Air quality is concerned with pollutants in both the gas phase and solid or liquid phases. The latter are referred to as aerosols, which are multifaceted agents affecting air quality, weather and climate through many mechanisms. Unlike gas pollutants, aerosols interact strongly with meteorological variables with the strongest interactions taking place in the planetary boundary layer (PBL). The PBL hosting the bulk of aerosols in the lower atmosphere is affected by aerosol radiative effects. Both aerosol scattering and absorption reduce the amount of solar radiation reaching the ground and thus reduce the sensible heat fluxes that drive the diurnal evolution of the PBL. Moreover, aerosols can increase atmospheric stability by inducing a temperature inversion as a result of both scattering and absorption of solar radiation, which suppresses dispersion of pollutants and leads to further increases in aerosol concentration in the lower PBL. Such positive feedback is especially strong during severe pollution events. Knowledge of the PBL is thus crucial for understanding the interactions between air pollution and meteorology. A key question is how the diurnal evolution of the PBL interacts with aerosols, especially in vertical directions, and affects air quality. We review the major advances in aerosol measurements, PBL processes and their interactions with each other through complex feedback mechanisms, and highlight the priorities for future studies.

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Aerosol and boundary-layer interactions and impact on air quality

Source apportionment of VOCs and the contribution to photochemical ozone formation during summer in the typical industrial area in the Yangtze River Delta, China

In this study, we present long-term near-surface measurements of sulfur dioxide (SO2), oxides of nitrogen (NOx), carbon monoxide (CO), and ozone (O3) carried out at an urban location, Kanpur (26.46°N, 80.33°E, 125 m amsl), in Northern India from June 2009 to May 2013. The mean concentrations of SO2, NOx, CO, and O3 over the entire study period were 3.0, 5.7, 721, and 27.9 ppb, respectively. SO2, NOx and CO concentrations were highest during the winter season, whereas O3 concentration peaked during summer. The former could be attributed mainly to the near-surface anthropogenic sources (e.g. automobiles, residential cooking, brick kilns, coal-burning power plants, agricultural land-clearing, and biomass burning) and low mixing height in winter, whereas the latter was clearly due to enhanced chemical production of O3 during the pre-monsoon (i.e. summer) season. The lowest concentration of all trace gases were observed during the monsoon season, due to efficient wet scavenging by precipitation. The averaged diurnal patterns also showed similar seasonal variation. NOx and CO showed peaks during morning and evening traffic hours and a valley in the afternoon irrespective of the seasons, clearly linked to the boundary layer height evolution. Contrarily, O3 depicted a reverse pattern with highest concentrations during afternoon hours and lowest in the morning hours. The mean rate of change of O3 concentrations (dO3/dt) during the morning hours (08:00 to 11:00 h) and evening hours (17:00 to 19:00 h) at Kanpur were 3.3 ppb h−1 and −2.6 ppb h−1, respectively. O3 followed a positive linear relationship with temperature, except in post-monsoon season while the strong negative with the relative humidity in all seasons. The ventilation coefficient was found to be highest in the pre-monsoon season (15,622 m2 s−1) and lowest during winter (2564 m2 s−1), indicative of excellent pollution dispersion efficiency during the pre-monsoon season. However, the low ventilation coefficient during winter and post-monsoon seasons indicated that the high-pollution potential occurs at this site.

Four-year measurements of trace gases (SO2, NOx, CO, and O3) at an urban location, Kanpur, in Northern India

Role of long-range transport and local meteorology in seasonal variation of surface ozone and its precursors at an urban site in India

Impacts of firecracker burning on aerosol chemical characteristics and human health risk levels during the Chinese New Year Celebration in Jinan, China

Atmospheric pollution in a semi-urban, coastal region in India following festival seasons

Influence of ozone precursors and PM10 on the variation of surface O3 over Kannur, India

Continuous measurements of surface ozone (O3), NOx (NO + NO2) and meteorological parameters have been made in Kannur (119 °N, 754 °E, 5 m asl), India from November 2009 to October 2010 It was observed that O3 and NOx showed distinct diurnal and seasonal variabilities at this site The annual average diurnal profile of O3 showed a peak of (303 ± 104) ppbv in the late afternoon and a minimum of (32 ± 07) ppbv in the early morning The maximum value of O3 mixing ratio was observed in winter (44 ± 31) ppbv and minimum during monsoon (1846 ± 35) ppbv The rate of production of O3 was found to be higher in December (101 ppbv/h) and lower in July (18 ppbv/h) during the time interval 0800–1000 h A correlation coefficient of 052 for the relationship between O3 and [NO2]/[NO] reveals the role of NO2 photolysis that generates O3 at this site The correlation between O3 and meteorological parameters indicate the influence of seasonal changes on O3 production Investigations were further extended to explore the week day weekend variations in O3 mixing ratio at an urban site reveals the enhancement of O3 The variations of O3 mixing ratio with seasonal air mass flows were elucidated with the aid of backward air trajectories This study also indicates how vapor phase organic species present in the ambient air at this location may influence the complex chemistry involving (VOCs) that enhances the production of O3 at this location

Variations in surface ozone and NOx at Kannur: a tropical, coastal site in India

Influence of solar eclipse of 15 January 2010 on surface ozone

Surface ozone is mainly produced by the photodissociation of nitrogen dioxide (NO2) by solar UV radiation. Subsequently, solar eclipses provide one of the unique occasions to explore the variations in the photolysis rate of NO2 and their significant impact on the production of ozone at a location. This study aims to examine the diurnal variations in the photodissociation rate coefficient of NO2, (j(NO2*)), and mixing ratios of surface ozone and NO
 X
 * (NO + NO2*) during the solar eclipse that occurred on 15 January 2010 at Kannur (11.9°N, 75.4°E, 5 m amsl), a tropical coastal site on the Arabian Sea in South India. This investigation was carried out on the basis of the ground level observations of surface ozone and its prominent precursor NO2*. The j(NO2*) values were estimated from the observed solar UV-A flux data. A sharp decline in j(NO2*) and surface ozone was observed during the eclipse phase because of the decreased efficiency of the ozone formation from NO2. The NO2* levels were found to increase during this episode, whereas the NO levels remained unchanged. The surface ozone concentration was reduced by 57.5%, whereas, on the other hand, that of NO
 X
 * increased by 62.5% during the solar eclipse. Subsequently a reduction of *% in the magnitude of j(NO2*) was found here during the maximum obscuration. Reductions in solar insolation, air temperature and wind speed were also observed during the solar eclipse event. The relative humidity showed a 6.4% decrease during the eclipse phase, which was a unique observation at this site.

M. K. Satheesh Kumar

Papers

Atmospheric pollution in a semi-urban, coastal region in India following festival seasons

Influence of ozone precursors and PM10 on the variation of surface O3 over Kannur, India

Variations in surface ozone and NOx at Kannur: a tropical, coastal site in India

Influence of solar eclipse of 15 January 2010 on surface ozone

Solar eclipse-induced variations in solar flux, j(NO 2 ) and surface ozone at Kannur, India