5 and 2 µm), 11 ng m⁻³ on stage 3 (particle sizes between 0.15 and 0.5 µm), and 29 ng m⁻³ on stage 4 (particle sizes between 0.05 and 0.15 µm). Interestingly, SO${}_{\mathrm{4}}^{\mathrm{2}-}$ was also found on all four stages (Fig. 3b): 14 ng m⁻³ on stage 1, 21 ng m⁻³ on stage 2, 15 ng m⁻³ on stage 3, and 12 ng m⁻³ on stage 4. Satellite analysis of aerosol extinction at 1020 nm from the Stratospheric Aerosol and Gas Experiment III (SAGE III) was conducted to understand the origin of those particles. We found high aerosol extinction values in the Northern Hemisphere from August 2017 to February 2018, consistent with the presence of smoke from the 2017 Canadian fire (Fig. S2).

Fires that intensify towering thunderstorms have the tendency to inject aerosols into the lower stratosphere that were otherwise thought to have originated from volcanic plumes (Fromm et al., 2010). The 2017 Canadian wildfire event led to the formation of multiple pyrocumulonimbus (PyroCb) episodes, resulting in a vast aerosol cloud. Within a few weeks, a portion of this initial plume was transported by the polar jet streams across the Atlantic Ocean in the Northern Hemisphere (Peterson et al., 2018), causing a strong perturbation of the stratospheric aerosol loads (Stocker et al., 2021). The quantity of smoke injected was enormous – so much so that it was observed for more than 8 months (Yu et al., 2019. The presence of the resultant aerosol layer was pointed out by high ultraviolet aerosol index values and confirmed with CALIOP lidar observations in the UTLS (Torres et al., 2020). The aerosol mass increase and subsequent adiabatic aerosol self-lofting as a result of absorption of solar radiation were also observed by the Earth Polychromatic Imaging Camera (EPIC) sensor aboard the Deep Space Climate Observatory (DSCOVR) satellite. Kloss et al. (2019) used SAGE III aerosol extinction values to show that the fire plume was transported within the AMA circulation in August 2017. Our analysis suggests that the smoke plume was still present at 18 km above Hyderabad between January and February 2018, indicating that aerosols sampled during the winter flight were influenced by this smoke plume.

6 Convective influence

Deep convection, emanating from Southeast Asia, and maritime convection over surrounding seas serve as a conduit for the transport of boundary layer (BL) pollutants (CO, HCN, CH₄) to the UTLS (Belikov et al., 2013; Park et al., 2007, 2006; Randel et al., 2010). Wind-driven physical processes lead to the accumulation of pollutants due to the limited exchanges of air between the interior and exterior of the Asian monsoon anticyclone (Fairlie et al., 2014, 2020; Ploeger et al., 2015).

Figure 5 Back trajectories initialized from ZF2 (15/08) and ZF3 (21/08) measurements between 150 and 70 hPa. Black dots along the trajectories are the position of convective systems intersecting air masses sampled during the balloon flight.

To study the impact of convection on our measurements, we calculate back trajectories from ZF2 and ZF3 using the Langley Trajectory Model (LaTM; Fairlie et al., 2014) driven by winds from the NASA Global Modelling and Assimilation Office (GMAO) Goddard Earth Observing System, Version 5, Forward Processing (GEOS-5 FP) product (Lucchesi, 2018). We locate the intersection with anvils and deep convective clouds observed through cloud-top brightness temperature from the Himawari-8 satellite (Vernier et al., 2018). Figure 5 shows the position of those 5 d back trajectories (colored lines) and deep convective influences (black dots). Air sampled during ZF2 on 15 August 2017 traveled along two branches influenced by convection over southern/eastern China and western China, respectively. Air masses sampled by ZF3 originated from convection over Laos, Myanmar, the Bay of Bengal, and possibly local convection over the Indian eastern shore close to the measurement location.

7 Nitrite measurements

The role of clouds in nitrite formation is further discussed in this section. To date, only a few nitrite measurements have been reported, mainly because of its low concentrations and the fact that nitrite ions are easily oxidized (Lammel and Cape, 1996). The first quantitative information on nitrite in cloud water was detected at Mt Tsukuba, Japan: values of 400–1050 µg L⁻¹ with pH levels of 5.7–6.5 were reported. In contrast, acidic cloud water samples (pH of 3.4–4.3) collected at significantly higher altitudes showed low nitrite values (15–104 µg L⁻¹) (Okita, 1968). Nitrite and nitrous acid are short-lived intermediates of reactive oxidized nitrogen in the atmosphere. There is limited information on nitrite measurements in the atmosphere (Lammel and Cape, 1996). Nitrite was also measured in fog water samples in a polluted region in Germany (Lammel and Metzig, 1998). Moreover, Bachmann et al. (1989) directly measured nitrite in rain and fog water samples using ion chromatography; respective values of 1.8 and 16 µmol L⁻¹ were found. Photolysis of particulate nitrate, hydrolysis of NO₂, and uptake of HNO₂ by particles are the sources of particulate nitrite in the atmosphere (Chen et al., 2019).

Figure 6 The two sets of six panels show the respective GEOS-Chem model-simulated carbon monoxide (CO, ppbv), sulfate (SO${}_{\mathrm{4}}^{\mathrm{2}-}$, ng m⁻³ at STP), nitrate (NO${}_{\mathrm{3}}^{-}$, ng m⁻³ at STP), ammonium (NH${}_{\mathrm{4}}^{+}$,ng m⁻³ at STP), black carbon (BC, ng m⁻³ at STP), and dust (Ca²⁺, ng m⁻³ at STP) concentrations averaged over 100–150 hPa at 22:00 UTC on 15 August 2017 (top six panels) and 21 August 2017 (bottom six panels). Standard temperature and pressure are 298 K and 1013.25 hPa, respectively. Arrows denote the wind direction, and the white circle indicates the sampling location in Hyderabad, India.

HNO₂ is an important precursor for nitrite formation, but there are challenges involved with making reliable HNO₂ measurements at desired concentrations, leading to a lack of information about HNO₂ in the troposphere. Firstly, due to its short lifetime (because of photolysis) and subsequent very low concentration from sources, HNO₂ is a challenging species to observe. Secondly, HNO₂, being sticky in nature, may be lost to the walls of sampling tubes or absorbed on filters. Thus, as nitrite is present in very low concentrations and is easily oxidized.

Intensive agricultural activities have led to the Indo-Gangetic Plain having the highest ammonia (NH₃) loading worldwide (Wang et al., 2020), as revealed by satellite observations (Van Damme et al., 2018; Warner et al., 2016) and ground-based measurements (Carmichael et al., 2003). Dentener and Crutzen (1994) reported ammonia (NH₃) emissions related to DMS (dimethylsulfide). Höpfner et al. (2016) reported the first detection of ammonia in the ATAL. Nitrite and nitrate are formed by the oxidation of NH₃ through the process of nitrification [NH₃ + O₂ → NO${}_{\mathrm{2}}^{-}$ + 3H⁻ + 2e⁻]. In addition, the existence of NH₃ in the presence of nitrate leads to the formation of ammonium nitrate which could neutralize aerosol particles and favor the persistence of nitrite, as revealed by a few existing measurements in the polluted region (Lammel and Metzig, 1998). The StratoClim campaign also revealed the presence of ammonium nitrate in the UTLS which would confirm that neutralization of nitrate is effective at high altitudes and may explain the persistence of nitrite found with our balloon measurements.

Figure 7 Time series of simulated 3-hourly CO, SO${}_{\mathrm{4}}^{\mathrm{2}-}$, and NO${}_{\mathrm{3}}^{-}$ concentrations averaged over 100–150 hPa at Hyderabad during the ZF2 and ZF3 flights on 15 and 21 August 2017. Also shown are concentrations of NO${}_{\mathrm{3}}^{-}$ due to lightning NO_x emissions (NO₃_LNO_x). See the text for details.

Figure 8 GEOS-Chem model-simulated NO${}_{\mathrm{3}}^{-}$, CO, and SO${}_{\mathrm{4}}^{\mathrm{2}-}$ concentrations (at STP) extracted along the trajectory lines during flights ZF2 and ZF3 (as shown in Fig. 5).

8 Comparison with GEOS-Chem simulations

We conducted GEOS-Chem model simulations to put our observations in the context of the large-scale transport and distribution of atmospheric composition. GEOS-Chem is a state-of-the-art global 3-D chemical transport model that includes fully coupled ozone–NO_x–volatile organic compound–aerosol chemistry for both the troposphere and stratosphere (Bey et al., 2001; Park et al., 2004; Eastham et al., 2014). Here, we use the model version 11-01 (http://wiki.seas.harvard.edu/geos-chem/index.php/GEOS-Chem_v11-01, last access: 29 July 2022). A previous version of the model was used to study the origins of aerosols in the ATAL by Gu et al. (2016) and Fairlie et al. (2020). The model simulates black carbon (Park et al., 2003), primary and secondary organic aerosols (POAs and SOAs, respectively; Pye et al., 2010), sulfate–nitrate–ammonium–aerosol thermodynamics coupled to ozone–NO_x–hydrocarbon–aerosol chemistry (Park et al., 2004), mineral dust (Fairlie et al., 2007; Ridley et al., 2014), and sea salt (Jaeglé et al., 2011), treated as an external mixture. SOA simulation uses the volatility-based scheme (VBS) of Pye et al. (2010). Sulfate–nitrate–ammonium thermodynamics is computed using the ISORROPIA II thermodynamic equilibrium model of Fountoukis and Nenes (2007). ISORROPIA has been evaluated with in situ measurements during several campaigns in the United States (Guo et al., 2016, 2020) and, more recently, in Korea (Ibikunle et al., 2020). Aerosol wet deposition includes rainout and washout due to large-scale precipitation as well as scavenging in convective updrafts (Liu et al., 2001). Scavenging of aerosols by snow and mixed precipitation is described by Wang et al. (2011) and Q. Wang et al. (2014). Dry deposition of dust and sea-salt aerosols uses the size-dependent scheme of Zhang et al. (2011). Dry deposition for other aerosols follows the resistance-in-series scheme of Wesely (1989). Anthropogenic emissions use the Emissions Database for Global Atmospheric Research (EDGAR; Olivier and Berdowski, 2001) with regional options, including the MIX inventory over East Asia (Li et al., 2014) and the EPA/NEI 2011 inventory over North America (Travis et al., 2016). Biofuel emissions are from Yevich and Logan (2003). Carbonaceous aerosol emissions are provided by Bond et al. (2007). Biogenic emissions are calculated by the Model of Emissions of Gases and Aerosols from Nature (MEGAN; Guenther et al., 2012). Biomass burning emissions use the Quick Fire Emissions Dataset (QFED; Darmenov and da Silva, 2015). Lightning NO_x emissions (LNO_x) are as described by Murray et al. (2012) and match the Lightning Imaging Sensor and the Optical Transient Detector (LIS/OTD) climatological observations of lightning flashes.  Volcanic SO₂ emissions are provided by the AeroCom project (data available from http://wiki.seas.harvard.edu/geos-chem/index.php/Volcanic_SO2_emission, last access: 29 July 2022). The model simulations are driven by the Modern-Era Retrospective analysis for Research and Applications (MERRA-2) reanalysis from the NASA GMAO (Gelaro et al., 2017). For computational efficiency, MERRA-2 fields have been mapped from the native grid to a 2.5^∘ × 2^∘ (longitude × latitude) horizontal resolution for input to GEOS-Chem. Further, we used the simulations with and without lightning NO_x emissions to understand the contribution of lightning to the formation of nitrate aerosol.

In situ chemical analysis are compared with GEOS-Chem simulations. Figure 6 shows the maps of CO, nitrate, sulfate, ammonium, black carbon (BC), and dust aerosol concentrations averaged over 100–150 hPa at 22:00 UTC for 15 and 21 August during the ZF2 and ZF3 flights, respectively (white circle on the map). On 15 August, CO, BC, nitrate, ammonium, and dust aerosol concentrations are enhanced over West China, Nepal, and northeastern India, with the center of the anticyclone positioned over West China. On the contrary, on 21 August during ZF3, the position of the anticyclone was shifted to the east, and the flight apparently sampled air at the edge of the anticyclone. The simulation shows a 20 % and 50 % increase in CO and BC on 21 August, respectively, compared with 15 August. Additionally, the ammonium concentration was decreased by 50 % and dust was reduced by 60 % on 21 August compared with the first flight on 15 August. However, the ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ concentration is seen to be stable at ∼80 ng m⁻³ for both 15 and 21 August above Hyderabad. The simulated NO₃ concentrations near the location of ZF2 and ZF3 are spatially inhomogeneous, with variations between 30 and 2700 ng m³ across South India. Figure 7 shows the time series of model 3-hourly CO, sulfate, and nitrate concentrations averaged over 100–150 hPa within the model grid point where Hyderabad is located during August 2017. The CO concentration shows a decrease of 14 %, while an increase of 21 % in the SO₄ concentration is seen in the ZF3 flight that took place on 21 August. The measured nitrate concentration during ZF2 and ZF3 (around ∼100 ng m⁻³) is within the range of values simulated within 24 h of the observations. The results of the GEOS-Chem model simulation indicate that lightning NO_x could significantly contribute (up to ∼75 % on 10 August) to the formation of nitrates during certain time periods. The lifetime of NO_x is approximately 3 h in the region of the outflow of thunderstorms due to the production of methyl proxy nitrate and alkyl, and multifunctional nitrate species and their lifetime is believed to increase downwind of the outflow (Nault et al., 2017). Also shown in Fig. 7 are nitrate concentrations attributed to lightning, as determined by the difference between simulations with and without lightning NO_x emissions. ZF2 and ZF3 occurred during a period where the levels of nitrate were ≈50 ng m⁻³ at STP on 15 August and ≈30 ng m³ at STP on 21 August. The influence of lightning NO_x emissions was minimal during this time. Nevertheless, CO levels are slightly higher during ZF2 (80 ppbv) than during ZF3 (60 ppbv), indicating that measurements made during ZF2 may have been more influenced by pollution. The latter is also reflected by the higher BC levels during ZF2 in the model.

We extracted CO, nitrate, and sulfate concentrations from the GEOS-Chem simulation along the calculated trajectories initialized from the ZF2 and ZF3 measurement locations in Fig. 8. The lines are colored according to the balloon GPS altitudes that are used to initialize the trajectory model. Figure 5 uses GEOS-FP winds (meteorology) to convey that GEOS-Chem could simulate convective activities reaching levels between 14 and 15 km. This is confirmed by cloud-top heights (black circle) derived from Himawari-8 crossed by trajectories originating from the troposphere for both ZF2 and ZF3. ZF2 was influenced by convective activities over Western China, whereas ZF3 sampled air masses originating from convection in Southeastern Asia (Myanmar, Laos). CO levels with initial altitudes near 14–15 km (green color) for ZF2 are shown to decrease from 120 to 80 ppbv along the back trajectories, confirming the influence of Chinese pollution and its progressive dilution. At the same initial altitudes, the CO levels along ZF3 back trajectories are significantly lower (near 50–80 ppbv), possibly indicating minimal impacts from polluted sources. The levels of NO${}_{\mathrm{3}}^{-}$ show significant variability along the trajectories for both cases but are more pronounced in ZF3, with levels above 400 ng m³, again emphasizing the likely importance of LNO_x in the production of nitrate aerosols.

Sulfate concentrations are much higher (100–200 ng m³ at STP) for air parcels initialized near 16–17 km for ZF2 and ZF3, likely indicating stratospheric sources, whereas air parcels near 14–15 km show levels below 100 ng m³ at STP. We note that sulfate along the trajectories influenced by Chinese pollution during ZF2 increases by 60 % approximately 50 h before our measurements, which could indicate the formation of sulfate aerosol from SO₂. It has previously been reported that sulfate has a lifetime of a few days in the troposphere (Hidy and Blanchard, 2016). The rather short lifetime of sulfate is due to absorption in precipitation or its solubility (Hidy, 1973). The global mean residence time of tropospheric sulfate against dry and wet deposition is approximately a few days (e.g., Park et al., 2004).

The GEOS-Chem model showed higher sulfate levels than the results from IC due to relatively weak scavenging of SO₂ and/or SO${}_{\mathrm{4}}^{\mathrm{2}-}$.

The aircraft field campaigns of the StratoClim project were held in July and August 2017 at the Tribhuvan International Airport (KTM; 27.70^∘ N, 85.36^∘ E; Katmandu, Nepal). In situ aerosol measurements within the AMA were carried out using the ERICA (European Research Council Instrument for Chemical composition of Aerosols) aerosol mass spectrometer. Flights KTM 01 and KTM 02 held on 27 and 29 July showed a low level of sulfate at 360 K (potential temperature), corresponding to an altitude of 15 km. The sulfate concentration was almost equal to zero on 10 August during flight KTM 08 at 16 km and 370 K (Stephan Borrmann, 4th ACAM workshop, 28 June 2019). The very low levels of sulfate sometimes observed in the StratoClim campaign near 360–380 K are consistent with our IC analysis results with respect to the sulfate ionic concentration during flight ZF2 held on 15 August at the same altitude and potential temperature.

9 Summary and conclusions

The chemical composition of the ATAL has been investigated using offline IC analysis of aerosol-impacted samples collected aboard zero-pressure balloon flights as part of the BATAL campaigns. The measurements of the 2017 summer campaign indicate the dominant presence of nitrate and nitrite aerosols with concentrations between 88 and 374 ng m⁻³ at STP. Our first flight (ZF2) on 15 August 2017 occurred within the AMA and, thus, sampled air masses therein. In situ measurements revealed the presence of NO${}_{\mathrm{3}}^{-}$, and NO${}_{\mathrm{2}}^{-}$ aerosols (60–200 ng m⁻³ at STP) with sizes ranging from 0.05 to 2 µm. The second flight (ZF3) on 21 August 2017, however, occurred at the edge of the anticyclone, and subsequent in situ measurements revealed the presence of larger-particle-size NO${}_{\mathrm{3}}^{-}$ and NO${}_{\mathrm{2}}^{-}$ aerosols at higher concentrations (87.3–343 ng m⁻³ at STP). Throughout the flights during the 2017 summer campaign, sulfate aerosol remained below the detection limit of the system (10 ng m⁻³ at STP) and were much lower than the results from the GEOS-Chem model simulation (80–120 ng m⁻³ at STP). The higher model sulfate levels compared with the values from IC are believed to be due to relatively weak scavenging of SO₂ and/or SO${}_{\mathrm{4}}^{\mathrm{2}-}$ in the model. Unlike the summer, Ca²⁺ and SO${}_{\mathrm{4}}^{\mathrm{2}-}$ were found on all four stages (sizes ranging between 0.2 and 0.05 µm) along with traces of NH${}_{\mathrm{4}}^{+}$, which could not be quantified in the winter campaign. The winter flight sampled residuals from the 2017 Canadian wildfires affected stratospheric aerosol loadings for several months.

We study the influence of convection on those measurements using back-trajectory calculations co-located with geostationary satellite observations. We show that ZF2 and ZF3 were influenced by convection over Western China, the Bay of Bengal, and Myanmar, Thailand, and Laos. The model was able to reproduce the convective transport from the mid-troposphere (9–12 km) to the upper troposphere (14–15 km). There was no indication of the transport of these air parcels from the boundary layer. Although Himawari-8 observations showed the convective transport reproduced in MERRA-2, the mixture between horizontal and vertical transport was not visible in trajectory calculations. Tropical convection could explain the rapid ascent of the air parcels to higher altitudes because other mechanisms, namely radiative heating, would delay the transport of air parcels from the middle to the upper troposphere. While the model seems to represent convection in the upper troposphere (14–15 km), with the rapid ascension of air parcels, the model's ability to simulate convective influence at higher altitudes seems to be limited.

We used the GEOS-Chem model simulations with and without lightning NO_x emissions to understand the contribution of lightning to nitrate aerosol. Flights ZF2 (15 August) and ZF3 (21 August) occurred during a period when the levels of nitrate were relatively small (<100 ng m⁻³ at STP) with minimal influence of lightning NO_x, in contrast with other periods largely affected by nitrate produced by LNO_x. As shown by trajectory calculations in Fig. 5, flights ZF2 and ZF3 sampled air masses localized at the border of the Asian anticyclone. Fairlie et al. (2020) showed that the eastern part of the ATAL anticyclone depicts a peak of ammonium contribution from Chinese emissions. The western core of the ATAL, on the other hand, is seen to be enriched with 80 % of anthropogenic sources from India, with the southern and eastern flanks of the anticyclone showing peaks of Chinese contribution wherein nitrate concentrations were found to be the highest.

As the ASM (Asian summer monsoon) varies with respect to spatial dimensions and methodology, inconsistencies in the seasonal and interannual contribution to the ATAL are expected. Mineral dust is considered to be the most abundant type in the troposphere, with its main emission source being from arid and semiarid regions (Huneeus et al., 2011). CaCO₃ is considered to be one of the most important components of mineral dust, and about 1.3 Tg of CaCO₃ is loaded in the troposphere, corresponding to approximately 8 % of the total loading of mineral dust (Scanza et al., 2015). During atmospheric transport, heterogeneous reactions occur with trace gases, forming more soluble species and resulting in the increased CCN (cloud condensation nuclei) activity of mineral dust particles. Flight ZF2 sampled air masses within a cloud showing the presence of Ca²⁺ and NO³⁻ on the same stage (particle size <0.15 µm). This implies the formation of Ca(NO₃)₂ in the presence of HNO₃.

Indeed, the atmosphere is an amalgamated den in which gaseous species, particulates, and liquid droplets coexist at the same time. Through our balloon campaigns during the ASM with simultaneous offline measurements of inorganic species and, subsequently, comparing these results with model simulations, we were able to understand, if not fully answer, the many unanswered questions regarding the existence and behavioral patterns of these ionic species of interest. We will continue to research this area with improved techniques and additional experimentation.