Dear authors

thank you for gathering this corpus of founding articles to understand the construction, evolution and fundamental discoveries of this discipline that is the chemistry of the atmosphere. This article will remain a reference and an entry point for the new generation of researchers, as well as for the older generation to understand its field and progress. 

As indicated at the end of the article, I would like to provide additional information. It seems to me that Lovelock's article (1) and his discovery of DMS (thanks to his invention of the ECD detector (2) which also allowed the first measurements of CFCs) as the reduced sulfur compound  emitted by themarine biota(and not H2S as thought previously) would have its place in this article. This article also advances the idea that the MSA resulting from the oxidation of DMS is a probable source of aerosols. It is on the basis of this article that the CLAW hypothesis was born. This article deserves to be mentioned.

In the same vein, I think Patterson's paper on the increase of lead in the air (3) (actually in snow) as well as its decrease since the elimination of lead in gasoline (4) would be worth mentioning. With the elimination of CFCs, this is the other major victory of atmospheric chemistry research over environmental policy. 

With regards

 

1- Lovelock, J. E., Maggs, R. J., and Rasmussen, R. A.: Atmospheric Dimethyl Sulphide and the Natural Sulphur Cycle, Nature, 237, 452-453, 10.1038/237452a0, 1972.

2- Lovelock, J. E., SR Lipsky,Electron Affinity Spectroscopy-A New Method for the Identification of Functional Groups in Chemical Compounds Separated by Gas Chromatography -Journal of the American Chemical, 82, 431, 1960. 

3-Murozumi, M., Chow, T. J., and Patterson, C. (2003): Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctic snow strata, Geochim. Cosmochim. Acta, 33, 1247-1294, https://doi.org/10.1016/0016-7037(69)90045-3, 1969.

4-Boutron, C. F., Görlach, U., Candelone, J.-P., Bolshov, M. A., and Delmas, R. J.: Decrease in anthropogenic lead, cadmium and zinc in Greenland snows since the late 1960s, Nature, 353, 153-156, 10.1038/353153a0, 1991.

Page 35, Section 2.17 - "A step-change in this area was made with the paper “Estimating 929 ground-level PM2.5 using aerosol optical depth determined from satellite remote sensing” by van Donkelaar et al. (2006) which was the first description of the derivation of surface PM2.5 from satellite AOD, and which has been extensively used to estimate the global impact of particulate matter (both PM2.5 and PM10) on health"

It appears that the above assessment is not correct.

The first paper showing quantitative PM2.5 estimation using satellite AOD was published in 2003 as 'Wang, J., and S. A. Christopher, Intercomparison between satellite-derived aerosol optical thickness and PM2.5 mass: Implications for air quality studies, Geophys. Res. Lett., 30(21), 2095, doi:10.1029/2003GL018174, 2003'

introduced a simple statistical method to estimate PM2.5 using satellite data, later on, the following paper by Liu et al., (2004)

"Liu, Y., Park, R. J., Jacob, D. J., Li, Q., Kilaru, V., and Sarnat, J. A. (2004), Mapping annual mean ground‐level PM_2.5 concentrations using Multiangle Imaging Spectroradiometer aerosol optical thickness over the contiguous United States, J. Geophys. Res., 109, D22206, doi:." 

first introduced the model scaling approach, which was later adopted by van Donkelaar et al. (2006) and resulted in global estimates of PM2.5 at annual mean levels. 

Over the last two decades, there have been several hundreds of papers published in assessing the satellite-derived AODs to estimate surface particulate matter air quality.

I want to congratulate the authors for putting together this interesting overview based on a series of suggestions from the science community.

In the spirit of collaboration that the authors ask for, I have a few comments.

continues from the previous submission

Thanks for this excellent overview of atmopheric chemistry research. It made really interesting reading.

I have a suggestion to make about section 2.13 on biogenic VOCs. Please see the linked document. 

Dear Paul, Ravi et al.,

Thank you for the considerable effort in putting this history together – to one active in the field since the late 1970s (when the curve in Fig 1 kicks off), it is an enjoyable read, and a trip down memory lane.

Inevitably, having solicited them, you will get lots of suggestions for things that you have left out or rated less important than judged by others.  Already there are several…  So I will add mine, for your consideration.  Two areas in my view deserve their own explicit subsection of section 2.  Some aspects are already mentioned in passing in several places, but my judgement would be that they deserved their own explicit subsection.

1. Measurement techniques. We might divide up the whole topic into two sub topics – the chemistry itself, and the methods used to probe it. Many advances in our understanding of atmospheric chemistry have come from stepwise advances in measurement (and modelling) capabilities.  There are already explicit sections for kinetics, models, meteorology and transport and satellite measurements, but nothing explicitly for chemical measurement techniques. The success of the high impact journal  AMT speaks for the importance. Examples of discoveries that might be included in this area might be (but are not limited to):
   1. The ECD detector (Lovelock), which enabled the detection of halocarbons
   2. PTR-MS – already mentioned in 2.12 in the VOC context
   3. OH measurements – LIF etc.
   4. The chemiluminescence technique for NO and NOx
   5. Dobson method for O3 (admittedly stratospheric)
   6. Satellite retrievals – satellite global mapping of composition has been a real revolution.
      
      
      
      
2. Greenhouse and climate-relevant gases (and aerosols). Given the current state and importance of climate change science and policy, a separate section on greenhouse and other climate-relevant species seems to me to be very desirable. There are some mentions throughout, eg to Keeling and to long term monitoring networks like ALE/AGAGE. GAW, NOAA, ICOS, but it would be valuable to bring them together into their own section. Also the techniques that enable reliable and accurate long term records to be maintained – NDIR, optical in situ and remote sensing techniques, isotopic methods, and satellite measurements (again).

This is an excellent and broad perspective on tropospheric chemistry. As such it will certainly become a valuable reference and resource.  As already pointed out by Nick Hewitt, it was actually Sanadze who first reported substantial isoprene emissions from plants in Russia (1957). Rasmussen independently found similar results in the US. I highly recommend the review article by Sharkey and Monson on 60 years of isoprene research. (Isoprene research – 60 years later, the biology is still enigmatic, doi: 10.1111/pce.12930, 2017). Further it is mentioned that "recent interest in deposition reemerged", but  that physio-chemical parameters are lacking. Physio – chemical parameters for most gases (e.g. HLC, vapor pressure, reactivity etc.) are very well established and an important reason why the ‘Wesely scheme’ works quite well for classic air pollutants over many surface types. I would argue that the complication stems more from a range of oxygenated VOC that can exhibit bi-directional exchange above vegetation (e.g. doi: 10.1126/science.1192534, doi: 10.1126/science.1235053). Moreover it was recently demonstrated that plants can also "reprocess" OVOCs, produced during atmospheric oxidation, through enzymatic reactions (doi: 10.1038/s43247-020-00041-2 ). The concept of bi-directional exchange is not included in the theoretical framework of emission and deposition models as there is little need for most gases. Yet, it is somewhat overlooked when discussing the fate of reactive organic carbon as many  oxygenated volatile species can reside in this no-man’s land.

I commend the authors for leading this nice effort and inspired by the call for contributions, I have a few suggestions. I am missing a discussion on nitrous acid (HONO) which has emerged as a major source of OH radical during the last 15 years.

Here is a relevant draft summary:

HONO photolysis is a major OH source, especially during the early morning, which was first identified by Perner and Platt (1979):

 HONO                        +          hν (λ < 400nm)           →        OH      +          NO

During the daytime, photolysis of HONO has been shown to be the most important OH initiation source under urban high NOx conditions (e.g., Elshorbany et al., 2009a, 2009b) and was found to have a significant impact on ozone photochemical formation (Elshorbany et al., 2010). HONO was also demonstrated to have large global impact on gas phase (Elshorbany et al., 2012) and aerosol formation (Elshorbany et al., 2014).

HONO is now recognized as an important species that is being measured routinely during field measurements. Attempts are currently underway to detect it from space, which is being challenged by its low near-surface concentrations but was recently detected in biomass burning emissions.

HONO research is driven by unexpected high daytime values of HONO (e.g. Neftel et al., 1996, Zhou et al., 2002; Kleffmann et al., 2002; Acker et al., 2006a, 2006b, Elshorbany et al, 2009). Several photochemical HONO sources have been proposed (Kleffmann, 2007), some of which have been identified in the laboratory (e.g., Zhou et al., 2003; George et al., 2005; Stemmler et al., 2006, 2007; Bejan et al., 2006; Li et al., 2008, Gustafsson et al., 2006; Ndour et al., 2008). HONO sources in the atmosphere are not yet well understood. While the dark heterogeneous conversion of NO2 on humid surfaces (Finlayson-Pitts et al., 2003) is commonly accepted as the dominant HONO sources during night (Alicke et al., 2002), the exact mechanism is still unclear:

2 NO₂ + H₂O               →        HONO + HNO₃

In addition, recent studies showed that the heterogeneous reaction of NO₂ with adsorbed hydrocarbons are also important under atmospheric conditions (Ammann, et al., 2005).

NO₂ + Organics(red)     →        HONO + Organics(oxi.)

For HONO daytime sources, five photochemical mechanisms were recently identified. Three of them dominate under high NOx urban conditions, namely heterogeneous conversion of gaseous NO2 on photosensitized solid surface organic compounds (George et al., 2005; Stemmler et al., 2006), photocatalytic conversion of NO2 on TiO2 (Gustafsson et al., 2006; Ndour et al., 2008) and the photolysis of the gaseous nitroaromatic compounds (Bejan et al., 2006) and should correlate well with j(NO₂). Under low NOx rural conditions, the photolysis of nitric acid (Zhou et al., 2003) adsorbed on solid surfaces (including vegetations) may dominate and would better correlate to j(O¹D), caused by the much lower wavelength range of the nitric acid photolysis.

Comment by:

Owen R. Cooper

Senior Research Scientist

Cooperative Institute for Research in Environmental Sciences (CIRES)

University of Colorado Boulder/NOAA Chemical Sciences Laboratory

February 19, 2021

I commend the authors for undertaking this endeavor, which will serve as an excellent historical reference for our field of research. I will limit my comments and suggestions to Section 2.16

Section 2.16 provides a short review of key papers on Chemical Transport. It starts out well with a discussion of STE and recognition of Danielsen 1968 and Holton et al. 1995, but it needs a better connection to the STE review article by Stohl et al. 2003, which appears much further down (on Line 887), after the concept of long range transport is introduced. This paper should appear in the paragraph on STE.

Before recognizing the important study by Jacob et al. 1999 (which is based on a model rather than observations) some key observational studies should be recognized, for demonstrating that long-range and intercontinental transport clearly occurs.

These studies are as follows:

Merrill et al. (1985) (cited 124 times) used isentropic back trajectories to link transport from Asia and North/Central America to the Marshall Islands

Moody et al., 1995 (cited 114 times) also used isentropic back trajectories to link transport from North America to Bermuda.

Stohl and Trickl, 1999 (cited 214 times) were the first to show that a warm conveyor belt can transport pollution from one continent to another, in this case, ozone from the boundary layer of North America to the free troposphere above Europe.

Forster et al., 2001 (cited 225 times) used kinematic back trajectories (FLEXTRA) to show that smoke from biomass burning in Canada could reach the boundary layer of Europe, and they showed that due to the thermal structure of the atmosphere, this transport has to occur at low levels behind cold fronts rather than in warm conveyor belts.

In addition to being great, groundbreaking studies, the papers by Moody et al. 1995 and Forster et al. 2001 are also notable because they were led by women at a time when the field was still dominated by men.

Regarding the discussion of trajectory methods, the earlier and more simplistic isentropic trajectory method (Merrill et al., 1985) should be mentioned before HYSPLIT.

Forster, C., Wandinger, U., Wotawa, G., James, P., Mattis, I., Althausen, D., Simmonds, P., O'Doherty, S., Jennings, S.G., Kleefeld, C. and Schneider, J., 2001. Transport of boreal forest fire emissions from Canada to Europe. Journal of Geophysical Research: Atmospheres, 106(D19), pp.22887-22906.

Merrill, J. T., Bleck, R., and Avila, L. (1985), Modeling atmospheric transport to the Marshall Islands, J. Geophys. Res., 90( D7), 12927– 12936, doi:10.1029/JD090iD07p12927.

Moody, J.L., Oltmans, S.J., Levy, H. and Merrill, J.T., 1995. Transport climatology of tropospheric ozone: Bermuda, 1988–1991. Journal of Geophysical Research: Atmospheres, 100(D4), pp.7179-7194.

Stohl, A. and Trickl, T., 1999. A textbook example of long‐range transport: Simultaneous observation of ozone maxima of stratospheric and North American origin in the free troposphere over Europe. Journal of Geophysical Research: Atmospheres, 104(D23), pp.30445-30462.

We thank all colleagues who has posted various suggestions.  We will attempt to incorporate all the comments to the best of our abilities.

We invited any other comments from colleagues.

I will add my support for some additional references to the chemiluminescence detection of NO/NO2. While Leighton's photostationary state hypothesis was fully established as you point out by the early 90's that establishment involved lots of measurements using the chemiluminescence detector. During the 70's and 80's the device was likely responsible for most of the NO/NO2 measurements used for determining the jNO2 which was crtical for model chemistry. I am not as widely read for this topic during this time period but papers by my former boss, Don Stedman, and others that I am aware of for a starting point are:

 D. H. Stedman, J. O. Jackson, "The photostationary state in photochemical smog" Intern. J. Chem. Kinetics Symp. I: 493

(1975).

 D. Kley, M. McFarland, "Chemiluminescence detector for NO and NO2," Atmos. Technol. 12:63 (1980)

R. E. Shetter, D. H. Stedman, and D. H. West, "The NO/NO2/O3 Photostationary State in Claremont, California," J. Air Poll. Cont. Assoc. 33:3, 212-214, 1983.

As a side note for the importance of the chemiluminescence detector for NO2. Don flew one on a commercial flight between Denver and Boston and included the data in the final report for the National Research Council entitled "The Airliner Cabin Environment Air Quality and Safety Committee on Airliner Cabin Air Quality" in 1986 which recommended a ban of smoking on commercial aircraft. That data highigted the absurdity that limiting the number of smoking rows to a few in the back of the plan spared others from the exposure. This report led to the eventual ban of smoking on US flights and laid the groundwork for all of the current limits on smoking because of the potentail negative health effects. 

The following comments are totally self-serving and I point this out so that the authors can choose to totally ignore them solely on that basis if they so choose. The paragraph in the emission section on vehicle emissions includes 4 references. The works of Calvert et al., Lawson and Singer and Harley use data either in part, Calvert et. al. and Lawson, or entirely (Singer and Harley) to form a basis of  their works that was collected using a remote sensing device invented by Don Stedman. The work described by Chaney was important in the development of the device Don invented as it gave Don the initial idea (he was a colleague of Chaney and involved in his work) but the method described by Chaney was a simple time series of a single species, carbon monoxide, captured alongside a road that implied that individual vehicle plumes could be capture if one had a fast enough detector. Chaney's work provides no way for those CO measurements to be turned into fuel specific emissions that could be used for vehicle emission comparisons or inventory calculations that Calvert, Lawson and Singer an Harley rely upon in their works. It was not until Don demonstrated the high speed measurement of pollution ratio's to carbon dioxide for vehicle exhaust emissions that made this possible.

Final comment on emissions the papers by Calvert et al., Lawson and Singer and Harley do not really "verify" the California Smog Control Program. If anything these papers call into question how well these control programs actually work and strongely suggest that it is significantly less that the State believes. At the time of writing of these papers California was in denial that a small portion of the fleet was responsible for a large proportion of the total emissions. In the state's mind high emitters did not exist because all of the control programs had eliminated them. Years of measurements subsequently have proven this idea wrong.

Thanks for the effort taken so far in providing a thourougly entertainting and educational look back at all the work that has combined to get us to where we are today in our understanding of Atmospheric Chemistry.

Gary Bishop

University of Denver