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Brown Carbon introduction

Introduction

Light-absorbing carbonaceous aerosols have significant climatic impact due to their direct and indirect aerosol forcing. Except the most potent absorber of Black carbon (BC) at the visible wavelengths (400-700 nm) in the atmosphere, there is a growing body of evidence showing that certain organic aerosols, which are referred to as “brown carbon (BrC)”, have strong absorption abilities at near-ultraviolet and blue wavelengths with active role in changing radiative balance. Unlike BC, BrC is more complex and is probably oxygenated or multifunctional, or with very high molecular weight. In fact, a large amount of directly emitted BrC may be internally mixed with BC and further enhance BC particles light absorption. BrC is composed of both primary and secondary organic carbons that are often associated to various anthropogenic activities as well as complicated atmospheric processes.

Recently, our group has carried out the development and measurement of aqueous-BrC method using 340 nm absorption on combustion emissions and ambient PM from Xi’an, China (Fig1 & 2). Thus, different emission sources have significantly different babs with biomass burning smoldering being the strongest in UV absorption compared with others (Table1). We also selected two sites in Hong Kong to investigate the “roadside -to-ambient” evolutions of BrC levels and related optical properties (in Fig.2). Additionally, babs also showed significant differences between Hong Kong (traffic-dominated) and Xi’an (mixed sources) region (Fig.3). Massive on-going investigations of BrC optical properties and its relation with chemical composition and their aging process.


Fig.1. The measurement of aqueous-BrC over Xi'an region


Table 1 Average AAC and babs in different combustion sources.

Source AAC Babs(Mm-1)
Biomass burning
(smoldering)
7.44 65655
Straw briquette 6.78 682.1
Coal burning 4.38 84.9

Fig.2. Sampling site in Hong Kong; Comparison of the optical parameters between aged and fresh particles, Absorption coefficient of the aged and fresh groups and fitted Absorption Ångström Coefficient (AAC).


Fig.3. Comparison of the optical parameters between Xi’an and HK particles



Project contact: Katie Qian ZHANG