Interactive comment on “ Long-term ( 2001 – 2012 ) trends of carbonaceous aerosols from remote island in the western North Pacific : an outflow region of Asian pollutants and dust ”

The manuscript titled “Long-term (2001-2012) trends of carbonaceous aerosols from remote island in the western North Pacific: an outflow region of Asian pollutants and dust”, is a well written paper. The methodology is sound and data analysis is convincing. The theme of the manuscript is well in accordance with the scope of the journal. The logic was explicit, and the content was comprehensive and integrated. However,

gested negligible contribution of local anthropogenic emissions as well as long-range influences over the sampling site.These results are consistent with previous studies, which reported that several times lower concentrations of organic compounds in summer compared to winter/spring over the same observation site (Kawamura et al., 2003;Mochida et al., 2003).Therefore, it is reasonable to believe that the sources of carbonaceous aerosols were transported from the adjacent Asian countries to the western North Pacific via long-range atmospheric transport.As described earlier, EC is primary particle and predominantly comes from biomass and fossil fuel combustion sources.On the contrary, OC is of either primary origin or secondary formation via gas-to-particle conversion in the atmosphere.The precursors of secondary OC may also come from biogenic sources in addition to fossil fuel and biomass burning combustions.The OC/EC ratios often used to distinguish the relative contribution of primary vs. secondary sources as well as biomass vs. fossil fuel burning sources (Turpin and Huntzicker, 1995;Castro et al., 1999;Rastogi et al., 2016).Atmospheric aerosols emitted from fossil fuel combustion are characterized by lower OC/EC ratios (<2.0) whereas higher OC/EC ratios (>5.0) are characteristic of biomass burning aerosols.The OC/EC ratios > 2.0 have been used to point out the presence of secondary organic aerosols (Cao et al., 2003;Chow et al., 1996;Kunwar and Kawamura, 2014).Table 2 summarizes OC/EC ratios reported for various sources of aerosol particles.Monthly mean OC/EC ratios in this study are greater than 2.0 for all months, suggesting the dominance of SOA in carbonaceous aerosol over the western North Pacific.The seasonal variation of OC/EC mass ratios was found maximum in summer (∼21 to 33) and minimum in winter-to-spring (3.9 to 7.7).The extremely high OC/EC ratios in summer indicate the secondary formation of OC via oxidation processes, while low OC/EC ratios in winter-to-spring suggest that both biomass burning and fossil fuel combustion are important sources for carbonaceous aerosols over the western North Pacific."Please see lines 217-244 as well as Table 2 and Figure 2 in the revised MS.
More explanation is needed for the tracers and their sources.For example, why is WSOC a tracer for SOA?It is stated that as the organics are oxidized, they become C3 more water soluble, but that does not necessarily make WSOC directly linked to SOA.
There is plenty of primary OA that is water soluble.This argument needs to be more clearly laid out with references and explanations for other possibilities.The aging of organic aerosols and SOA are two different things, which are both represented by WSOC in this paper.Additionally, the attribution of low WSOC to OC ratios to primary marine OC is not justified .This is only one sentence with no discussion of emissions correlated to wind speed or OC correlated with any sea salt tracers.This should be removed or discussed in more detail.Response: Following the reviewer's suggestion, we explained more about the WSOC and WSOC/OC ratios in the revised MS as follows."Previous studies have shown that SOA is largely composed of oxygenated compounds that are highly water-soluble (Kanakidou et al., 2005;Kondo et al., 2007 and references therein).Thus, measurements of WSOC have been used to estimate the SOA in ambient aerosols (Weber et al., 2007;Snyder et al., 2009;Sudheer et al., 2015;Decesari et al., 2001;Docherty et al., 2008).Because major fraction of biomass burning products is highly water-soluble (Sannigrahi et al., 2006;Saarikoski et al., 2008), WSOC/OC ratio has been used as an unique tracer to better understand the photochemical activity and/or aging of aerosols and to discuss SOA formation mechanism in the atmosphere during long-range transport (Miyazaki et al., 2007;Ram et al., 2010b;Ram and Sarin, 2011;Kondo et al., 2007;Weber et al., 2007;Gilardoni et al., 2016;Boreddy et al., 2017).The WSOC/OC ratios exceeding 0.4 have been used to indicate the significant contribution of SOA (Ram et al., 2010a) and aged aerosols.
The WSOC/OC ratios ranged from 0.06 to 0.19 in diesel particles (Cheung et al., 2009) and 0.27 for vehicular emissions (Saarikoski et al., 2008).In this study, monthly mean WSOC/OC ratios are >4.0 for all months except for September, indicating a significant contribution from SOA over the western North Pacific.The seasonal variation of WSOC/OC showed higher values (monthly mean: 0.44 to 0.62) during winter-spring months (Figure 3f), implying that the SOA formation is enhanced due to increased photochemical activity and/or aging of East Asian polluted aerosols during long-range atmospheric transport.The high WSOC/OC ratios are traditionally attributed to the at-C4 mospheric oxidation of various VOCs in the presence of oxidants such as ozone and hydrogen peroxide radicals via gas and/or aqueous phase reactions in the atmosphere (Miyazaki et al., 2007;Ram and Sarin, 2012).However, the atmosphere over the western North Pacific is always characterized by high relative humidity (>80%) and air temperature (∼24 • C) during the whole year (Figure S1).Therefore, higher WSOC concentrations in winter-to-spring over the western North Pacific were largely attributed to the aqueous-phase oxidation of anthropogenic and/or biogenic VOCs (Gilardoni et al., 2016;Youn et al., 2013), which are emitted over East Asia and long-range transported to the western North Pacific.On the other hand, lower ratios of WSOC/OC in summer may suggest that the primary emission of OC from the ocean surface via sea-to-air flux because the low speed easterly winds originated from the central Pacific are dominant in summer over the western North Pacific (Figure 2).Miyazaki et al. ( 2010) reported the presence of significant water-insoluble organic matter in the western North Pacific during summer, which may be produced by bubble-bursting processes at the ocean surface.Similarly, Ovadnevaite et al. ( 2011) reported higher contributions of primary organic matter to marine aerosols over the Northeast Atlantic.Further, laboratory studies have revealed a high abundance of primary organic matter in sea-spray aerosols (Facchini et al., 2008;Keene et al., 2007)."Please see lines 264-299 in the revised MS.
The discussion of the time series in Figure 4 is confusing.It is unclear why an increase in OC/EC and OC/TC (which are not shown) suggest an "enhanced formation of SOA" over the measurement time.An increase in OC/EC may suggest different sources, but not all OC is SOA.This needs a clearer explanation.Additionally, it may be clearer if the data was binned by year and/or just the first and last years were shown.With all of the variability in the time series, even the trends that are statistically significant do not appear to be actual trends.Response: To better understand the discussion of the trends, we revised whole section 3.3 and Figure 4 in the revised MS.We quantified all the trends in this study and reported in the text where it is necessary and Figure 4. Following the reviewer's comment, the annual mean variations of all chemical species C5 and their ratios are shown as a Figure S2 in the supporting information.Please see the text in section 3.3 as well as Figure 4 and Figure S2 in the revised MS.
OC is not purely "scattering" or "cooling" as suggested here.Brown carbon is OC that can absorb solar radiation.More explanation is needed on how the ratio of OC/EC can be used to understand the relative contributions of scattering and absorbing of aerosols.Additionally, at line 341, there is no mention of radiative forcing by other types of aerosols.The ratio of OC/EC only applies to the organic fractions.And, the size of the particles plays a role as well as their morphology and composition.This needs to be included in the discussion.Response: In light of the reviewer's comment, we briefly discussed following points in the revised MS. "It is well know that atmospheric aerosols play a key role in the climate system as they can act as cloud condensation nuclei (CCN) and impact cloud formation, thus, radiative forcing (RF) (IPCC, 2013).
The RF of aerosol is estimated based on the aerosol optical depth (AOD), absorption and scattering coefficients and asymmetry parameters.OC (except for brown carbon) and SO42-particles majorly scatter the solar radiation whereas EC particles strongly absorb the radiation in the atmosphere.The single scattering albedo (SSA), defined as the ratio of scattering to the extinction coefficient of aerosols (Pani et al., 2016), is an important property for determining the direct RF (Gopal et al., 2017;He et al., 2009).
The SSA is highly sensitive to the nature (scattering and/or absorption) of aerosols in the atmosphere.Therefore, although OC has certain uncertainty because of light absorbing brown carbon, the OC/EC ratios can be used to understand the relative contributions of scattering or absorbing aerosols in the atmosphere (Ram and Sarin, 2015).Further, knowledge of the OC/EC ratios in aerosols (for example, biomass burning) may also help to improve model representation of the absorption caused by organic compounds constituting the so called brown carbon, which contributes to the aerosol RF (Chung et al., 2012;Saleh et al., 2014;Kirchstetter and Thatcher, 2012).In this study, atmospheric aging may make OC more scattering during long-range transport over the western North Pacific.A significant increasing trend of OC/EC ratios suggests that scattering aerosols are increased significantly over the western North Pacific.""It C6 should be noted that all these ratios are applicable to organic fractions that are derived from the bulk measurements only; however, the size of the particle also plays a role on RF as well as their morphology, chemical composition and mixing state (Jacobson, 2001;Lohmann and Feichter, 2005;Zhang et al., 2008)."Please see lines 374-392 and 413-416 in the revised MS.
Along these lines, the "Atmospheric implications" section is far reaching for the data shown and needs to be revised to reflect the actual measurements and analyses.The correlation between WSOC and CCN is very short and not fully explained.There needs to be much more discussion, if that is included.Figure 5 just shows a correlation between total particles activated (CCN) and the organic concentration.There is no description of initial particle sizes or compositions.It is possible that WSOC also correlates with NaCl in the particles, which are driving the CCN activity.No direct link can be shown with this correlation alone.Response: Following the reviewer's comment, we revised this section 3.4 as follows." Novakov and Corrigan (1996) found that pure organic components from biomass smoke emissions can form cloud condensation nuclei (CCN) without the presence of sulfate (SO42-) and other inorganic compounds.Roberts et al. (2002) showed that biomass burning derived organic aerosol does serve as CCN.Further, large loadings of CCN in continental air masses were observed over the western North Pacific (Matsumoto et al., 1997;Boreddy et al., 2015).In this study, the enhanced WSOC concentrations and WSOC/OC ratios in continental air masses suggest an important role of WSOC in CCN activity over the western North Pacific in addition to other particles such as SO42-and sea-salts.To better understand the impact of WSOC on cloud forming potential, we performed regression analysis between WSOC and CCN concentrations as shown in Figure 5. CCN data were downloaded from the MODIS satellite over the region (140 water-soluble organic matter also plays an important role in CCN formation over the C7 western North Pacific.This point is consistent with previous studies, which explain the contribution of water-soluble organic matter to CCN (Matsumoto et al., 1997;Zhao et al., 2016).It should be noted that all these ratios are applicable to organic fractions that are derived from the bulk measurements only; however, the size of the particle also plays a role on RF as well as their morphology, chemical composition and mixing state (Jacobson, 2001;Lohmann and Feichter, 2005;Zhang et al., 2008)."Please lines 396-416 in the revised MS.
Specific Comments.Lines 56-49: References are necessary for this statement.While it is true that OC may warm less than EC, it does not necessarily provide a "cooling effect" as written here.This is still a major topic of study and should be written to reflect that, along with the necessary references.Response: Following the reviewer's comment, we rephrased this sentence along with necessary references in the revised MS.Please see lines 62-65 in the revised MS.Line 92: Some general statements are made about increasing trends over time periods throughout the paper.These need to be rewritten or better explained.For example, at Line 92, it states "an increasing trend of biogenic emissions in northern China during 1982-2010."It is unclear from this statement if the emissions were simply higher in 2010 than 1982 or if emissions increased every year during that period.Response: Following the reviewer's comment, we quantified (increasing rate per year) all the trends in this study and reported in the text.Please see lines 96, 313-322, 325 etc., as an example.

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Line 98: Is marine aerosol the main focus of this paper?It needs to be defined here.Just sampling at a remote island does not necessarily mean that the sampled air masses are marine in origin and can include polluted air masses, as the authors discuss.Response: We rephrased this sentence in the revised MS as "To better understand the long-range transport of Asian pollutants and their atmospheric processing over the western North Pacific, we continuously collect total suspended particulate (TSP) samples since 1990 at Chichijima Island."Please see lines 105-107 in the revised MS.
Figure 1: Why does this figure show the concentration of chlorophyll?Why is the winter 2008 chlorophyll map shown?That is not representative of the summer season.Is it representative of all of the years included in the study?That seems unnecessary to include.Also, the color bar is too small.Response: We modified Figure 1 according to the reviewer's comment.Please see Figure 1 in the revised MS.
Line 133: How good of an assumption is it to assume that the carbonate carbon is insignificant.The IMPROVE network may be observing different sources and particle types that those in this study.Provide a reference to show that carbonate carbon is expected to be low in this region or with these sources.Response: Provided a reference as suggested.We also briefly added the following sentence in the revised MS. "Previous studies have also shown that carbonate, particularly calcium carbonate, levels are low or negligible in most ambient samples, which are analyzed by IMPROVE protocol (Wang et al., 2005;Clarke and Karani, 1992;Chow et al., 2001)."Please see lines 139-142 in the revised MS.
Line 154: The whole section on statistical analysis should be condensed or moved to the supplement.Also, at line 163, this equation is unnecessary to list here.Response: We modified this section in the revised MS.Please see lines 164-170 and also text in SI.
Figure 2: The text says 2001 to 2013, and the caption says 2001 to 2012.And an aver-C9 age over 11 years is not that interesting without some sort of error bars or something to show the lack of change with time.If this is the general trend during these four times of year, that is fine, but it seems unnecessary to show in a figure then.Additionally, there is no label on the color bar.The main point of this figure is to show that in the winter the air masses measured at the sampling site are from East Asia while the weaker winds in the summer transport air masses from the central Pacific.These figures and the wind show the general patterns, but it would be more relevant for this study to show the actual air mass backtrajectories for the samples.The wind pattern does not show origin or destination but rather a general pattern.Additionally, the arrows in many regions are too small to actually determine their direction, and there is no scale for their size.Response: Following the reviewer's comment, we replaced Figure 2 by air mass back trajectories for each month and shown as Figure 2 in the revised MS.Please see Figure 2 in the revised MS.
Line 205: This is not new.Change "we found" to "there is" to be consistent.Lines 283-284: Panels d and e show the opposite trend.Response: We rephrased this sentence in the revised MS as "It is seen that all the annual trends of chemical species and WSOC/OC ratios showed clear seasonal patterns with higher values in winter-spring and lower values in summer.In contrast, the OC/EC and nss-K+/EC ratios showed higher values in summer."Please see lines 305-307.
Line 289: Why are OC/EC and OC/TC not shown in Figure 4, especially since their trends are significant?Response: Following the reviewer's comment, we included the

Line 68 :
Check this reference.Response: Checked as suggested.Please see line 72 in the revised MS.Line 71: The particles are what act as CCN.Change "act" to "aid in particles acting" Response: Changed as suggested.Please see line 76 in the revised MS.Line 85: Contribution to what?Response: Global contributions.We mentioned already in the manuscript as "on a global scale.." Please see lines 87-88 in the revised MS.
Figure 3: What are the small open, grey boxes (means?) and the X grey markers (outliers?)?So the EC is pollution and the OC is not?Response: Changed as suggested.Please see line 190 in the revised MS.We also explained all symbols in Figure 3. Please see Figure 3 caption in the revised MS.Line 268: Needs a reference.Response: Provided references as suggested.Please see line 284 in the revised MS.
• -145 • E, 25 • -30 • N) in the western North Pacific for the period 2002-2012.The results show a significantly good correlation (r=0.69,p<0.001) between WSOC and CCN concentrations.This result suggests that, although nss-sulfate is a major contributor to CCN activity (Mochida et al., 2011);