Ambient Nitro-Aromatic Compounds – Biomass Burning versus Secondary Formation in rural China

Abstract. Nitro-aromatic compounds (NACs) were measured hourly at a rural site in
China during wintertime to monitor the changes due to local and regional
impacts of biomass burning (BB). Concurrent and continuous measurements of
the concentrations of 16 NACs in the gas and particle phases were performed
with a time-of-flight chemical ionization mass spectrometer (CIMS) equipped
with a Filter Inlet for
Gases and AEROsols (FIGAERO) unit using iodide as the
reagent ion. NACs accounted for <2 % of the mass concentration of
organic matter (OM) and total particulate matter (PM), but the total
particle mass concentrations of these compounds can reach as high as 1000 ng m−3 (299 ng m−3 avg), suggesting that they may contribute
significantly to the radiative forcing effects of atmospheric particles.
Levels of gas-phase NACs were highest during the daytime (15:00–16:00 local
time, LT), with a smaller night-time peak around 20:00 LT. Box-model
simulations showed that this occurred because the rate of NAC production
from gas-phase sources exceeded the rate of loss, which occurred mainly via
the OH reaction and to a lesser degree via photolysis. Data gathered during
extended periods with high contributions from primary BB sources (resulting
in 40 %–60 % increases in NAC concentrations) were used to characterize
individual NACs with respect to gas–particle partitioning and the
contributions of regional secondary processes (i.e. photochemical smog). On
days without extensive BB, secondary formation was the dominant source of
NACs, and NAC levels correlated strongly with the ambient ozone
concentration. Analyses of individual NACs in the regionally aged plumes
sampled on these days allowed precursors such as phenol and catechol to be
linked to their NAC derivatives (i.e. nitrophenol and nitrocatechol).
Correlation analysis using the high time resolution data and box-model
simulation results constrained the relationships between these compounds and
demonstrated the contribution of secondary formation processes. Furthermore,
13 of 16 NACS were classified according to primary or secondary formation
process. Primary emission was the dominant source (accounting for 60 %–70 %
of the measured concentrations) of 5 of the 16 studied NACs, but secondary
formation was also a significant source. Photochemical smog thus has
important effects on brown carbon levels even during wintertime periods
dominated by primary air pollution in rural China.


Action: Grammatical issues were addressed, and the sentence was divided and do now read: "BB is considered a major driver of atmospheric NAC formation (Kahnt et al., 2013;Laskin et al., 2015).Here, the combustion of coal and wood leads to thermal degradation and pyrolysis of lignins, that results in strong emission of substituted phenols including 1,2-benzenediols (catechols) and cresol/methylphenols that in turn are precursors for the formation of NACs" 2. Page 4, Line 16: please also cite [Lee et al., 2014] Action: Lee et al.,2014 is now included in the main text.

Methods:
1. Section 2.1: I'd be curious to see HYSPLITs and also know if any information is available regarding boundary layer height both of which I think will help interpret the data.As the authors correctly point out, the AtChem model does not factor in meteorology and adding this information will help augment the chemical interpretation of their work.

Reply:
A backward trajectory analysis using the NOAA HYSPLIT Model were done.However, the results did not add significant insight into explaining the chemical evolution of the investigated compounds.There was a slight difference in air mass origin between regime 1 and 2 where most of the arriving airmasses arriving in Dezhou was coming from the northwest and stayed mostly inland.During regime 1 (Dec 14-19) most of the airmasses that arrived in Dezhou came from inner Mongolia creating little time for regional secondary formation.On the other hand, airmasses during regime 2 generally had extended time in the region which could explain an aging effect on primary emission in the region before reaching Dezhou.(see Figure R1).However, we prefer not to add these speculations to the manuscript.Furthermore, Weather Research and Forecasting (WRF) calculation indicated average highest BL at ~1300 m (usually at noon, can be up to 1500), and average lowest ~800 m (usually at night, can be as low as <300m).Again, these are estimates that potentially do not completely match real condition and we after this exploration judge the usefulness of these to explain the results to be limited and rather keep the focus on the chemical indicators defining the two regimes.
Action: No action 2. Page 5, Line 15: is this a WHO limit or a national limit for PM2.5?Please clarify.
Reply: The PM 2.5 limit presented was based on the Air Quality Standards set by the European Union.
Action: This was clarified in the main text with following statement …with an aerosol mass spectrometer typically exceeded the European Air Quality allowable limit for PM 2.5 (25 μg m -3 )

Page 6, line 3, please rename to Filter Inlet for Gases and Aerosols (FIGAERO) coupled to a chemical ionization time-of-flight mass spectrometer (TOF-CIMS).
Action: Done.The terms "coupled to a" was included in the description of the main instrument and the sentence now reads: A Filter Inlet for Gas and Aerosol (FIGAERO) coupled to a time-of-flight mass spectrometer (ToF-CIMS) was utilized to characterize the NAC content of the gas and particle phases.

Page 7, were any reference gases added to monitor drift in signal? Were any gas phase zeros performed?
Reply: There were no reference gases used instead, the signal of the reagent ion (Iodide, m/z= 126.904) provided the information on the drift of the signal of the mass spectrometer (MS).The variabilities of the raw iodide signal during the field measurement were less than 10 and 20% for the gas and particle phase analysis, which indicated the minimal drift of the CIMS signal.During the post-processing of the data, all signals from MS were normalized to the signal of the reagent ion to account for the daily variations/drifts.Gas-phase zero was performed during the post-campaign calibration of the instrument.

Action:
The following statements that detail the drift in signal and gas phase zeroes were included in the experimental section The signal of the reagent ion (Iodide, m/z= 126.904) provided the information on the drift of the signal of the mass spectrometer (MS).The variabilities of the raw iodide signal during the field measurement were less than 10 and 20% for the gas and particle phase analysis, which indicated the minimal drift of the CIMS signal.During the post-processing of the data, all signals from MS were normalized to the signal of the reagent ion to account for the daily variations/drifts.Gas-phase zero was performed during the post-campaign calibration of the instrument 5. Page 7, Line 21: some of these compounds are light sensitive, were they used fresh for calibration?
Reply: Freshly prepared calibrants were utilized during the standard compound analysis to prevent any degradation of the NACs.

Action:
The statement in the experimental section was modified accordingly.
"The NACs were quantified by doping the PTFE filter of the FIGAERO with known amounts of freshly prepared authentic standards"

Results
1. Figure 2: I suggest pointing out the levoglucosan peak in the spectra and other markers used to indicate BB events.

Reply:
We wish to include the peaks of levoglucosan and HONO in the figure, however, the signals of these two compounds were not as prominent in the graph compared to nitrocatechol and nitrophenol due to the difference in the sensitivity of the compounds to the instrument.

Action: No action
2. Page 10, Line 9: your diurnal peaks in particle phase NACs are similar to those from residual wood burning seen in Gaston et al 2016.
Reply: We would like to thank the reviewer for highlighting the similarity of our results with emissions from residual wood burning.

Action:
The following statement was included in the main text "The diurnal profile of particle-phase NACs was comparable with the observed profile of nitrocatechol detected from residential wood smoke (Gaston et al., 2016)"

Page 17, Lines 20-22, I'd be curious to know if the secondary production of NACs was also correlated
with OOA from the AMS measurements.

Reply:
We observed poor correlation (r= -0.1 to -0.4) for most nitro-aromatic compounds with OOA, as illustrated in the time series profiles of major NACs and OOA in figure R2.In particular, a strong enhancement of OOA was measured in the later period (Nov 28-30), when the production of NACs was the lowest.This negative relationship between NACs and OOA may indicate an efficient loss rate through partitioning to the particle phase with subsequent condensed phase reactions.Reply: Indeed, the model was not able to capture the variability of the measured nitrocatechol for those days.As indicated in the previous comment, aerosol mass increased drastically between December 27 to 31 (also humidity) which would favor the partitioning of the gas phase NACs to the particle phase and potential a loss by condensed phase process/deposition.

Action:
The following statement was included in Section 3.5 to explain the overestimation of the model between Dec 17 to 31.
"The modelling procedure overestimated the observed concentration from Dec 27 to 31, which was accounted to elevated mass aerosol mass concentration and increased RH which would favor the partitioning of the gas phase NACs to the particle phase and a potential loss by condensed phase processes/deposition"

Response to Reviewer 2
General.This is an interesting study on the occurrence of nitroaromatic compounds (NAC) in particles which differentiates between biomass burning (BB) and secondary formation (SF) in the city of Dezhou, Shandong province in China in the winter of 2017.The paper combines NAC measurements in the gasand particle phase with box modelling.Main measurement instrumentation was a Figaero inlet coupled to a Api-ToF-CiIMS operated in iodide cluster ion mode.Overall, this is a very interesting study extending our knowledge on NACs significantly by state-of-the-art measurements.It is fully in the focus of ACD and, in my view, could be accepted after minor revision.

Reply:
The authors are grateful that the reviewer recognized the main relevance and highlights of our study.

Details
Section 3.1 If BB periods are to be identified, was there the possibility to use levoglucosan measurements?
Reply: The negative ion (Iodide) ToF-CIMS with FIGAERO inlet is capable of measuring levoglucosan both in gas and condensed phase.The measurement of the levoglucosan was highlighted in figure 4 where the concentration of levoglucosan was enhanced during NAC formation regime 1 when BB episodes were evident.NACs may also form in aqueous phases, particularly when the atmosphere has a high liquid water content (Harrison et al., 2005b;Vidovic et al., 2020;Vidović et al., 2018).
A reference to aqueous phase processes where added in section 3.5 as plausible explanation for deviation between model and measurements for the period 27/12-31/12.
"The modelling procedure overestimated the observed concentration from Dec 27 to 31, which was accounted to elevated mass aerosol mass concentration and increased RH which would favor the partitioning of the gas phase NACs to the particle phase and a potential loss by condensed phase processes/deposition"
Additional key aqueous phase focused studies were referred to in the following statement(at  P12, line 19) Action: no actionComment: P 12, line 19: Aqueous chemistry could be important involving BB compounds -what about this possibility?Do days with the high RH give indications for contributions of such pathways?Maybe such consideration can be included here.Reply:The contribution of aqueous chemistry to the formation of NAC in this study was expected to the be limited based on the weak association between relative humidity and the mixing ratio of NACs in gas and particle phase.During the measurement, the condition in Dezhou was relatively dry (Mean R.H.: 50%) with only a few days with R.H. exceeding 70%.However, we further acknowledge the importance of aqueous chemistry in the formation of nitroaromatic compounds by citing relevant studies that provided important implications of atmospheric water content.In addition, there were one exception where the RH reach 100% and a noticeable loss ofNAC were observed,Action: