An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog, mist, dust, forest exudates and geyser steam. Examples of anthropogenic aerosols are particulate air pollutants and smoke. aerosol alter rainfall and monsoon patterns, it have negative impact on agricultural lands and it is harmful to human health.

**Explanation:**

**Aerosols** are tiny particles suspended in the atmosphere. They can be either solid or liquid particles and can originate from natural sources (such as dust and volcanic emissions) or human activities (such as industrial processes and burning of fossil fuels).

The given options state various effects of aerosols, and we need to determine which statement is not valid.

1. **a) They alter rainfall and monsoon patterns:**
Aerosols can have a significant impact on precipitation patterns. They can act as cloud condensation nuclei (CCN) and ice nuclei (IN), influencing cloud properties and processes. This can result in changes in cloud lifetime, cloud cover, and cloud microphysics, ultimately affecting rainfall and monsoon patterns. Therefore, this statement is valid.

2. **b) They cause increased agricultural productivity:**
This statement is not valid. While aerosols can have indirect effects on agriculture by altering rainfall patterns, the direct impact on agricultural productivity is generally negative. High concentrations of aerosols can reduce the amount of solar radiation reaching the Earth's surface, affecting photosynthesis and crop growth. Additionally, some aerosols can deposit on plants, leading to damage and reduced yields.

3. **c) They have a negative impact on agricultural land:**
This statement is valid. Aerosols can contribute to soil degradation through the deposition of particles. This can affect soil fertility, nutrient availability, and water-holding capacity, leading to reduced crop productivity and deterioration of agricultural land.

4. **d) They are harmful to human health:**
This statement is valid. Inhalation of aerosols, especially those containing toxic substances or fine particulate matter (PM2.5), can have detrimental effects on human health. Fine particles can penetrate deep into the respiratory system, causing respiratory and cardiovascular diseases, and even premature death.

Therefore, the correct answer is **option B**: They cause increased agricultural productivity.