Molecular characteristics and diurnal variations of organic aerosols at a rural site in the North China Plain with implications for the influence of regional biomass burning

Field burning of crop residue in early summer releases a large amount of pollutants into the atmosphere with significant impacts on the air quality and aerosol properties in the North China Plain (NCP). In order to investigate the influence of this regional anthropogenic activity on molecular characteristics of organic aerosols, PM2.5 filter samples were collected with a 3 h interval at a rural site of NCP from 10 to 25 June 2013 and analyzed for more than 100 organic tracer compounds, including both primary (n-alkanes, fatty acids/alcohols, sugar compounds, polycyclic aromatic hydrocarbons, hopanes, and phthalate esters) and secondary organic aerosol (SOA) tracers (phthalic acids, isoprene-, α-/β-pinene, β-caryophyllene, and toluenederived products), as well as organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC). Total concentrations of the measured organics ranged from 177 to 6248 ng m−3 (mean 1806± 1308 ng m−3) during the study period, most of which were contributed by sugar compounds, followed by fatty acids and fatty alcohols. Levoglucosan (240±288 ng m−3) was the most abundant single compound and strongly correlated with OC and WSOC, suggesting that biomass burning (BB) is an important source of summertime organic aerosols in this rural region. Based on the analysis of fire spots and backward trajectories of air masses, two representative periods were classified, which are (1) Period 1 (P1), 13 June 21:00–16 June at 15:00 CST (China Standard Time), when air masses were uniformly distributed from the southeast part of NCP, where intensive open-field biomass burning occurred; and (2) Period 2 (P2), 22 June at 12:00 to 24 June at 06:00 CST, which is representative of local emission. Nearly all the measured PM components showed much higher concentrations in P1 than in P2. Although n-alkanes, fatty acids, and fatty alcohols presented similar temporal–diurnal variations as those of levoglucosan throughout the entire period, their molecular distributions were more dominated by high molecular weight (HMW) compounds in P1, demonstrating an enhanced contribution from BB emissions. In contrast, intensive BB emission in P1 seems to have limited influence on the concentrations of polycyclic aromatic hydrocarbons (PAHs), hopanes, and phthalate esters. Both 3-hydroxyglutaric acid and βcaryophyllinic acid showed strong linearly correlations with levoglucosan (R2 = 0.72 and 0.80, respectively), indicating that BB is also an important source for terpene-derived SOA formation. A tracer-based method was used to estimate the distributions of biomass-burning OC, fungal-spore OC, and secondary organic carbon (SOC) derived from isoprene, α/β-pinene, β-caryophyllene, and toluene in the different periods. The results showed that the contribution of biomassburning OC to total OC in P1 (27.6 %) was 1.7 times that in P2 (17.1 %). However, the contribution of SOC from oxidaPublished by Copernicus Publications on behalf of the European Geosciences Union. 10482 J. Li et al.: Influence of regional biomass burning on molecular characteristics of organic aerosols in NCP tion of the four kinds of volatile organic compounds (VOCs) increased slightly from 16.3 % in P1 to 21.1 % in P2.

For the selection of the period 2 (P2).We agree that it's not easy to select a period that is representative of a pure local emission in field study.However, in this study, the 72 h backward trajectories clearly show that air masses came predominantly from the surrounding areas of the sampling site during P2 (Figure 1).Although the winds in sometimes from the north, the wind speed in most time of P2 are <2 m s -1 (Figure S2).
In addition, all the chemical compositions present very low concentrations during P2 except 2 samples affected by a near-site biomass burning emission.These results confirm that local fresh emissions dominated the chemical compositions of aerosols during P2.
For the OC/EC ratios, as suggested by the referee, we compared the OC/EC ratios in this study with those reported by some chamber or filed studies.We concluded that the relatively low OC/EC ratio in our study is related to more complex source contributions and different combustion conditions of open biomass burning.More explanation can be found in Line 225-237.

Anonymous Referee #3:
This manuscript presents measurement results from a field study conducted in the North-China Plain, which is notorious for high aerosol pollution.While suffering slightly from a relatively short measurement period, this study presents an impressive suite of organic aerosol component concentrations at a rather high time resolution.Two specific periods were singled out, including one episode with high biomass smoke impact.The results presented in this paper are helpful for a better understanding of the sources and characteristics of organic aerosols in this highly polluted part of China.Prior to publication of the manuscript in ACP, the authors should address the comments and suggestions below.

Response:
We thank the referee's comments, which are very helpful for us to improve our work.Detailed revision and response to the comments are list below.

Specific comments:
1. Line 136: The total sampling period is rather short, especially when dividing it into 2 special periods, requiring caution in the discussion of the measurement results.The authors may want to add a statement regarding how representative the data are.
Response: Suggestion taken.We revised related description, please see Line 134-140.

Lines 161-163:
The authors corrected the data for the field blanks, although the blank values were relatively low, in contrast to the recoveries which introduced larger errors for certain species.Why were the recoveries not taken into account as well?
Response: In our study, averaged recoveries of the target compounds were better than 70%.Recovery experiment is a method for QA and QC.However, compounds used in a recovery experiment are usually pure agent while those in real samples are a mixture with other organic and inorganic components, which means that the recovery experiment could not entirely reflect the conditions of target compounds in the atmosphere.Thus, many documents report the data without a correction by recovery.
For example, US ASTM method D 6209-98 for atmospheric PAH, the section 16.4.2at page 12 notes that "Typically, measured PAH analyte concentrations are not corrected for surrogate recovery".Therefore, in this paper the data reported were not corrected by the recoveries.
3. Lines 220-223: Do the authors have a possible explanation for the rather low OC/EC ratios measured during the biomass burning period?Previous studies, especially those investigating burns which were dominated by smoldering combustion, were characterized by emissions with significantly higher OC/EC ratios.Is it possible that the wheat straw combustion during the study period occurred to some extent in the flaming phase?
Response: By comparing the OC/EC ratios in this study with those reported in some chamber or filed studies, we do agree with the referee's comment that wheat straw combustion during the study period occurred to some extent in the flaming phase.On the other hand, we think the contribution of fossil fuel combustion in NCP is also a reason for lower OC/EC ratio in this study.Thus, we added more explanation at Line 225-237 in the manuscript.4. Lines 241-245: Indeed, the regional biomass burning activities contributed to the elevated WSOC/OC fractions, but it may also be worthwhile mentioning here (as the authors do later on in the paper) that SOA was likely produced in the biomass burning plumes (especially considering the transport distance/time to the sampling site), and thus contributed to the higher degree of oxygenation of the organic aerosol as well.
Response: We do agree with the referee's comment, and revised the related description as "many SOA could be produced in the biomass burning plumes during the long-range transport (detailed discussions are given in Section 3.3), these results indicate that particulate WSOC in the region is mostly contributed from biomass burning activities including direct emission and secondary oxidation" (Line 256-260).
5. Lines 287-288: It would be helpful for the readers who are not familiar with these diagnostic ratios to at least briefly explain how the high L/M ratios indicate straw burning.
Response: Suggestion taken.We gave an explanation at Line 302-307.
6. Lines 288-289: How are the anhydrosugar emission ratios of lignites relevant to this study?Wouldn't it be more useful to mention results from some of the previous studies which specifically investigated anhydrosugar emissions from burning of straw or similar types of biomass?
Response: We thank the referee's comment.We clarified the statement as " Fabbri et 7. Lines 290-294: It would be helpful if the authors showed more quantitative results, e.g., state what is considered "higher".And how specifically do these results confirm the contribution of wheat straw burning?
Response: Suggestion taken.We added quantitative results of L/G+M and L/M ratios in the statement, and reevaluated results as "average ratios of L/G+M and L/M during P1 (10.1±3.41 and 6.77±1.97,respectively) and P2 (29.7±12.2 and 18.0±4.28)suggest that biomass burning is always the dominated contributor for these compounds in NCP" (Line 307-310).
8. Lines 503-505: Why do the authors mention measurements from this area, as there seems to be no relation to this study region?Why not show data from other areas in Asia?
Response: Suggestion taken.We revised the related discussion, and cited a new data measured in the PM2.5 samples emitted from burning of three main kinds of cereal straws (wheat, maize, and rice) in China.Please see Line 517-519.

Technical corrections:
al. (2009) compared the concentrations of the three anhydrosaccharides in the smokes from different fuel types, and proposed that levoglucan/(galactosan+mannosan) (L/G+M) and levoglucan/mannosan (L/M) values range in 0.2-18 and 0.23-33 for various source tests for biomass burning as compared to the average of 54 and 54 for lignites" (Line 303-307).