The optical properties and in-situ observational evidence for the formation of brown carbon in cloud

Atmospheric brown carbon (BrC) makes a substantial contribution to aerosol light-absorbing and thus the global radiative forcing. Although BrC may change the lifetime of the cloud and ultimately affect precipitation, little is known 15 regarding the optical properties and formation of BrC in the cloud. In the present study, the light-absorption properties of cloud droplet residual (cloud RES) were measured by coupled a ground-based counterflow virtual impactor (GCVI) and an Aethalometer (AE-33), in addition to the cloud interstitial (cloud INT) and ambient (cloud-free) particles by PM2.5 inlet-AE33, at Mt. Tianjing (1690 m a.s.l.), a remote mountain site in southern China, from November to December 2020. Meanwhile, the light-absorption and fluorescence properties of water-soluble organic carbon (WSOC) in the collected cloud water and 20 PM2.5 samples were also obtained, associated with the concentration of water-soluble ions. The mean light-absorption coefficient (Abs370) of the cloud RES, cloud INT, and cloud-free particles were 0.25 ± 0.15, 1.16 ± 1.14, and 1.47 ± 1.23 Mm , respectively. The Abs365 of WSOC was 0.11 ± 0.08 Mm in cloud water and 0.40 ± 0.31 Mm in PM2.5, and the corresponding mass absorption efficiency (MAE365) was 0.17 ± 0.07 and 0.31 ± 0.21 m·g, respectively. A comparison of the light-absorption coefficient between BrC in the cloud RES/cloud INT and WSOC in cloud water/PM2.5 indicates a considerable 25 contribution (48-75%) of water-insoluble BrC to total BrC light-absorption. Secondary BrC estimated by minimum R squared (MRS) method dominated the total BrC in cloud RES (67-85%), rather than in the cloud-free (11-16%) and cloud INT (9-23%) particles. It may indicate the formation of secondary BrC during cloud processing. Supporting evidence includes the enhanced WSOC and dominant contribution of secondary formation/biomass burning factor (> 80%) to Abs365 in cloud water provided by Positive Matrix Factorization (PMF) analysis. In addition, we showed that the light-absorption of BrC in cloud water was 30 closely related to humic-like substances and tyrosine/proteins-like substances (r > 0.63, p < 0.01), whereas only humic-like substances for PM2.5, as identified by excitation-emission matrix fluorescence spectroscopy. https://doi.org/10.5194/acp-2021-945 Preprint. Discussion started: 25 November 2021 c © Author(s) 2021. CC BY 4.0 License.

and PM 2.5 . We aim to explore: 1) the optical properties of BrC in cloud-processed, cloud-free particles and WSOC in PM 2.5 and cloud water; 2) the possible contribution of in-cloud production to BrC light-absorption, and 3) the characteristics of fluorescent chromophores in cloud water and PM 2.5 and their relationship with light-absorption properties of BrC.

Sampling setup
Measurements of the cloud-free, cloud RES, and cloud INT particles were performed at Mt. Tianjing (24°41′56″N, 112°53′56″E, 1690 m a.s.l.) in Guangdong province, China during 18 November to 5 December 2020. This site is located at a national forest reserve and is less affected by anthropogenic sources. The cloud event determination threshold was set as visibility less than 3 km and relative humidity (RH) larger than 95%. During the cloud events, the cloud RES and cloud INT 80 particles were alternately introduced into the instruments through ground-based counterflow virtual impactor (GCVI, model 1205, Brechtel Mfg., Inc., USA) and PM 2.5 cutoff, respectively, at a frequency of one hour. The GCVI cut size was set to 7.5 μm, where the transmission efficiency of cloud droplets is 50% (Shingler et al., 2012). The collected cloud droplets passed through an evaporation chamber (40℃), resulting in the cloud RES particles for downstream analysis. An Aethalometer (model AE-33, Magee Scientific., USA) was used to measure the light-absorption coefficients of particles at wavelengths of 85 370,470,520,590,660,880, and 950 nm. AE-33 uses two parallel spot measurement technology to compensate for the light attenuation due to the filter loading effect (Drinovec et al., 2015). The BC concentration was calculated by the light-absorption coefficient at 880 nm. The detection limit of BC is less than 10 ng· m -3 (equal to 0.077 Mm -1 at 880 nm) and the uncertainty is ~ 2 ng· m -3 (equal to 0.015 Mm -1 at 880 nm), with a time resolution of 1 minute.
Cloud water samples were collected by a Caltech Active Strand Cloud water Collector, Version 2 (CASCC2) (Demoz et al., 90 1996;Yang et al., 2021b) when the visibility was less than 200 m (during 14 November to 4 December 2020). The collection efficiency was 50% at a cut size of 3.5 μm. During the sampling period, 53 cloud water samples were collected. The 0.22 μm quartz fiber filter was used immediately to remove insoluble components after collection of cloud water and then frozen at -20℃ until analysis. Meanwhile, PM 2.5 samples were collected by a mid-volume (300 L· min -1 ) aerosol sampler Mingye,China). Daily samples (during 14 November to 8 December 2020) were collected on the quartz fiber filters, which 95 were prebaked at 450℃ for 4 h in a muffle furnace to remove residual organics before use. After collection, all samples were frozen at -20℃ until analysis. In this study, PM 2.5 samples collected at the same time with cloud water samples were regarded as INT-PM 2.5 (n = 13), and the others as FREE-PM 2.5 (n = 19). It should be noted that some FREE-PM 2.5 samples also experienced short cloud events during collection. Blank samples of the cloud water and PM 2.5 were collected and processed following the same procedure as the samples.
The light-absorption coefficient (Abs BrC (λ), Mm -1 ) of BrC in different wavelengths can be obtained by AE-33, assuming that the absorption Ångstrӧm exponent (AAE) of BC is 1 and the light-absorption at 880 nm only due to BC (Drinovec et al., 2015).
The cloud RES, cloud INT, and cloud-free particles were generally located in submicron size (Fig. S1), and thus unlikely originated from non-combustion sources are mostly biogenic and mainly exist in the coarse mode (Perrino and Marcovecchio, 105 2016). The Abs BrC (λ) contributed by the combustion sources can be estimated through a BC-tracer method : Where Abs(λ) is the total light-absorption coefficient of carbonaceous aerosol that measured by AE-33, ( ) , which is assumed 110 to be step increasing from 0 to 120 with a rate of 0.1. The target ( ( ) ) value can be retrieved when the correlation coefficient (R 2 ) between Abs BrC,sec (λ) with BC concentration reaches the minimum (see Fig. S2). Previous studies showed that the bias of MRS method is less than 23%, when the measurement uncertainty is less than 20% (Wu and Yu, 2016). It should be noted that when the measured ratio of ( ) is lower than the retrieved ( ( ) ) , the Abs BrC,sec (λ) could be negative. In these cases, Abs BrC,sec (λ) is set to zero for subsequent analysis (Kaskaoutis et al., 2021;Wang et al., 2019a). These cases account 115 for less than 5% in the cloud RES and 28-70% in the cloud INT and cloud-free particles 2.3 Measurements of PM2.5 and cloud water PM 2.5 samples were ultra-sonically extracted with ultrapure water (resistivity: 18.2 MΩ cm) for 30 min, then filtered by the 0.22 μm polytetrafluoroethylene (PTFE) filters to obtain the PM 2.5 aqueous extract. The concentrations of water-soluble ions, water-soluble heavy metals, WSOC in PM 2.5 aqueous extract and cloud water samples were analyzed by ion chromatography 120 (Metrohm 883 IC plus, Switzerland), inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher, USA), and total organic carbon analyzer (TOC-V, Shimadzu, Japan), respectively. Parallel analyses showed that the relative standard deviation of each analysis was generally less than 15%. The reported concentration data herein was after blank subtraction.
The light-absorption coefficient (Abs WSOC,λ ) of WSOC can be obtained (Hecobian et al., 2010) by the measurement of cloud water and PM 2.5 aqueous extract, with UV-Vis (UV1901, Kejie, China): Where is the absorbance of the sample, A700 is used to account for any drift; Vl is the volume of ultrapure water used to extract the sample (for cloud water it is the total sample volume), Va is the volume of sampled air through the PTFE filter (for cloud water it is the total volume of sampled air), and L is the cuvette path length (0.01 m).
The AAE values describing the spectral dependence of WSOC light-absorption can be further deduced by exponential fitting (μg·m -3 ). The E 250 /E 365 (the ratio of absorbance at 250 nm to that at 365 nm) is used to describe the humification of organic matter, which is inversely related to aromaticity and molecular weight of WSOC (Kristensen et al., 2015). Specific UV absorbance (SUVA, m 2 ·g -1 ,) at 254 and 280 nm had been proved to be qualitatively related to the structural characteristics (aromaticity and molecular weight) of WSOC to a certain extent (Weishaar et al., 2003), which can be calculated using the contribution (BIX), and humification index (HIX) were described in the supporting information (SI) text S1.

The optical properties of BrC during cloud events
The presence of BrC could be indicated by the AAE values derived from AE-33 data, which are 1.30 ± 0.12 for cloud-free,  (Kirillova et al., 2016), and much lower than those in urban areas (as summarized in Table S1) (Chen INT-PM 2.5 , cloud water-Day, and cloud water-Night, which are 6.01 ± 0.81, 5.37 ± 1.08, 5.81 ± 1.47, and 6.31 ± 1.51, respectively, within the reported range. The MAE 365 of WSOC in FREE-PM 2.5 , INT-PM 2.5 , cloud water-Day, and cloud water-Night are 0.31 ± 0.17, 0.31 ± 0.26, 0.17 ± 0.07, and 0.17 ± 0.07 m 2 ·g -1 , respectively. The MAE 365 of WSOC in cloud water and PM 2.5 are much lower than those in urban/alpine areas and various source emission samples (Table S1)

180
The contribution of water-insoluble BrC to the light-absorption is estimated to be ~75% for the cloud INT particles and ~48% for the cloud RES particles on average, based on these differences (Fig. 3). It is also noted that the light-absorption of WIOC might still be underestimated by ~16% when sampling size is considered for the GCVI and cloud sampler (as discussed in SI text S1). High contributions of WIOC to BrC light-absorption have also been observed in the Indo-Gangetic plain (77%) (Satish et al., 2020), Beijing (62%), and Xi'an (51%) (Huang et al., 2020).

190
RH was generally higher than 70% (Fig. S1). The contribution of secondary BrC in cloud INT and cloud-free particles are in the low range of reported values (as summarized in Table S2) (Gao et al., 2022;Kaskaoutis et al., 2021;Lin et al., 2021;Wang et al., 2019aWang et al., , 2019bWang et al., , 2020bWang et al., , 2021Zhang et al., 2020Zhang et al., , 2021Zhu et al., 2021). Differently, the contribution of secondary formed BrC to the total BrC light-absorption is 67-85% in the cloud RES particles, surprisingly higher than those in the cloud-free and cloud INT particles. Such a high contribution may suggest the critical role 195 of cloud processing in the formation of BrC. Compared with the relative contributions for the cloud-free and cloud INT particles, the importance of such a process in cloud droplets remarkably overrides that in wet particles. The significance of secondary water-soluble BrC formation in cloud droplets may also be reflected by the significant correlation between the Abs 365 of cloud water and PM 2.5 aqueous extract with SNA (sulfate, nitrate, and ammonium) (r > 0.77, p < 0.01), and NOx (r > 0.58, p < 0.01), as shown in Fig. S4. The SNA and NOx concentrations are higher at night than the daytime (Fig. S5), consistent 200 with higher Abs 365 of cloud water at night. NOx may enhance the formation of nitrogen-containing organics (Seinfeld and Pandis, 2016;Yang et al., 2021a), potentially contributing to the light-absorption of cloud water (Desyaterik et al., 2013). Incloud aqueous processes leading to more CHON compounds in cloud water than below-cloud atmospheric particles has also been observed (Boone et al., 2015). In addition, a comparison between the WSOM (WSOM = WSOC*1.8) normalized by K + (as a primary source tracer) in cloud water than INT-PM 2.5 (Fig. S6) also clearly indicates the enhanced formation of WSOM 205 in cloud water. It is consistent with that the light-absorption of WSOC contributed more to the cloud RES (~52%) than the cloud INT (25%) particles, as estimated in Fig. 3.
Consistently, the source and contribution apportionment of BrC in cloud water (i.e., Abs 365 ) evaluated by the PMF model (see SI for data analysis and evaluation methods) also supports the critical role of aqueous process on the formation of BrC, as shown in Fig. 4. Factor 1 is associated with relatively higher K + , NH 4 + , NO 3 -, SO 4 2-, and C 2 O 4 2-, contributing 64.3% to WSOC and 86.9% to Abs 365 . It may be appropriately recognized as secondary products with contribution from biomass burning, as K + represents a tracer for biomass burning, and NH 4 + , NO 3 -, C 2 O 4 2-, and SO 4 2are regarded as secondary species (Cheng et al., 2015;Wang et al., 2012). Note that C 2 O 4 2is generally considered as a tracer of aqueous-phase processes (Zhang et al., 2017b).
As previously observed, the aqueous SOA formed from biomass burning might contributed to the BrC budget in fog water (Gilardoni et al., 2016). Factor 2 is characterized by high levels of crustal trace elements such as Mg 2+ , Ca 2+ , Mn, and Zn, and 215 thus identified as crustal materials, contributing 21.9% to WSOC and 8.7% to Abs 365 . Factor 3 shows extremely high loading with Na + and relatively high Mg 2+ , Cl -, and Ni, which may originate from marine, contributing 13.8% to WSOC and 4.4% to Abs 365 .

Fluorescence properties of BrC in PM2.5 and cloud water
The results from the EEMs measurements further indicate the different characteristics of WSOC/WS-BrC in PM 2.5 and cloud 220 water. Based on the PARAFAC model calculation (Fig. 5), two independent fluorescence components (P1-P2) assigned as humic-like substances are found in PM 2.5 , whereas four independent fluorescence components (C1-C4) assigned as humic-like substances (C1-C3) and tyrosine/protein-like substances (C4) are found in cloud water (Catalá et al., 2015;Coble, 2007). The fluorescence components of cloud water are similar to those in Mt. Tai (Zhao et al., 2019) and France (Bianco et al., 2016b), where humic-like and protein-like substances are the main chromophores in cloud water. Compared with PM 2.5 , tyrosine/protein-like substances are unique to cloud water in the present study, which may be partly due to their relative enrichment in cloud water (Kristensson et al., 2010;Zhang and Anastasio, 2003).
In addition, the relative contribution of individual chromophores indicated by F max in PM 2.5 and cloud water also exhibits different characteristics, although humic-like substances are the dominant fluorescent fraction in both PM 2.5 and cloud water.
The relative contribution shows no obvious difference between P1 and P2 components in
Therefore, it is most likely that the organic components in cloud water may be significantly affected by in-cloud aqueous formation, consistent with the PMF results. With respect to the secondary processes, humic-like substances may be formed through Maillard reaction involving carbonyls and ammonium/amines (Bones et al., 2010;Hawkins et al., 2016), and also the photo-transformation of tyrosine (Berto et al., 2016).

4 Conclusions and implications
In the present study, the light-absorption properties of the cloud RES, cloud INT, and cloud-free particles were simultaneously investigated at a remote mountain site in southern China. Coupled with the measurements of light-absorption and fluorescence properties of WSOC in the collected cloud water and PM 2.5 , it is evident that in-cloud aqueous processing facilitates the 245 formation of BrC (i.e., 67-85% secondary BrC in cloud RES particles by MSR method). As potential contributors to lightabsorption of BrC, only two fluorescence fractions of humic-like substances are found in PM 2.5 , whereas four fluorescence fractions (three types of humic-like substances and one type of tyrosine/protein-like substances) are identified in cloud water, most likely attributed to secondary production. While extensive laboratory evidence indicated the possible formation of BrC in aqueous phase (Hems et al., 2021), our study represents the first attempt to show the possibility under real cloud condition.

250
The results could support a previous hydrolysis that in-cloud formation of BrC might contribute to the enhanced BrC/BC in the attitude between 5-12 km (Zhang et al., 2017c). Such process might also have potential implication for the lifecycle of BrC (Liu et al., 2020).
In order to evaluate the influence of BrC formation in the light-absorption properties of cloud water, the imaginary part of the refractive index for cloud water was calculated according to Gelencsé r et al. (2003), as detailed in the SI text S1. The average 255 imaginary part of cloud water was 5.5×10 -8 at 365 nm (Fig. S7), ~10 times that of pure water. The imaginary part (3.4×10 -8 at 475 nm) is a magnitude higher than previous laboratory simulation results (5.2×10 -9 at 475 nm), involving 3,5-dihydroxybenzoic acid reaction with FeCl 3 (Gelencsé r et al., 2003). It should also be noted that it is the lowest estimation since only WSOC is included in the calculation. As previously indicated, the overall light-absorption of WIOC cannot be negligible. cloud events could be classified as weakly absorptive BrC (Saleh, 2020). The measured optical properties and suggested incloud formation of BrC would help better understand the atmospheric evolution and the radiation forcing of BrC.