If you do decide to resubmit, please include with your revised manuscript a statement indicating
how you have dealt with the review comments. List the comments individually, explain how you
responded to each comment, and provide the location of the response within the manuscript (page
and paragraph). Should you choose not to make a change recommended by a reviewer, please
justify that decision.
Naohiro Yoshida
Editor
Geophysical Research Letters
Reviewer #1 Evaluations:
Science Category: Science Category 3
Presentation Category: Presentation Category A
Reviewer 1
1. The reviewer doubted that the results of our work are significantly different from what can be
expected based on previous studies of the impacts of boreal fires on atmospheric trace gases
and claims that similar results were already documented in numerous studies.
We disagree that the results of this study are not different from the previously published works. In
fact, the reviewer 2 even believes that our results contradict to what is known about the impact of
boreal fires on ozone. Below we list unique features of this work.
First, up to date there are no studies on the large-scale impact of boreal fires on ozone in the free
troposphere involving long-term measurements. The existing studies use the measurements made
at the ground level (e.g. Jaffe04, Simmonds05). Therefore, the current work is a significant addition
to the scientific knowledge since it utilizes simultaneously made CO and O3 measurements
obtained at the free tropospheric PICO-NARE station. Thus, by analyzing these data we can learn
about the large-scale impact of boreal fires on the free troposphere, before this signal gets watered
down within the boundary layer. Such signal is expected to be very different from the signal
detected in the BL.
Second, impact of boreal fires on ozone is currently a topic of huge interest in the atmospheric
chemistry community. On the one hand, there is anecdotal evidence that the impact of boreal fires
on ozone can be significant supported by chemical transport model simulations. On the other – such
observations are sparse compared to the multiple observations of low or no ozone enhancements in
the fire plumes.
This is the probably the reason for reviewer 2 to think about the conclusions of this paper as far
from what could be expected. Thus, work like this will definitely the state of the knowledge in the
field.
To emphasize the comments above, we added
2. The reviewer holds an opinion that this manuscript presents results of an incremental
improvement from the previous Honrath et al. (2004) study.
We disagree with this statement. Honrath et al. (2004) presented no analysis of fire impact on ozone
levels, but only on CO and ozone enhancement ratios, covering just two out of studied in this paper
three years of measurements. On the other contrary, the whole emphasis of the current paper is the
impact of boreal fires on O3 levels. In addition to previously analyzed in Honrath et. al (2004)
2001 and 2003 measurements, we have included the observations made in 2004, which is the most
complete measurement year up to date.
3. The reviewer criticized the lack of details on the specific time periods during which 2003 and
2004 contained the anomalously high CO signatures.
We expanded our analysis by including chemical transport model (CTM) MOZART simulations for
2004 as well as the results of Honrath et. al (2004) to .and have revised our text ..to add
paragraphs …
In our earlier work (REH04) we analyzed time periods with anomalously high CO levels for the
summers of 2001, 2002 and 2003. In that work we used NRL Aerosol Analysis and Prediction
system (NAAPS) model simulations and showed events were the result of fires. Here we use the
results of REH04 to identify the periods at the station affected by fires.
4. Reviewer noticed that on page 3, 3rd line from the bottom we should cite Kasischke et al.
(2005) instead of Kasischke and Bruhwiler (2002)
Changed as suggested.
5. Missing citations included: Draxler and Rolph (2003), Stunder (1997)
References were added.
6. Page 7, end of 1st paragraph - should also cite Kasischke et al. (2005), which can be argued to
contain the most extensive analysis of impacts of boreal fires on high northern hemisphere
atmospheric CO
Citation was added as suggested.
7. Reviewer warned against using a reference that has not been submitted (e.g., Owen et al., Val
Martin et al.)
We replaced a reference to Val Martin’s paper by reference to her AGU abstract. We kept a
reference to Owen’s paper since it has been submitted.
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Reviewer 2
Science Category: Science Category 3
Presentation Category: Presentation Category C
1. The reviewer questions results as he thinks they are contrary to what is expected from
boreal forest fire plumes and suggests to perform more in-depth data to build a case for
elevated O3 originating from boreal forest fires.
1.1.
Reviewer argues that enhanced CO means smoldering fires which produce low NOx and
therefore low potential produce O3 ( Wofsy et al, JGR, 97, 1992 and several papers
[including Mauzerall et al.] in the ABLE 3A special issue, JGR, 99, 1994).
Enhanced CO is not necessarily the result of smoldering fires. Cofer 98 observed that during very
intense crown fires combustion was shown to be less efficient than during typical flaming, with
emission factors for CO approaching those for smoldering in slash/tramp fires, while many other
species were not affected.
Mauzerall et al (1996) observed a very wide range of ozone enhancement ratios during ABLE 3B.
While the average ratio was 0.1, the highest was 0.66. This indicates a large variability in ozone
forming potential within boreal fire plumes and supports the ability of ozone production.
Finally, we updated our analysis by including MOZART simulations, and previous work that used
NRL model, as an additional tool to identify periods at the station affected by fires. Thus, we are
not relying on CO levels as the sole way to tell the fire plume’s presence.
Cofer III, W. R., E. L. Winstead, B. J. Stocks, J. G. Goldammer, and D. R. Cahoon (1998), Crown
fire emissions of CO2, CO, H2, CH4, and TNMHC from a dense jack pine boreal forest fire,
Geophys. Res. Lett., 25, 3919–3922.
1. 2. The reviewer cites ABLE 3A & B and Goode et al. (2000) studies and claims that the
NOx/CO ratio is low in boreal fires and O3 production occurs mainly in the early stages of
the plume lifetime, with NOx quickly converted to reservoir NOy compounds as the plumes
age. Therefore he thinks that whether further O3 production occurs is still an open issue.
The reviewer also mentions PAN decomposition as the only available mechanism for
producing NOx far downwind provided large amount of PAN and warm temperatures
available. But he questions the possibility of such temperatures in the free troposphere.
Finally, he questions the high O3/CO enhancement ratio and compares it to the low ones
reported in some papers.
First, these campaigns cannot be directly compared to our study because
a) they were performed mostly in the boundary layer (BL) while our measurements are taken in
the free troposphere (FT)
b) Goode et al (2000) – measured fresh plumes, less than 3 hours old while we measure 5-15
days old plumes
Second, Val Martin et al. (2005) observed large NOy in the boreal fire plumes at PICO (identified
using MOZART simulations and high CO levels observed at the station), which indicate significant
NOx emitted from fires. They also observed frequent NOx enhancements in such plumes
suggesting continuing photochemistry. In our previous work (Honrath et al. 2004) we wrote about
PAN decomposition as a possible source for the enhanced NOx levels in the fire plumes. While
PAN dissociation requires warm temperatures, they can be achieved when airmasses subside
towards the station on their way from high-latitude regions.
Finally, large ozone enhancement ratios in the boreal fire plumes in the regions far downwind were
observed on a few occasions (Honrath et al. 2004, Bertschi and Jaffe 2005, Forster et al. 2003, Law
et al. 2005), and significant ozone production is supported by model simulation of fire plumes
subsiding over Europe (Real at al 2005, Pfister et al 2005).
2. The reviewer suggested to … (still working on this one)
The calibration correction factors that were applied it should be stated which ones were
multiplied or divided to obtain the corrected data. It appears that it was a combination of the
two. Three years is a very long time between calibrations and I don t see where the 7%
correction factor comes from in the mix. Also, was the data from January 1, 2002 to March 14,
2003 not corrected? This large time period seems to have been left out of the discussion. There
is far more detail on the O3 calibrations than for CO. A bit more information needs to be added
to the text, since the CO is also key to the story. Especially when looking at the O3/CO or
NOx/CO ratios. Errors in anyone of these species could lead to a wrong interpretation of the
data.
What should we add here?
3. Does the number of North American boreal fires per year agree with the trends in CO and
O3 during the specific years of this study?
As we wrote in the Introduction, several works showed that trends in boreal fires (both - North
American and Siberian) affect CO background. Some papers also showed that during the years of
large boreal fires ozone background was affected, too.
In this paper we argue that the trends in CO and O3 reflect the intensity of burning in the boreal
regions, i. e. , during the years with large areas burned in the boreal region
We included the following text in section 3, paragraph 1:
As we showed previously [Honrath et al., 2004], the highest CO levels in 2003
were the result of extreme Siberian fires that year. These fires burned the largest area in
more than ten years. 2004 was a high-fire year, too, with the largest fire season on record
in Alaska, while 2001 was a relatively low-fire year.
4. During the ICARTT campaign in summer 2004, the NASA DC-8 sampled what they thought
were Alaskan fire plumes in the upper troposphere on a few occasions. These data could be
explored to check NOx/ CO and O3/ CO ratios and compare them to the PICO data set.
The NOAA P3 may have also sampled similar plumes in the mid-troposphere
If the flights took place over the eastern US, such comparison would be probably meaningless, as
during such flights very little O3 was observed, while CO can be still very high. ADD MORE
5. The reviewer argues that the Hysplit trajectories are best over 2-3 day travel time periods
and questions the suitability of using them for 10-day periods.
Bertschi et al. (2005) used 10-day HYSPLIT backward trajectories to identify the source region
for several pollution events observed during their aircraft campaign. They compared the
obtained results to satellite images, TOMS aerosol index, which showed that the locations of
smoke plumes from fires were consistent with the paths of back trajectories. NAAPS model
also confirmed the fire source. In addition, we successfully used HYSPLIT trajectories in our
previous work (Honrath et al. 2004)
6. If input for the Hysplit model was on a 6-hourly basis, why were new trajectories run every
hour?
We added the following text in the Section 2.2, paragraph 1:
The model uses hourly wind vectors interpolated from 6-hourly National Weather
Service’s National Center for Environmental Prediction FNL data set [Stunder, 1997].
7. Figure 1 - Why was the low fire year of 2001 (low CO and O3) used to illustrate the high fire
impact travel routes? For consistency, it seems data from 2004 would be the best to use even
though it will look very similar.
We agree with the reviewer. However, it appeared to us that Figure 1 is not needed here, since the
explanation in the text is sufficient. Therefore, Figure 1was removed.
8. It would be better to generate a statistical summary table which provides details on the number
and percentage of total trajectories starting >50 N, start and end altitudes, etc.
Such analysis is not related to this paper. It was not our purpose to perform a flow analysis, but
rather to demonstrate that ozone is enhanced in fire plumes.
9. It s not clear how the individual trajectories relate to the 1-hour averaged mixing ratios of O3
and CO. How long do these plume events last? What is their seasonal distribution; mainly in
summer? What is there frequency of occurrence over time and when is it highest. This type of
information should be added to the text.
We added the following sentence to the Section 2.2, paragraph 2.
The fire-affected periods at the station typically last for several hours.
The text was added in the Section 1, paragraph 2:
However, northerly regions can be a substantial source of the O3 precursors nitrogen
oxides (NOx, NO + NO2), CO and volatile organic compounds (VOCs) [Goode et al.,
2000] during the boreal fire season which typically lasts from May till September.
10. Figure 2 & 3 - the use of fraction of observations for the y-axis is vague . Is this the fraction of
the total 1-hour averaged data? This needs to be more specific and I would suggest using the
actual number of measurements
Figures 2 and 3 are standard relative frequency histograms, where y-axis represents the fraction of
all observations. Because of the large difference in the number of observations for each year the
histogram in absolute numbers will not work.
11. Figure 2 Caption- How can there be a small number of observations greater than the maximum
value for the data set?
Caption to the Figure 2 says: A small number of observations greater than the maximum value
SHOWN are included in the rightmost bin. That is, several observations exceeding the values in the
rightmost bin, are included in it.
12. Reviewer suggested to test whether the elevated O3 and CO in 2003 and 2004 were caused by
the potential fire source or if there is more to the story. The distribution should shift toward the
one in 2001 if you take out the high O3 and CO data points from the fire periods.
We performed this analysis, but with minor modification. Instead of removing fire-affected periods
from all data, we selected from all data the non-fire periods. That is, we applied the non-fire criteria
discussed in the paper. Such approach excludes fire data plus the periods not related to either. The
resulting plots are below. The difference between all data O3 mean in 2003 and 2001 was 6 ppbv,
and between the O3 mean in 2004 and 2001 – 8 ppbv. For the data selected using the non-fire
criteria, these differences dropped to 3 and 4 ppbv respectively, that is, by half. This indicates that
the half of the interannual differences in O3 distributions at PICO could be explained by impact of
fires.
13. Reviewer found the figures hard to read because of their size and gray scale usage.
We modified Figure 2, by making lines thicker and replacing hatching with gray color.