Aerosol absorption by in-situ filter-based photometer and ground-based sun-photometer in an urban atmosphere

Abstract

Abstract. Light Absorbing Aerosols (LAA) are short-lived climate forcers with a significant impact on Earth radiative balance and include dust aerosols, Black Carbon (BC) and organic light-absorbing carbonaceous aerosol (collectively termed as Brown Carbon, BrC), which have been also proven to be highly toxic. Aerosol absorption at 5 wavelengths (UV, B, G, R, IR) by filter photometer was monitored continuously during two winter seasons in 2020 and 2021 in the city of Modena (South-central Po valley, Northern Italy) at the two regulatory air quality monitoring sites, along with other pollutants (PM10, PM2.5, O3, NO, NO2, C6H6) and vehicular traffic rate. Columnar levels of AOD and of other aerosol optical properties were concurrently monitored at multiple wavelengths by a local sun-photometer at urban background conditions and within the AERONET network. In-situ absorption levels were apportioned both to sources (fossil fuel, biomass burning) and to species (BC, BrC), while columnar absorption was apportioned to BC, BrC and mineral dust. The combined analysis of the atmospheric aerosol and gas levels and of the meteorological conditions (in-situ and by ERA5 reanalysis) identified the location of potential urban sources for BC and BrC, most likely traffic and biomass burning. In-situ data shown different diurnal/weekly patterns for BrC by biomass burning and BC by traffic, with minor differences among the background and the traffic urban conditions. AERONET version 3 Absorption Aerosol Optical Depth (AAOD) retrievals at 4 wavelengths allowed the estimate of the Absorptive Direct Radiative Effect by LAA over the same period under the reasonable assumption that the AOD signal is built up within the mixing layer. AERONET retrievals showed a modest correlation of columnar absorption with PBL-scaled in-situ observations, although the correlation improves significantly during a desert dust transport event, affecting both in-situ aerosol and columnar absorption, particularly in the Blue spectrum range. Low correlation occurred between the contribution of BrC to aerosol absorption for the in-situ and the columnar observations, with this contribution being generally larger at the former. Finally, evidences of a strongly layered atmosphere during the study period, featured by large spatial mixing and modest vertical mixing, were shown by ERA5-based atmospheric temperature profiles and by the large correlation of concurrent AERONET AOD retrievals in Modena and in Ispra (on the NW side of the Po valley, ca. 225 km distant from Modena).

Reply on RC2

“…In this simulation, the burning of wood for domestic heating was mainly attributed to rural areas, whereas some of these emissions may actually occur in more central locations of the city and contribute to local pollution, even during night time, as noted by Bigi et al (2023). In addition, NO x emissions from domestic heating due to the combustion of compressed natural gas (CNG) have been spatially distributed using the volume of each building as a spatial proxy, which may not accurately reflect the actual distribution.…”

Description and validation of VERT 1.0, an R-based framework for estimating road transport emissions from traffic flows

“…In particular, the valley often suffers from low wind conditions, preventing the effective dispersion of ground emissions and contributing to the accumulation of pollutants. This is further exacerbated in the winter months by atmospheric inversion, which reduces the extent of vertical mixing and thus the part of the atmosphere where pollutants are diluted and mixed (Bigi et al, 2012(Bigi et al, , 2023Pernigotti et al, 2012). These meteorological characteristics, together with the high population density and the presence of busy commercial and industrial activities, places Modena among the largest European cities that exceed the air quality limits set by both the European regulation (European Council, 2008) and the latest WHO guidelines (WHO, 2021).…”

“…The emission inventory for the Emilia-Romagna region (INEMAR, 2019) estimates that vehicular traffic serves as the predominant source of NO x emissions in Modena, contributing with 78% to the total emissions, followed by domestic heating (12%), other mobile machinery (3%), waste treatment management (3%) and the industrial sector (2%). Previous studies (Bigi et al, 2023;Veratti et al, 2021Veratti et al, , 2020aVeratti et al, , b, 2017 have evaluated the impact of different sources on air quality in the city and its surrounding areas. This paper, however, focuses specifically on transport activities within the city and examines the influence of traffic on urban air quality using an integrated modelling approach.…”