Identification

Title

Can 3-D models explain the observed fractions of fossil and non-fossil carbon in and near Mexico City?

Abstract

A 3-D chemistry-transport model has been applied to the Mexico City metropolitan area to investigate the origin of elevated levels of non-fossil (NF) carbonaceous aerosols observed in this highly urbanized region. High time resolution measurements of the fine aerosol concentration and composition, and 12 or 24 h integrated ¹⁴C measurements of aerosol modern carbon have been performed in and near Mexico City during the March 2006 MILAGRO field experiment. The non-fossil carbon fraction (fCNF), which is lower than the measured modern fraction (fCM) due to the elevated ¹⁴C in the atmosphere caused by nuclear bomb testing, is estimated from the measured fCM and the available source information. The fCNF contained in PM1 total carbon (fCNFTC) ranged from 0.37 to 0.67 at the downtown location (T0), and from 0.50 to 0.86 at the suburban site T1. Substantially lower values (i.e. 0.24-0.49) were found for PM10 filters at T0 by an independent set of measurements, which are inconsistent with the modeled and known differences between the size ranges, suggesting higher than expected uncertainties in the measurement techniques of ¹⁴C. An increase in the non-fossil organic carbon (OC) fraction (fCNFOC) by 0.10-0.15 was observed for both sets of filters during periods with enhanced wildfire activity in comparison to periods when fires were suppressed by rain, which is consistent with the wildfire impacts estimated with other methods. Model results show that the relatively high fraction of non-fossil carbon found in Mexico City seems to arise from the combination of regional biogenic SOA, biomass burning OA, as well as non-fossil urban OA. Similar spatial and temporal variations for fCNFOC are predicted between the urban vs. suburban sites, and high-fire vs. low-fire periods. The absolute modeled values of fCNFOC are consistent with the PM10 dataset but lower than the PM1 filters. Resolving the ¹⁴C measurement discrepancies is necessary for further progress in model evaluation. The model simulations that included secondary organic aerosol (SOA) formation from semi-volatile and intermediate volatility (S/IVOC) vapors showed better skill in explaining both total OA mass and fCNFOC compared to simulations which only included SOA from VOCs. Urban sources of modern carbon are important in reducing or closing the gap between model and measurements, even though they are often neglected on the interpretation of ¹⁴C datasets. The fCNF of urban POA and SOA precursors is an important parameter that needs to be better constrained by measurements. Performing faster (≤3 h) ¹⁴C measurements in future campaigns is critical to further progress in this area. To our knowledge this is the first time that radiocarbon measurements are used together with aerosol mass spectrometer (AMS) organic components to assess the performance of a regional model for organic aerosols.

Resource type

document

Resource locator

Unique resource identifier

code

http://n2t.net/ark:/85065/d7j67hfv

codeSpace

Dataset language

eng

Spatial reference system

code identifying the spatial reference system

Classification of spatial data and services

Topic category

geoscientificInformation

Keywords

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keyword value

Text

originating controlled vocabulary

title

Resource Type

reference date

date type

publication

effective date

2016-01-01T00:00:00Z

Geographic location

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Temporal reference

Temporal extent

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End position

Dataset reference date

date type

publication

effective date

2010-11-25T00:00:00Z

Frequency of update

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Conformity

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Use constraints

Limitations on public access

None

Responsible organisations

Responsible party

contact position

OpenSky Support

organisation name

UCAR/NCAR - Library

full postal address

PO Box 3000

Boulder

80307-3000

email address

opensky@ucar.edu

web address

http://opensky.ucar.edu/

name: homepage

responsible party role

pointOfContact

Metadata on metadata

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