3.1 Optical Parameters at Lahore and Karachi
Aerosol optical depth and Ångstroṁ exponent are two primary columnar aerosol parameters used to characterize aerosols. AOD, which is a commonly obtained optical measurement through satellites and ground-based devices, indicates the amount of aerosol loading and specifies the column-integrated aerosol abundance in the atmosphere (Goudie., 2006) and AE is the aerosol particle size indicator (ÅngstrÖm., 1961). Atmospheric aerosol burden and their size distributions are usually described by AOD and AE.
AOD and AE have been investigated to get more information about aerosols in the Lahore and Karachi sites for 1173 and 988 days, respectively. At the Lahore site, the daily average value of AOD-440nm (AE 440-870nm) is found to be 0.81 ± 0.49 (1.02 ± 0.32), while for Karachi it is 0.49 ± 0.20 (0.66 ± 0.32). Both optical parameters (AOD and AE) have comparatively large mean values at the Lahore site which is attributed to the more aerosol burden due to vehicular, and industrial emissions, and dust (both from local sources and transported from other regions) aerosols at Lahore, especially during the spring and summer seasons.
Figure 3 (a − e) shows the trend of monthly mean and seasonal AOD and AE values for Lahore and Karachi sites with standard deviation bars. AOD values sharply vary between spring (Mar, Apr) and autumn (Oct, Nov) seasons indicating the presence of dust activities during pre-monsoon, while AE decreases at the same time from Mar to Nov, which is attributed to the greater contribution of coarse mode aerosols, especially dust transported from Cholistan and Thar deserts to the atmosphere of Lahore and Karachi. Over Lahore, AOD (AE) values are in the range of 0.68 − 0.92 (1.16 − 1.28) in winter, 0.53 − 0.70 (0.72 − 0.96) in spring, 0.91 − 1.09 (0.75 − 1.02) in summer, and 0.84 − 1.01 (0.97 − 1.23) in autumn, respectively. A decrease in AE value between Apr to Jun, at both sites, is linked mainly with dust loadings. At the Lahore and Karachi sites, aerosols are not only produced through local sources but are also transported from the transboundary, which results in smog production and reduced atmospheric visibility (especially in autumn and winter) that may contribute to deteriorating air quality. Furthermore, it has been seen that with increasing AOD values, PLDR values decrease which means that particle dimensions become smaller. There is a positive correlation coefficient (r = 0.35) between monthly mean AOD and AE at Lahore, while a negative correlation coefficient (r = − 0.30) has been seen for the Karachi site. There are more contributions of coarse mode (86% and 99%) aerosols than fine mode (14% and 1%) at both Lahore and Karachi sites, respectively. In Fig. 4, the variation in AOD-AE is shown with PLDR and SSA at 1020nm which shows that with an increase in PLDR, AOD decreases as the dimensions of atmospheric particles become larger i.e. more coarse mode aerosols. Also is observed that there is a strong negative correlation coefficient (r = − 0.94) between PLDR (1020) and AE at both Lahore and Karachi sites. Table 3 shows the statistical presentation of direct and inversion products during the whole study period.
Table 3
Monthly mean statistic variations in AOD, AE, SSA, and PLDR for both sites during the study period
| Lahore | Karachi |
Parameters | Max | Min | SD (±) | Max | Min | SD (±) |
AOD (440nm) | 1.08 | 0.52 | 0.18 | 0.59 | 0.39 | 0.07 |
AE (440-870nm) | 1.28 | 0.72 | 0.19 | 0.99 | 0.35 | 0.23 |
SSA (1020nm) | 0.96 | 0.89 | 0.02 | 0.87 | 0.96 | 0.02 |
PLDR (1020nm) | 0.17 | 0.02 | 0.05 | 0.10 | 0.26 | 0.05 |
3.2. PLDR Correlation with SSA and VPSD
The spectral behavior of SSA and VPSD is investigated for various intervals of PLDR taken from the Lahore and Karachi site measurements. Investigations are carried out into how sensitive these parameters are to PLDR variations and how well they could categorize atmospheric particles. The distinguishable patterns of spectral changing of the average SSA value for various PLDR intervals for Lahore and Karachi, respectively, for the measurement period between 2015 and 2023 are shown in Fig. 5. A positive (negative) linear relationship between SSA and wavelength for PLDR ⩾ 0.29 (PLDR < 0.10) indicates the mineral dust (biomass-burning smoke and anthropogenic pollution (Giles et al., 2012). For Lahore (Karachi), SSA values are in the range between 0.86 (0.88) and 0.92 (0.94) for PLDR < 0.10, and between 0.93 (0.94) and 0.97 (0.97) for higher PLDR values. Moreover, the highest range of SSA, for different intervals of PLDR, is observed at 440 nm at Lahore, in comparison to Karachi where the range of SSA is relatively equal at all wavelengths. Soot (or black carbon) has significant light absorption property in anthropogenic pollution, and it is the main absorber of light all over the radiation spectrum, whereas the composition of mineral dust − contain iron oxides, leading to strong light absorption at short wavelengths (Bergstrom et al., 2002; Derimian et al., 2008).
Figure 6 clearly shows the variation in fine (radius ≤ 0.6µm) and coarse-mode (radius ≥ 0.6µm) parts of the volume size distribution parameter for the different intervals of PLDR. The inversion technique returns the size distribution of aerosols in the range of 0.05 µm to 15 µm. Fine-mode aerosols (< 0.07 µm3/µm2) dominate at lower PLDR (< 0.10) values, while the contribution of coarse mode-aerosols to the total volume concentration (0.21 µm3/µm2 at Karachi, and 0.27 µm3/µm2 at Lahore) increases on increasing PLDR. PLDR values higher than 0.3 (lower than 0.08), with the Lidar measurements at 532 nm, indicating pure dust (biomass burning/smoke, anthropogenic pollution, or marine) aerosols (Burton et al., 2012; Shin et al., 2018). A range of PLDR from 0.08 to 0.30 indicates the presence of non-spherical particles and mixtures of mineral dust (Groß et al., 2011).
3.3. Characteristics of aerosol types
The spatial distributions of eight types of atmospheric components are shown in Fig. 7, considered for Lahore and Karachi sites for the period of 9 years (2015–2023). The occurrence rate of dust-free (MA + NA + SA + WA) pollution-type aerosols over Lahore is equal to 46% of weakly absorbing (WA), 25% of moderately absorbing (MA), 24% of non-absorbing (NA) and just 1% of strongly absorbing (SA) aerosol, and other types which contain pure dust (PD) and polluted dust (PDM, and DDM) are as 43% of Pure dust (PD), 34% of Dust-Dominated Mixture (DDM), 22% of Pollutant-Dominated Mixture, and just 1% of Pollution aerosols, respectively. The lowest percentage of aerosol type belongs to strongly absorbing (SA: <1%) aerosol-type in the dust-free category and pollution aerosol (PA: 1%) among the polluted-dust category over Lahore, while weakly absorbing (WA: 46%) and pure dust (PD: 43%) aerosols have the highest percentage occurrence. Less light-absorbing (NA + WA) aerosols are detected more frequently over Lahore. The occurrence rate of all aerosol types over Lahore in descending order is WA > PD > DDM > MA > NA > PDM > SA > PA.
Similarly over Karachi, the percentage of dust-free aerosol type is 44% for WA and NA, 11% for MA, merely 1% for SA, 49% for PDM, 30% for DDM, 13% for PA, and 8% for PD, respectively. Karachi site is mostly affected by pollutant-dominated mixture (PDM: 49%) aerosol-type among the pure-polluted dust category of aerosols. Similar to Lahore, the Karachi site also has a higher occurrence rate of less light-absorbing aerosols than more light-absorbing (MA + SA) aerosols. Similar to Lahore, the occurrence rate over Karachi is PDM > NA = WA > DDM > PA > MA > PD > SA. The high occurrence rate for PDM and DDM and relatively lower occurrence rate for PD over Karachi indicate that dust particles over Karachi are typically mixed with anthropogenic pollution types that will show the optical signature of a mixture in the column-integrated AERONET measurements.
On monthly and annual bases, (Fig. 8), it has been observed that the PD aerosol type is dominated over Lahore in autumn and winter seasons with the highest occurrence rate in winter (88% in Dec, 93% in Jan, 74% in Feb) and autumn (23% in Sep, 32% in Oct, 86% in Nov) season, and contributes 46% on annual average, respectively, and have a relatively lower concentration in spring (Mar: 19%, Apr: 4%) and summer (Jun: 7%, Jul: 31%, Aug: 26%) seasons. Prevailing of pure dust aerosols is the major cause of poor visibility during autumn and winter in Lahore. NA aerosol type sharply increases from May (10%) to Aug (87%) and have highest occurrence in summer (Jun: 31%, Jul: 64%, Aug: 87%) season and then decreases in autumn (Sep: 40%, Oct: 7%, Nov: 1%) and spring (Mar: 25%: Apr: 27, May: 10%), followed by winter (Dec: 5%, Jan: 10%, Feb: 15%) season, respectively, and contributes 27% yearly in total aerosol burden. Spring (Mar: 40%, Apr: 60%, May: 65%) and autumn (Sep: 51%, Oct: 53%, Nov: 51%) seasons have a high occurrence rate of WA aerosols, followed by summer (Jun: 65%, Jul: 33%, Aug: 9%) and winter (Dec: 43%, Jan: 37%, Feb: 28%) seasons. WA aerosol has a 45% occurrence rate on the annual average. DDM aerosol varies between 20%−60% during spring (Mar: 54%, Apr: 45%, May: 47%) and summer (Jun: 40%, Jul: 20%, Aug: 27%) season and have relatively higher concentrations during autumn (Sep: 49%, Oct: 59%, Nov: 13%) and lower in winter (Dec: 12%, Jan: 7%, Feb: 17%) season. It contributes 33% to the annual mean. PDM aerosol type varies between 20 to 55% throughout the study period over Lahore with more concentration in summer and spring and less concentration during winter and autumn. PDM has a 33% annual average aerosol contribution to the atmosphere of Lahore. Other types of aerosol (MA: 23%, SA: 8%, PA: 7%) have relatively low annual mean concentrations. Annual mean occurrence of all aerosol types in descending order over Lahore is PD (46%) > WA (45%) > PDM (33%) > DDM (32%) > NA (27%) > MA (23%) > SA (8%) > PA (3%). Higher concentrations of PD (winter + autumn), PDM (spring + summer), and DDM (spring + summer) indicate that dust particles are usually mixed with urban industrial pollution, while among the dust-free pollution particles; aerosols with weak absorption have the largest contribution. Dust-free pollution particles are observed in all seasons, with more occurrences as WA and NA in spring − summer, and MA and SA in autumn − winter, respectively.
At the Karachi site, among dust-free aerosols type, WA particles are persistent in winter (Dec: 90%, Jan: 17%, Feb: 21%), NA in spring (Mar: 60%, Apr: 40%, May: 68%) and summer (Jun: 55%, Jul: 50%, Aug: 67%), WA in all seasons especially in spring (Mar: 33%, Apr: 57%, May: 30%) and autumn (Sep: 36%, Oct: 48%, Nov: 65%), while SA rarely seen during the studied period, which indicates a possible combination of dust, urban/industrial aerosols, and sea salt over the Karachi region. Among the dust-containing categories, PDM aerosol-type are largely seen in all seasons, especially in spring (Mar: 66%, Apr: 86%, May: 55%) and autumn (Sep: 82%, Oct: 33%, Nov: 11%) seasons followed by winter season, while PD particles are dominated in winter (Dec: 20%, Jan: 50%, Feb: 7%) and autumn (Sep: 4%, Oct: 2%, Nov: 27%), respectively. on annual average, the occurrence rate of all aerosol types over Karachi is NA (47%) > PDM (44%) > WA (40%) > DDM (38%) > PA (22%) > MA (21%) > PD (17%) > SA (3%). Table 4 shows seasonal percentage occurrence rates of all aerosol types for both sites.
Collectively, the higher occurrence rates of light scattering aerosols i.e. NA and WA over Karachi and Lahore, respectively, indicate that both these sites are mainly affected by sulfates (SOx), nitrates (NOx), anthropogenic particles, dust, organic matter, and sea salt aerosols which are originated from industrial emissions, vehicle exhaust, manufacturing, agricultural activities especially during harvesting season, waste burning, and the use of biomass for cooking or heating purposes. Further, MA and SA aerosols i.e. light absorbing, are less in concentrations over both sites relative to polluted aerosols. Sulfates and other secondary inorganic aerosols mostly scatter light, while carbonaceous aerosols have considerable light absorption characteristics (Bergstrom, 2006).
Table 4
Seasonal frequency rates of dust-free and dust-mixed aerosol types over Lahore and Karachi AERONET sites during the studied period
Aerosol Type | | Winter | Spring | Summer | Autumn |
| | Lahore | Karachi | Lahore | Karachi | Lahore | Karachi | Lahore | Karachi |
MA | | 41.26% | 42.70% | 19.65% | 4.85% | 2.27% | 16.67% | 29.51% | 15.42% |
NA | | 10.35% | 33.04% | 20.44% | 55.10% | 60.94% | 57.22% | 16.28% | 38.62% |
SA | | 12.14% | 4.17% | 6.57% | | 1.82% | | 3.96% | 2.4% |
WA | | 36.25% | 33.89% | 55.53% | 40.05% | 36.18% | 37.22% | 51.56% | 49.46% |
DDM | | 11.59% | 54.29% | 48.88% | 14.15% | 29.11% | 32.96% | 40.49% | 45.55% |
PD | | 85.42% | 25.71% | 11.99% | | 25.04% | 26.67% | 49.16% | 10.92% |
PDM | | 8.96% | 20.00% | 42.60% | 68.61% | 43.01% | 10.83% | 15.01% | 40.95% |
High occurrence rates of PD (over Lahore) and PDM (over Karachi) mainly between Mar to Sep are due to the large amount of desert and mineral dust particles including dust from local sources and transported from Thar (India), since Karachi site is located near to Arabian Peninsula. Another factor is that both sites experience high levels of air pollution because of fog and haze carried by excessive aerosol loading as a result of increased anthropogenic emissions and dense population. Furthermore, aside from variations in the climatic phenomena, local emissions are also responsible for the seasonality of aerosol characteristics over the areas (Khan et al., 2019).
Lahore city and its divisions are under the influence of intense smog in autumn of 2023, with an average Air Quality Index (AQI) of 122.77µg/m3 in Sep, 185.26µg/m3 in Oct, and 288.00µg/m3 in Nov, respectively. Such high values of particulate matter (PM2.5) over Lahore and its neighboring districts have very “unhealthy” effects on human health. Every November or December for the past few years, pollution has descended on Lahore and the surrounding areas. This haze has grown much worse over the past five years due to high levels of pollution from industry and cars, as well as poor air quality. A notification for the deployment of a smart lockdown has been issued by the Provincial Disaster Management Authority (PDMA), Punjab (punjab.gov.pk). Ten districts—Lahore, Nankana Sahab, Sheikhupura, Kasur, Gujranwala, Gujrat, Sialkot, Narowal, Hafizabad, and Mandi Bahauddin—remain under lockdown, as per orders.