Primary nature of brown carbon absorption in a frigid atmosphere with strong haze chemistry

https://doi.org/10.1016/j.envres.2021.112324Get rights and content

Highlights

  • Secondary BrC formation was enhanced at high RH despite low temperatures.

  • Secondary BrC accounted for 28% of brown carbon mass.

  • The contribution of secondary BrC to total brown carbon absorption was only 6%.

  • Though sources varied, total BrC had relatively constant optical properties.

  • BrC was inferred as an important contributor to aerosol absorption in Harbin.

Abstract

Severe haze hovered over Harbin during the heating season of 2019–2020, making it one of the ten most polluted Chinese cities in January of 2020. Here we focused on the optical properties and sources of brown carbon (BrC) during the extreme atmospheric pollution periods. Enhanced formation of secondary BrC (BrCsec) was evident as relative humidity (RH) became higher, accompanied with a decrease of ozone but concurrent increases of aerosol water content and secondary inorganic aerosols. These features were generally similar to the characteristics of haze chemistry observed during winter haze events in the North China Plain, and indicated that heterogeneous reactions involving aerosol water might be at play in the formation of BrCsec, despite the low temperatures in Harbin. Although BrCsec accounted for a substantial fraction of brown carbon mass, its contribution to BrC absorption was much smaller (6 vs. 28%), pointing to a lower mass absorption efficiency (MAE) of BrCsec compared to primary BrC. In addition, emissions of biomass burning BrC (BrCBB) were inferred to increase with increasing RH, coinciding with a large drop of temperature. Since both the less absorbing BrCsec and the more absorbing BrCBB increased as RH became higher, the MAE of total BrC were largely unchanged throughout the measurement period. This study unfolded the contrast in the source apportionment results of BrC mass and absorption, and could have implications for the simulation of radiative forcing by brown carbon.

Introduction

Light-absorbing organic aerosol (OA), which is commonly referred to as brown carbon (BrC; Andreae and Gelencsér et al., 2006; Laskin et al., 2015), is increasingly involved in climate models to predict the radiative effects of particulate matter. BrC has both primary and secondary sources. Biomass burning (BB) is a major contributor to primary BrC (BrCpri), while precursors of secondary BrC (BrCsec) include both anthropogenic and biogenic volatile organic compounds (VOCs). It has been widely accepted that BrCsec is generally less absorbing than BrCpri and in addition, secondary BrC produced by anthropogenic precursors typically have stronger light absorption capacities than those from biogenic VOCs (Laskin et al., 2015). However, it remains difficult to properly constrain the emissions and subsequent atmospheric processes (e.g., transport and transformation) of BrC, adding substantial challenges to BrC simulation (Feng et al., 2013; Saleh et al., 2014; Zhang et al., 2017; Wang et al., 2018). The difficulties are partially due to the different optical properties observed in ambient aerosols versus laboratory generated BrC, which are usually produced by combustion of various fuels (fossil or biomass) or by oxidation of various precursors. For example, the field observation by Forrister et al. (2015) and the chamber experiment by Cappa et al. (2020), both of which focused on biomass burning smoke, showed opposite patterns for the evolution of BrC absorption, i.e., decreasing by more than 90% vs. increasing by a factor of 1.6 after 1 day of aging. Actually, comparison of BrC results across studies is frequently not straightforward, given the multitude of measurement methods. In general, two types of techniques have been widely used to determine BrC, i.e., spectrophotometric measurement of BrC solution (with water and methanol as the most commonly used solvents; Liu et al., 2013, 2015; Zeng et al., 2020) and attribution of wavelength-resolved aerosol absorption, either filter-based (Sandradewi et al., 2008) or in-situ (Lack et al., 2012), to BrC and black carbon (BC) absorption. While there are studies evaluating uncertainties of the respective approach (e.g., Lack and Langridge, 2013), comparisons between BrC measurement methods are still lacking and the inter-method difference (e.g., regarding its magnitude) has not been well addressed with limited studies (Shetty et al., 2019; Moschos et al., 2021).

Recently, research on BrC received a boost from a growing interest in incorporating OA absorption into climate models (Samset et al., 2018; Saleh, 2020). In China, BrC has been intensively explored in laboratory studies focusing on primary and/or secondary OA (e.g., Tang et al., 2020; Ni et al., 2021; Zhang et al., 2020, 2021), and in field observations covering not only megacities such as Beijing (Yan et al., 2015), Nanjing (Liu et al., 2019), Guangzhou (Qin et al., 2018) and Xi'an (Wu et al., 2020) but also remote sites such as those in or around the Tibetan Plateau (Li et al., 2016; Wu et al., 2018; Wang et al., 2019b). Despite the progress, there are still gaps to be filled, e.g., observational results on ambient BrC are far from being enough, considering the large spatial variations of primary emissions and meteorological conditions in China. This is particularly the case for the Harbin-Changchun (HC) metropolitan area, the northernmost national-level city cluster in China. HC consists of 11 cities located in the severe cold climate region in the Northeast Plain, and the extremely cold winter, with daily-average temperatures down to < −20 °C, differs it from other city clusters such as those located in the North China Plain, the Yangtze River Delta and the Fenwei Plain. However, there are much fewer studies focusing on the aerosols, including BrC, in HC (Tao et al., 2017), e.g., for the heating season when massive air pollutants are emitted (e.g., by burning of fossil fuels and biofuels; Li et al., 2020a) and transformed in the frigid atmosphere.

In this study, we investigated the optical properties and sources of BrC during a recent heating season in Harbin, the central city of HC. As can be seen from the air quality data published by the China National Environmental Monitoring Center (CNEMC), a unique feature of this campaign was that it covered January of 2020, during which Harbin was listed in the bottom 10 Chinese cities with worst air quality. The monthly-average concentration of fine particulate matter (PM2.5) reached as high as 155 μg/m3 in Harbin. Thus, this study provided observational results on BrC during heavily-polluted winter haze events in a largely unexplored city cluster, which could enrich the understanding of BrC in China.

Section snippets

Field observation

Field measurements were conducted following the procedures described in Cheng et al. (2021b). Briefly, PM2.5 samples were collected on a daily basis in the campus of Harbin Institute of Technology (HIT) from 16 October 2019 to 4 February 2020 (N = 112), using a low volume sampler (MiniVol; Airmetrics, OR, USA) operated with quartz-fiber filters (2500 QAT-UP; Pall Corporation, NY, USA) at a flow rate of 5 L/min. The samples were cut into four punches, and the following laboratory analyses were

Enhanced secondary BrC formation at high RH

MSOC concentrations apportioned to the SA-1 and SA-2 factors were specified as the masses of secondary BrC (BrCsec). Considerable variations were identified in the time series of BrCsec during the measurement period. As shown in Fig. 2a, the first half of the campaign, i.e., mid-October to early December 2019, exhibited relatively low BrCsec concentrations, with an average of 1.78 ± 1.18 μgC/m3. However, a large increase of BrCsec occurred for the second half of this study, with persistently

Conclusions

As a recently-identified driver for climate change, BrC remains largely unexplored for Northeast China, with few observational results available to constrain the simulation of BrC for this region. To address this lack of understanding, field measurements were conducted in the central city of the Harbin-Changchun metropolitan area, which differs from other city clusters in China due to its extremely cold winter. The campaign covered a month representing one of the worst atmospheric pollution

Credit author contribution statement

Yuan Cheng: Conceptualization, Methodology, Writing-Original Draft. Xu-bing Cao, Qin-qin Yu, Zhen-yu Du and Lin-lin Liang: Investigation. Jiu-meng Liu: Conceptualization, Methodology, Writing-Review & Editing. Peng Wang: Validation. Cai-qing Yan: Formal analysis. Qiang Zhang and Ke-bin He: Supervision, Writing-Review & Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (41805097), the Natural Science Foundation of Heilongjiang Province (YQ2019D004), the State Key Laboratory of Urban Water Resource and Environment (ES202006), Longfengshan Regional Atmospheric Background Station and Heilongjiang Touyan Team.

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