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npj Climate and Atmospheric Science volume 6, Article number: 20 (2023) Cite this article

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The implementation of air pollution reduction measures has significantly reduced the concentration of atmospheric fine particles (PM2.5) in Beijing, among which the "coal-to-gas" conversion measures may play a crucial role. However, the effect of this conversion measure on brown carbon (BrC) is not well known. Here, the chemical composition of BrC in humic-like fraction (HULIS-BrC) and water-insoluble fraction (WI-BrC) were characterized for ambient PM2.5 samples collected in Beijing before and after the "coal-to-gas" conversion measure. After the conversion measure, the number of HULIS-BrC compounds increased by ~14%, while the number of WI-BrC compounds decreased by ~8%. The intensity of over 90% of HULIS-BrC compounds also increased after the conversion measure, and correspondingly the O/C ratios of CHO and CHON compounds in HULIS-BrC fraction generally increased with the increase of intensity ratios after/before the conversion measure, indicating that there were more water-soluble highly oxygenated BrC compounds after "coal-to-gas" conversion measure. On the contrary, the intensity of more than 80% of WI-BrC compounds decreased after the conversion measure, and the O/C ratios of CHO and CHON compounds in WI-BrC fraction generally decreased with the decrease of intensity ratios after/before the conversion measure, indicating that after the "coal-to-gas" conversion measure the water-insoluble low oxygenated BrC compounds decreased. This work sheds light on the differences in the chemical composition of BrC between before and after the "coal-to-gas" conversion measure and suggests that future studies on the residential coal combustion BrC and secondary BrC deserve further exploration.

Haze pollution with a high concentration of fine particle matter (PM2.5) pollution is a notable environmental problem in China, especially during the wintertime heating period, leading to negative effects on air quality and public health1,2,3,4 as well as the potential impact on climate5,6. To mitigate air pollution, a variety of legislative actions have been applied from the central to local governments, including the "Air Pollution Prevention and Control Action Plan" implemented in 2013, the "Air Pollution Prevention and Control Law" established in 2016, and some regulatory measures, e.g., the "odd-even vehicular ban" and "staggered-peak production" in many cities. With these efforts, the air quality in North China has strikingly improved over the past years, with the annual average PM2.5 concentration reducing from 86 μg m−3 in 2013 to 37 μg m−3 in 2021. Nevertheless, haze pollution still occurred less frequently in wintertime North China, leaving a question about the culprit.

In recent years, there has been mounting evidence suggesting that residential coal combustion is a major emission source responsible for wintertime haze pollution in North China1,7,8,9,10,11,12. For example, direct emissions from residential coal combustion were found to contribute 25% of organic aerosol (OA) in PM1 during extremely polluted days in winter 2014 in Shijiazhuang9 and 19% of OA in PM1 during high pollution days in winter 2014 in Beijing11. A residential "coal-to-gas" conversion measure was therefore initiated for the region of Beijing–Tianjin–Hebei and the surrounding area, which requires the replacement of coal with gas for residential heating and cooking by the end of 2018. Consequently, coal consumption for heating in Beijing decreased by over 70%, from 3.47 million tons in 2015 to 0.97 million tons in 2019 (http://tjj.beijing.gov.cn/), while the concentration of PM2.5 decreased from 84.0 μg m−3 in 2015 to 47.9 μg m−3 in 2019 in wintertime Beijing (http://sthjj.beijing.gov.cn/). This may affect the light absorption of OA, i.e., the light-absorbing OA fraction, which is also known as brown carbon (BrC), and thus affect the atmospheric photochemical reactions and radiative forcing12,13,14,15,16, because residential coal combustion has recently been recognized as an important BrC source in China, especially during the heating season17,18,19. As the light absorption properties of BrC is determined by its chemical composition, the residential "coal-to-gas" conversion measure provides a unique scientific platform to gain insights into how the conversion measure affects the BrC chemical composition.

In this study, we present the detailed chemical characterization of different BrC fractions in PM2.5 samples collected in urban Beijing before and after the residential "coal-to-gas" conversion measure, with a high-performance liquid chromatograph (HPLC) coupled with a photodiode array (PDA) and a high-resolution Orbitrap mass spectrometer (HRMS) in both negative (ESI−) and positive mode electrospray ionization (ESI+). The differences in chromophoric number, intensity, and oxidation status for humic-like BrC fraction (HULIS-BrC) and water-insoluble BrC fraction (WI-BrC) are discussed.

The identified formulas are classified into five major compound categories, including CH, CHO, CHN, CHON, and S-containing (CHOS and CHONS), based on their elemental composition. CHO refers to compounds that contain carbon, hydrogen, and oxygen elements. Other compound categories are defined analogously. Supplementary Fig. 1 shows the plots of DBE versus the number of C atoms for all assigned formulas in HULIS and WISOC fractions before (2015) and after (2019) the "coal-to-gas" conversion measure, along with the reference boundary line characteristics of fullerene-like hydrocarbons with DBE = 0.9 × C20,21, cata-PAHs with DBE = 0.75 × C − 0.520,22, and linear polyenes CxHx+220,22. The chromophores of BrC have efficient light absorption in UV–vis wavelengths, which requires uninterrupted conjugation across a significant part of the molecular structure. Therefore, compounds with a DBE/C ratio greater than that of polyenes are potential BrC chromophores (see the shaded area in Supplementary Fig. 1)20,23. As shown in Table 1, before and after the measure, a total of 9708 and 11,032 BrC chromophores were detected in the HULIS fraction, while 11,150 and 10,239 BrC chromophores in WISOC fraction, respectively.

Figure 1 shows the number and intensity of different chromophore categories in HULIS-BrC and WI-BrC fractions before and after the "coal-to-gas" conversion measure. In the HULIS-BrC fraction, CHON chromophores have the largest contribution in number (>50%) before and after the measure, followed by CHO (~32%), CHN (~6%), and S-containing (~ 1.5%) chromophores (Supplementary Fig. 2). For CHON chromophores in HULIS-BrC fraction, more were detected in ESI+ mode than in ESI−, and for CHO chromophores, more were identified in ESI− mode. This result is in line with a previous study of HULIS in the Pearl River Delta Region, China, by Lin et al.24. Unlike the number distribution, the intensity of CHON and CHO chromophores were both higher in ESI- than in ESI + mode. In the WI-BrC fraction, CHON chromophores also have the largest contribution in number (> 50%) before and after the measure, followed by CHO (~ 23%), CHN (~16%), CH (~ 5%), and S-containing (~ 0.5%) chromophores (Supplementary Fig. 2). Different from the HULIS-BrC fraction, the number and intensity of different chromophore categories in WI-BrC fraction are both higher in ESI+ than in ESI− mode. The differences in chromophore number and intensity between HULIS-BrC and WI-BrC can be ascribed to the difference in chromophore composition, and the differences in chromophore composition of HULIS-BrC or WI-BrC between ESI− and ESI+ is mainly due to the different ionization mechanisms24,25.

a Number and b intensity of chromophores.

In the HULIS-BrC fraction, the number and intensity of CHON, CHO, CHN, and S-containing chromophores were generally higher after the "coal-to-gas" conversion measure than before the measure (increased by about 14%, 15%, 7%, and 3% in number and 80%, 81%, 24% and 51% in intensity for CHON, CHO, CHN and S-containing chromophores, respectively). However, in the WI-BrC fraction, the number and intensity of CHON, CHO, CHN, CH, and S-containing chromophores were generally lower after the measure than before the measure (decreased about 8%, 6%, 12%, 9%, and 13% in number and 34%, 35%, 44%, 52%, and 19% in intensity for CHON, CHO, CHN, CH, and S-containing chromophores, respectively) (Supplementary Fig. 3). These results indicate that after the "coal-to-gas" conversion measure the HULIS-BrC chromophores increased while the WI-BrC chromophores decreased, which could provide a guide for future study on the influences of the measure on optical properties and radiative forcing of atmospheric BrC.

Supplementary Figure 4 shows the reconstructed mass spectra of HULIS-BrC and WI-BrC chromophores before and after the "coal-to-gas" conversion measure, which show obvious differences. It can be seen that after the "coal-to-gas" conversion measure, the intensity of some chromophores increased while others decreased. For example, in the WI-BrC fraction assigned in ESI+ mode, the total intensity was higher for CHN+ chromophores than CHO+ chromophores before the measure, while the total intensity of CHO+ chromophores was similar to that of CHN+ chromophores after the measure. The differences in mass spectra of BrC fractions before and after the "coal-to-gas" conversion measure indicate that the change in emission sources and, thus, atmospheric processing alter the chromophore composition of atmospheric BrC. In the following sections, the differences for all the five chromophore categories in BrC fractions before and after the "coal-to-gas" conversion measure are discussed.

CHO compounds are a class of common compounds identified in atmospheric organic aerosol26,27,28,29, which could be from both primary emissions30,31 and secondary formation32,33. A total of 3099 and 3556 CHO chromophores were assigned in HULIS-BrC fraction before and after the "coal-to-gas" measure, respectively, and correspondingly 2551 and 2410 were assigned in WI-BrC fraction (Fig. 1). Figure 2 shows that van Krevelen diagram for CHO chromophores in HULIS-BrC and WI-BrC fractions in ESI- mode (CHO-). The peak intensity-weighted average molecular formulas (MFavg) for CHO- chromophores in HULIS-BrC and WI-BrC were C9.85H8.21O3.43 and C11.56H8.81O2.93, respectively, before "coal-to-gas" conversion measure, and C9.66H8.05O3.60 and C11.09H8.56O3.04, respectively, after the measure (Supplementary Table 1). The CHO− chromophores in HULIS-BrC fraction before and after the measure were dominated by monocyclic aromatics (2.5 ≤ Xc < 2.7) (44% and 45% in terms of peak intensity, respectively) and polycyclic aromatics (2.7 ≤ Xc) (44% and 42% in terms of peak intensity, respectively) (Fig. 2a, b). The corresponding intensity of most CHO− chromophores in the HULIS-BrC fraction was higher after the measure than before the measure, and the O/C ratios generally increased with the increase of intensity ratios after/before the measure (Fig. 2e). For example, the average O/C ratios of CHO- chromophores in HULIS-BrC fraction was non-aromatics (Xc < 2.5) (0.49) > monocyclic aromatics (0.30) > polycyclic aromatics (0.24), and the average intensity ratios of after/before the measure was also non-aromatics (2.6 times) > monocyclic aromatics (2.1 times) > polycyclic aromatics (1.9 times). Besides, only 5 extra CHO- chromophores in the HULIS-BrC fraction were identified before the measure, but none after the measure, and their O/C ratios were all less than 0.3 (0.18 on average). In comparison, 229 extra CHO− chromophores in the HULIS-BrC fraction were identified after the measure but none before the measure, with the O/C ratio of 0.34 on average. The peak intensity-weighted O/C ratio (O/Cavg) of CHO− chromophores in HULIS-BrC fraction were 0.36 and 0.39 before and after the "coal-to-gas" conversion measure, respectively (Supplementary Table 1). These results indicate that more highly oxygenated CHO- chromophores existed in the HULIS-BrC fraction after the "coal-to-gas" conversion measure than before the measure. These increased CHO- chromophores in HULIS-BrC could possibly be explained by the increase of secondary BrC34,35. Although the concentrations of PM2.5 and other pollutants (e.g. CO and NO2) were indeed decreased in Beijing after the "coal-to-gas" conversion measure (52 and 38 μg m−3 of PM2.5 and NO2, respectively, 0.8 mg m−3 of CO) compared with those before measure (84 and 52 μg m−3 of PM2.5 and NO2, respectively, 1.4 mg m−3 of CO), the concentration of O3 (60 and 67 μg m−3 before and after "coal-to-gas" conversion measure, respectively) (http://sthjj.beijing.gov.cn/) was not decreased, which is one of the major sources of OH radicals. Siemens et al.35 reported the formation of secondary CHO- chromophores from the photooxidation of anthropogenic aromatic volatile organic compound (VOC).

a, b Van Krevelen diagram for CHO− chromophores in HULIS-BrC fraction. c, d Similar to (a, b), but in WI-BrC fraction. e Corresponding intensity ratios of after/before (2019/2015) the measure of CHO- chromophores in HULIS-BrC fraction. f Similar to (e), but in the WI-BrC fraction. The area of the circles is proportional to the fourth root of the intensity of individual chromophores, and the color bar denotes the aromaticity equivalent (black with Xc < 2.5, red with 2.5 ≤ Xc < 2.7, and blue with Xc ≥ 2.7). The insert plot represents the chromophores identified only before or after the measure.

The CHO− chromophores in WI-BrC fraction before and after the "coal-to-gas" conversion measure were dominated by polycyclic aromatics (47% and 46% in terms of peak intensity, respectively) and monocyclic aromatics (40% and 40%, respectively) (Fig. 2c, d). In contrast to CHO- chromophores in the HULIS-BrC fraction, the corresponding intensity of most CHO- chromophores in the WI-BrC fraction was higher before the measure than after the measure, and the O/C ratios of CHO- in WI-BrC fraction generally increased with the decrease of intensity ratios of before/after the measure (Fig. 2f). For example, the average O/C ratios of CHO− chromophores in WI-BrC fraction was non-aromatics (0.48) > monocyclic aromatics (0.28) > polycyclic aromatics (0.17), and the average intensity ratios of before/after the measure were polycyclic aromatics (1.8 times) > monocyclic aromatics (1.5 times) > non-aromatics (1.2 times). There were 97 extra CHO− chromophores in the WI-BrC fraction identified before the measure but none after the measure, with the average O/C ratio of 0.16, while only 15 extra CHO- chromophores in WI-BrC fraction were identified after the measure but none before the measure, with the average O/C ratios of 0.44. The O/Cavg of CHO− chromophores in WI-BrC fraction were 0.29 and 0.31, respectively, before and after the "coal-to-gas" conversion measure (Supplementary Table 1). These results indicate that less low-oxygenated CHO- chromophores existed in WI-BrC fraction after the "coal-to-gas" conversion measure than before the measure, which may be related to the reduction of primary emissions (e.g. coal and biomass burning)36,37.

The CHO + chromophores in HULIS-BrC and WI-BrC fractions have similar changing characteristics with CHO− chromophores in HULIS-BrC and WI-BrC fractions before and after the "coal-to-gas" measure (Supplementary Figure 5). These results indicate that after the "coal-to-gas" conversion measure, the water-insoluble CHO chromophores decreased, while the water-soluble CHO chromophores increased.

CHON chromophores have the highest number in BrC fractions before and after the "coal-to-gas" conversion measure. A total of 5852 and 6674 CHON chromophores were assigned in the HULIS-BrC fraction before and after the measure, respectively, while 6162 and 5676 were assigned in the WI-BrC fraction correspondingly. The MFavg for CHON- and CHON+ chromophores in HULIS-BrC were C7.67H7.21O3.50N1.11 and C10.21H9.96O1.95N1.30, respectively, before "coal-to-gas" conversion measure, and C7.62H7.06O3.59N1.18 and C10.13H10.12O2.24N1.38, respectively, after the measure. The MFavg for CHON- and CHON + chromophores in WI-BrC were C8.80H8.81O3.47N1.10 and C13.06H12.14O1.78N1.35, respectively, before "coal-to-gas" conversion measure, and C8.56H8.33O3.62N1.21 and C12.52H12.12O1.95N1.44, respectively, after the measure (Supplementary Table 1). These results suggest that CHON chromophores in BrC after the "coal-to-gas" conversion measure contain more O and N atoms on average than those before the measure. Figure 3 shows the van Krevelen diagram for CHON chromophores in HULIS-BrC and WI-BrC fractions assigned in ESI− mode. The CHON− chromophores in the HULIS-BrC fraction were dominated by monocyclic aromatics (59% and 61% in terms of peak intensity before and after the measure, respectively) (Fig. 3a, b), and the corresponding intensity of most of CHON− chromophores in HULIS-BrC fraction was higher after the measure than before the measure (Supplementary Figure 6a). We further classified CHON- chromophores in HULIS-BrC fraction into different subgroups according to their O/N ratios (Fig. 4a, b); it can be seen that most CHON− chromophores in HULIS-BrC fraction (56% and 56% in terms of formula numbers before and after the measure, respectively) have O/N ≥ 3, which may contain one nitro (–NO2) or nitrooxy (–ONO2) group. Further, more than 35% of CHON- chromophores in the HULIS-BrC fraction have O/N ≥ 4, which may indicate the presence of further oxygenated functional groups, such as a hydroxyl group (–OH) or a carbonyl group (C = O)24,38. There were 12 CHON- chromophores in the HULIS-BrC fraction identified only before the measure but none after the measure and 75% of them have O/N < 3. On the contrary, 302 CHON− chromophores in the HULIS-BrC fraction were assigned only after the measure but none before the measure and over 50% of them have O/N ≥ 3. These results indicate that most of these increased CHON- chromophores in HULIS-BrC fraction after the "coal-to-gas" conversion measure may be nitroaromatic and organonitrate compounds, which could be from secondary formation24,39,40,41,42. Wang et al.42 reported the formation of CHON- chromophores from oxidation of anthropogenic VOCs in the presence of NOx".

CHON− chromophores in a, b HULIS-BrC and c, d WI-BrC fractions. The area of the circles is proportional to the fourth root of the intensity of individual chromophores, and the insert plot represents the chromophores identified only before or after the measure. The color bar denotes the aromaticity equivalent (black with XC < 2.5, red with 2.5 ≤ XC < 2.7, and blue with XC ≥ 2.7).

a, c Number of different CHON− chromophore subgroups in HULIS-BrC and WI-BrC fractions before (2015) the "coal-to-gas" conversion measure. b, d Similar to (a, c), but after (2019) the "coal-to-gas" conversion measure.

The CHON− chromophores in WI-BrC fraction before and after the "coal-to-gas" conversion measure were also predominated by monocyclic aromatics (53% and 54% in terms of peak intensity, respectively) (Fig. 3c, d). Different from CHON- chromophores in the HULIS-BrC fraction, the corresponding intensity of a majority of CHON− chromophores in the WI-BrC fraction was higher before the measure than after the measure (Supplementary Fig. 6b). CHON− chromophores with O/N ratio of 2 dominated the CHON- chromophores in WI-BrC fraction and most CHON− chromophores in WI-BrC fraction have O/N < 3 (57% and 53% in terms of formula numbers before and after the measure, respectively) (Fig. 4c, d), suggesting they may contain reduced nitrogen function groups (e.g., amines)24,38. There were 223 CHON− chromophores in the WI-BrC fraction assigned only before the measure but none after the measure, and about 79% of them have O/N < 3, while only 43 CHON− chromophores in the WI-BrC fraction were assigned after the measure, but none before the measure and about 81% of them have O/N < 3. These results indicate that after the "coal-to-gas" conversion measure, CHON− chromophores with less-oxidized structures in WI-BrC fraction decreased a lot, which may be due to the decrease of combustion emissions36,43,44.

CHON+ chromophores in HULIS-BrC fraction before and after the "coal-to-gas" conversion measure have similar characteristics with CHON- chromophores in HULIS-BrC fraction, which were also dominated by monocyclic aromatics (55% and 53% in terms of peak intensity, respectively) (Supplementary Figure 7a, b). The corresponding intensity of most CHON + chromophores in the HULIS-BrC fraction was also higher after the measure than before the measure (Supplementary Fig. 8a), and more extra CHON + chromophores in HULIS-BrC fraction were assigned only after the measure (Supplementary Figure 7a, b). The chromophore composition of CHON+ was different from CHON− in the HULIS-BrC fraction, which could be due to the difference in ionization efficiency between the positive and negative modes24,25. A majority of CHON+ chromophores in HULIS-BrC fraction have O/N < 3 (69% and 68% in terms of formula numbers before and after the measure, respectively) (Supplementary Fig. 9a, b), suggesting they could contain reduced nitrogen function groups24,38. Different from CHON+ chromophores in the HULIS-BrC fraction, the CHON+ chromophores in the WI-BrC fraction before and after the measure was mainly composed of polycyclic aromatics (47% and 44% in terms of peak intensity, respectively) and monocyclic aromatics (34% and 34%, respectively) (Supplementary Fig. 7c, d). Like the CHON− chromophores in the WI-BrC fraction, most CHON+ chromophores in the WI-BrC fraction also have O/N < 3 (62% and 60% in terms of formula numbers before and after the measure, respectively) (Supplementary Figure 9c, d). The corresponding intensity of most CHON+ chromophores in the WI-BrC fraction was higher before the measure than after the measure (Supplementary Fig. 8b), and more extra CHON+ chromophores were assigned in WI-BrC fraction only before the measure (Supplementary Fig. 7c, d). These results suggest that there was a large number of CHON + chromophores with low oxidation degrees in both water-soluble and water-insoluble BrC, and after the "coal-to-gas" conversion measure, the water-soluble CHON+ chromophores increased, while the water-insoluble CHON + chromophores decreased.

In ESI+ mode, a total of 603 and 643 CHN chromophores in HULIS-BrC fraction were assigned before and after the "coal-to-gas" conversion measure, respectively (6.2% and 5.8% in terms of formula numbers, respectively), and 1811 (16.2%) and 1587 (15.5%) were identified in WI-BrC fraction, respectively. The CHN + chromophores assigned in this study were composed of aromatics (Xc ≥ 2.5), implying a high degree of aromaticity. The van Krevelen diagram of CHN+ chromophores in HULIS-BrC and WI-BrC fractions before and after the "coal-to-gas" measure is shown in Fig. 5. The CHN+ chromophores in HULIS-BrC fraction were mainly composed of polycyclic aromatics (69% and 67% in terms of peak intensity before and after the measure, respectively) (Fig. 5a, b). The corresponding intensity of most CHN+ in the HULIS-BrC fraction was higher after the measure than before the measure (Supplementary Fig. 10a), and more extra CHN+ chromophores in the HULIS-BrC fraction were assigned only after the measure. The CHN+ chromophores in the WI-BrC fraction were also mainly composed of polycyclic aromatics (86% and 85% in terms of peak intensity before and after the measure, respectively) (Fig. 5c, d). However, the corresponding intensity of a majority of CHN+ chromophores in the WI-BrC fraction was higher before the measure than after the measure (Supplementary Fig. 10b), and more extra CHN+ chromophores in WI-BrC fractions were assigned only before the measure. It should be noted that in the HULIS-BrC fraction before and after the "coal-to-gas" measure, the CHN+ chromophores generally have carbon atoms <15 and DBE 5–12 (Supplementary Fig. 11a, b), which were suggested to be alkaloids44, and in WI-BrC fraction, there were a large number of CHN + chromophores with carbon atoms >15 and DBE 12–24 (Supplementary Fig. 11c, d), which were assigned to fused aromatic structures and N-heterocyclic rings24,44. According to previous studies, the increased CHN+ chromophores in the HULIS-BrC fraction may be formed from the photooxidation of aromatic hydrocarbons45,46, and the deceased CHN+ chromophores in WI-BrC fraction could be due to the reduction of fossil fuel combustion (especially coal combustion)38,47,48.

a, c Van Krevelen diagram for CHN+ chromophores in HULIS-BrC and WI-BrC fractions before (2015) the "coal-to-gas" conversion measure. b, d Similar to (a, c), but after (2019) the "coal-to-gas" conversion measure. The area of the circles is proportional to the fourth root of the intensity of individual chromophores, and the color bar denotes the aromaticity equivalent (black with XC < 2.5, red with 2.5 ≤ XC < 2.7, and blue with XC ≥ 2.7). The insert plot represents the chromophores identified only before or after the measure.

A total of 154 and 159 S-containing chromophores were identified in HULIS-BrC fraction before and after the "coal-to-gas" measure, respectively, while they were 62 and 54 in the WI-BrC fraction. The S-containing chromophores include CHOS and CHONS. For CHOS chromophores, they were dominated by aliphatic compounds (Xc < 2.5), and most of them were detected in the HULIS-BrC fraction in ESI− mode. The majority of CHOS chromophores have O/S ratios of 4–11 (Supplementary Fig. 12), indicating the possibility of at least one –OSO3H functional group and thus a possible classification to organosulfates (OSs)29,38. The corresponding intensity of most CHOS chromophores in BrC was higher after the "coal-to-gas" conversion measure (7.0 × 107 in total) than before the measure (5.5 × 107 in total) (Supplementary Fig. 13), which could be formed by secondary reactions of primary precursors24,38,49,50,51. For CHONS chromophores, in the HULIS-BrC fraction, most of them were identified in ESI− mode, while in the WI-BrC fraction, a majority of them were measured in ESI+ mode (Supplementary Fig. 14). Like CHOS chromophores, more CHONS chromophores were assigned in HULIS-BrC fraction than in WI-BrC fraction. More than 70% of CHONS chromophores in HULIS-BrC and WI-BrC fractions have O atoms <7, suggesting that they may contain reduced S (e.g., aromatic S-containing compounds) or reduced N functional groups (e.g., nitrile)36. Besides, before and after the measure, about 24% and 26% of CHONS chromophores in HULIS-BrC fraction and about 21% and 20% of CHONS chromophores in WI-BrC fraction were assigned with O atoms ≥ 7, suggesting they may contain one –OSO3H and one –NO3 functional group and they are likely nitrooxy-organosulfates27,29,36,38, which can be formed by the photochemical oxidation of VOCs under high NOx condition49,52. Similar to CHOS chromophores, the corresponding intensity of most CHONS chromophores in BrC was also higher after the "coal-to-gas" conversion measure than before the measure (Supplementary Fig. 13).

All CH chromophores identified in this study have Xc ≥ 2.5, among which over 95% have Xc ≥ 2.7 (Supplementary Fig. 15), suggesting they are likely polycyclic aromatic hydrocarbons (PAHs) that have been identified as important chromophores in WI-BrC fraction in ambient aerosols53. In this study, a total of 564 and 512 CH+ chromophores were assigned in the WI-BrC fraction before and after the "coal-to-gas" measure, respectively. The corresponding intensity of most CH+ chromophores (>95%) was higher before the measure than after the measure, indicating that after the "coal-to-gas" conversion measure, the CH + chromophores in atmospheric aerosols decreased significantly, which can be ascribed to the reduction of combustion emissions54,55.

This study reveals obvious changes in the chemical composition of atmospheric BrC in Beijing during the heating period before and after the "coal-to-gas" conversion measure, providing new insights into the importance of coal combustion in the real world as a BrC source. After the measure, the number and intensity of WI-BrC chromophores decreased, while the number and intensity of HULIS-BrC chromophores increased. Besides, more highly oxygenated chromophores were found in the HULIS-BrC fraction, and fewer low-oxygenated chromophores existed in the WI-BrC fraction after the measure than before the measure, likely due to the reduction in residential coal emissions as well as the change of atmospheric oxidative capacity. These results suggest that the impact of emission reduction measures on the chemical composition of BrC is complex, and more research is needed in the future. For example, some studies have shown that coatings of BrC on BC can enhance BC's light absorption56,57,58; it is important to understand the influence of the "coal-to-gas" conversion measure on the mixing state of carbonaceous aerosol and thus the effects on radiative forcing. Besides, the enhancement of water-soluble BrC after the measure may lead to an increase of the aerosol and liquid water content and thus the formation of secondary BrC via aqueous photoreactions, which consequently affect the lifetime of BrC in the atmosphere59,60,61. The changes in the chemical composition of BrC could also transform the size distribution of atmospheric particles, thereby affecting the light absorption and radiative forcing of atmospheric particles54,62,63.

Daily ambient PM2.5 filter samples were collected from 17 January to 15 March in 2015 and 2019 on the rooftop (~20 m above the ground; 39.99°N, 116.32°E) of the National Center for Nanoscience and Technology in Beijing, China, which was surrounded by residential, traffic and commercial areas. Filter samples were collected on pre-baked (780 °C, 3 h) quartz-fiber filters (20.3 × 25.4 cm; Whatman, QM-A, Clifton, NJ, USA) using a high-volume PM2.5 sampler (1.05 m3 min−1; Tisch, Cleveland, OH). After collection, the samples were wrapped in baked aluminum foils and stored in a freezer at −20 °C until further analysis.

The combined filter samples (3.526 cm2 for each of the 6 high light absorption samples) were extracted for different organic carbon (OC) fractions. In order to get a more meaningful result, the selected samples all have light absorption coefficient (Abs) above the average value (average Abs at 365 nm (Abs365) of 23.0 ± 14.2 and 10.0 ± 6.2 M m−1 before and after "coal-to-gas" conversion measure, respectively), and the Abs365 and OC concentration of selected samples were shown in Supplementary Table 2. The water-soluble fraction was extracted twice with 5 mL of ultrapure water (>18.2 MΩ cm) in an ultrasonic bath for 30 min. Then, the solution was filtered with a polyvinylidene fluoride (PVDF) syringe filter (Whatman, 0.45 μm; 3 mm) and treated following the protocol described by Lin et al.64 to obtain HULIS fraction, which is the primary light absorption component of water-soluble BrC (WS-BrC)53. The residual filter after water extraction was dried in a fume hood and further extracted twice with 5 mL methanol (HPLC grade, J.T. Baker, Phillipsburg, NJ, USA) in an ultrasonic bath for 30 min and filtered with a PTFE syringe filter for water-insoluble organic carbon (WISOC) fraction. The HULIS and WISOC fractions were concentrated to 300 μL for further analysis with an HPLC-PDA-HRMS. More details about HPLC-PDA-HRMS analysis were described elsewhere53.

Xcalibur 4.0 software (Thermo Scientific) was used to acquire raw data, and the open-source software toolbox MZmine version 2.52 (http://mzmine.github.io/) was used to perform peak deconvolution and chromatogram construction30,53. The formula assignment was performed with the following constraints: C1-30H0-60O0-15N0-3S0-1Na0-1 with a mass tolerance of ± 3 ppm for the ESI+ model and ±2 ppm for the ESI− model53. The recognized formulas were restricted by setting 0.3 ≤ H/C ≤ 3.0, O/C ≤ 3.0, N/C ≤ 0.5, S/C ≤ 0.2 to eliminate the chemically unreasonable formulas24,53. Further, the compounds with DBE/C ≥ 0.5 were considered candidates for BrC chromophores20,53. The double-bond equivalent values of the neutral chemical formula of CcHhOoNnSsNax were calculated as DBE = (2C + 2 – H + N)/2, where C, H, N are the number of carbon, hydrogen, and nitrogen, respectively20,53, and the aromaticity equivalent (Xc) was obtained using the equations: Xc = [3(DBE – (mO + nS)) – 2]/[DBE – (mO + nS)], where m and n refer to the fraction of oxygen and sulfur atoms involved in the π-bound structure of a compound26,38. In this study, m = n = 0.5 was employed for chromophores detected in ESI− mode, and m = n = 1 was used for chromophores assigned in ESI+ mode38. The neutral molecular formulas which did not conform to the nitrogen rule or with non-integer or negative DBE were excluded. All reported results have been corrected for blank. The peak intensity of the chromophores was used to compare their relative contents24,36,38,44. It should be noted that different types of chromophores may exhibit distinct sensitivity in the mass spectrometer due to the differences in ionization and transmission efficiencies38,65. Therefore, uncertainties may exist when comparing the peak intensity among chromophores. In this work, we assume that all chromophores have the same peak abundance response in the mass spectrometer.

Raw data used in this study are archived at the Institute of Earth Environment, Chinese Academy of Sciences, and are available on request by contacting the corresponding author (rujin. [email protected]).

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This work was supported by the National Natural Science Foundation of China (NSFC) under Grant No. 41925015, the Key Research Program of Frontier Sciences from the Chinese Academy of Sciences (no. ZDBS-LY-DQC001), the Strategic Priority Research Program of the Chinese Academy of Sciences (no. XDB40000000), and SKLLQG (no. SKLLQGTD1801).

State Key Laboratory of Loess and Quaternary Geology, Center for Excellence in Quaternary Science and Global Change, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, 710061, China

Wei Yuan, Ru-Jin Huang, Lu Yang, Ting Wang, Yuquan Gong, Wenjuan Cao, Jie Guo, Haiyan Ni & Jing Duan

University of Chinese Academy of Sciences, Beijing, 100049, China

Wei Yuan, Ru-Jin Huang, Lu Yang & Yuquan Gong

Key Laboratory of Detection Technology R&D on Food Safety, Food Inspection and Quarantine Technology Center of Shenzhen Customs, Shenzhen, 518045, China

Jincan Shen

Key Laboratory of Plant-Soil Interactions of MOE, College of Resources and Environmental Sciences, National Academy of Agriculture Green Development, China Agricultural University, Beijing, 100193, China

Kai Wang

Department of Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany

Thorsten Hoffmann

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Formal analysis, writing-original draft, W.Y., R.-J.H.; conceptualization, validation, supervision, project administration, funding acquisition R.-J.H.; writing-review & editing W.Y., R.-J.H., K.W., L.Y., T.W., Y.G., H.N., J.D., T.H.; data curation W.Y., R.-J.H., J.S., W.C., J.G.

Correspondence to Ru-Jin Huang.

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Yuan, W., Huang, RJ., Shen, J. et al. More water-soluble brown carbon after the residential "coal-to-gas" conversion measure in urban Beijing. npj Clim Atmos Sci 6, 20 (2023). https://doi.org/10.1038/s41612-023-00355-w

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Received: 08 July 2022

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Published: 23 March 2023

DOI: https://doi.org/10.1038/s41612-023-00355-w

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