Methods based on the absorption Ångström exponent (AAE) are widely used to estimate the absorption by brown carbon (BrC), and the estimated absorption by BrC can be significantly different from 0, even for pure black carbon (BC). However, few studies have systematically quantified the effects of BC microphysical properties. Moreover, the conditions under which AAE-based methods are applicable are still unclear. In this work, we used BC models partially coated with non-absorbing materials to calculate the total absorption. Since the total absorption is entirely due to BC, the estimated BrC absorption should be 0 if the retrieval methods are accurate. Thus, the ratio of the estimated BrC absorption to BC absorption (ABS

Carbonaceous aerosols are a major contributor to climate change

However, our understanding of BrC absorption is still quite limited. BrC strongly absorbs light in the UV region, while its absorption strongly decreases with increasing wavelength from the UV region to the visible region

Remote sensing can provide regional/global measurements and is an effective complementary method to address the above issues. Recently, researchers have attempted to derive the absorption by BrC based on the absorption of multiple wavelengths from remote sensing

An effective technique for separating out the contribution of BC is based on the different spectral absorption dependences of BC and BrC

Using the BC AAE value, the absorption by BC in the UV region can be estimated based on the absorption at NIR wavelengths. When estimating the absorption by BrC, a BC AAE of 1 has often been used, but more recent studies based on measurements and simulations have shown a wide range of AAE values

It is well known that the AAE method does not provide accurate results. However, there is a lack of understanding of the uncertainties caused by the microphysical properties of BC. Using morphologically realistic models,

How large are the uncertainties caused by BC microphysical properties in the estimation of BrC?

How do the microphysical properties of BC affect estimates of BrC?

What are the implications for estimates of BrC on a global scale?

When calculating the optical properties, the shape of BC was largely assumed to be spherical, so we could calculate the optical properties using Mie theory

With atmospheric aging, BC can be reconstructed into a condensed structure

The DLCA algorithm was developed to characterize the morphology of fresh BC, and the aggregates generated generally have a

After the BC cores were generated, we added the coating materials onto the surfaces of the BC cores. Similar to previous studies

The spherical coating materials were generated similarly to the method used in

Once the BC volume fraction (

Typical BC morphologies assumed in this work, which are similar to those in

Similar to our previous study

Once the BC structures were generated and the refractive index was specified, the MSTM method was used to calculate the absorption by each BC particle. The MSTM method directly outputs an effective absorption efficiency (

Numerous BC aerosols exist in the atmosphere, and the optical properties of BC should be the average properties over all the particles. Thus, we calculated the bulk optical properties by assuming different size distributions. Assuming a lognormal distribution for the size distribution of BC cores, we have

We first calculated the

The

We estimated the absorption by BC which was incorrectly attributed to BrC after calculating the BC absorption. In AERONET, the wavelengths 440, 675, and 870 nm were most commonly used to estimate the BrC absorption, and so we mainly considered these three wavelengths. At 675 and 870 nm, all of the absorption was assumed to be entirely due to BC. The

The

We used a global atmospheric chemical transport model, GEOS-Chem

The BC AAOD in each GEOS-Chem grid was calculated using

The global AAOD of BC incorrectly attributed to BrC can be determined using the following method:

ABS

Figure

ABS

The variation in ABS

The bare, fluffy BC aggregates are gradually coated by other materials with atmospheric aging, so BC with a larger

Similar to Fig.

With atmospheric aging, the BC cores are reconstructed into a more compact structure. We used a larger

Similar to Fig.

ABS

Because the AAE

The WDAs of BC with different morphologies and different

The WDAs of BC with different morphologies and different

The variation in ABS

However, with a more compact structure and a larger

The variations in the WDAs of BC with different morphologies with

Because fixed AAE methods cannot always provide accurate estimates of BrC,

The global mean BC AAOD was calculated using different models. The error bars in the figures represent the upper and lower limits when

The applicability of the WDA method is also related to the atmospheric aging status. As shown in Fig.

Figure

The global mean BC AAOD (

To explain why the above phenomenon occurs, we also calculated the WDAs of BC with different

As the BC cores become compact (

The global mean BC AAOD that is misattributed to BrC for different morphological configurations. The error bars in the figures represent the upper and lower limits when

The global mean BC AAOD (

Recent studies have generated increasing interest in estimating the global distribution of BrC

Global distributions of the BC AAOD (

Similar to Fig.

By comparing five models,

In general, the AAODs of BC with a fluffy structure are higher than those of BC with a compact structure, which is consistent with the results of previous studies

The BC DRF is an important parameter for assessing climate change.

Figure

Figures

AAE-based methods have been widely used to estimate the absorption by BrC, but they are subject to large uncertainties due to the properties of BC. We quantify the effects of the microphysical properties of BC based on numerical simulations and investigate how the applicability of AAE-based methods varies under different aging conditions. From the above, it is clear that using a BC AAE of 1 can provide reasonable estimates for BrC absorption, while the deviation from the “true” BrC absorption becomes significant as the particles age. This means that the AAE

The AAE-based method is commonly used to estimate the absorption by BrC, but it may provide inaccurate estimates due to the effects of the microphysical properties of BC. The goal of this work was not to discuss the use of the AAE-based method but to assess the uncertainties of the AAE-based method. We find that an AAE of 1 can provide a reasonable estimate when the BC is freshly emitted. Therefore, an AAE of 1 is suggested for regions close to the emission source, such as a vehicle emissions region. However, we should also note the uncertainties associated with using an AAE of 1. We estimate an ABS

This work represents the aging condition by assuming a more compact structure, the presence of more coating materials, and a larger

At the global level, the use of a BC AAE of 1 can lead to a global mean misassigned AAOD of about

The optical properties of black carbon were calculated using the MSTM code, which was downloaded from

The data can be requested from the corresponding author.

The supplement related to this article is available online at:

JL conceived the presented idea. JL developed the models, performed the computations, and wrote the paper. JQ verified the simulation methods and results, revised the paper, and supervised the findings of this work. All authors discussed the results and contributed to the final paper.

The contact author has declared that none of the authors has any competing interests.

Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors.

We gratefully acknowledge financial support from the National Key R&D Program of China (grant no. 2022YFB3902802) and the National Natural Science Foundation of China (grant nos. 42305148 and 41871269). We particularly thank Daniel W. Mackowski and Michael I. Mishchenko for making the MSTM code publicly available.

This research has been supported by the National Key Research and Development Program of China (grant no. 2022YFB3902802) and the National Natural Science Foundation of China (grant nos. 42305148 and 41871269).

This paper was edited by Eduardo Landulfo and reviewed by two anonymous referees.